Custom Bag Dimensions: Waterproof Bag Sizing

A custom bag size is not three numbers you choose; it is three numbers you derive, from the load it has to carry, from the body it has to fit, and from the carton it has to ship in. Get the order wrong and the bag fails in one of three predictable ways: the contents do not fit, the bag does not carry comfortably, or the freight tier moves and the landed cost changes after the price was agreed. Capacity in litres is the least useful of the numbers involved, because two bags of identical volume and completely different proportions pack and carry nothing alike, and because the industry has no agreed method for measuring it — so a twenty-five litre claim from one supplier and a twenty-five litre claim from another can differ by a fifth.

This guide covers why litres mislead and what to specify instead, the three carry geometries that set the real dimensions, how to dimension from the load rather than from a competitor, how tolerances stack through cutting, welding and foam into a cumulative error nobody specified, the measurement method that has to be written down or every dispute is lost, what disappears into seam and weld allowances, the roll-top trap that turns a twenty litre dry bag into a sixteen litre one, the packaging and freight ceilings that cap maximum size, what a dimension change actually triggers in tooling and cost, and why a sealed physical sample settles arguments that drawings never do. Production reference for this guide — QUANZHOU JUNYUAN BAGS, custom waterproof bags since 2014, 4,950 m² SGS-verified facility, MOQ 500 pieces per style, sampling in 6–10 working days, bulk in 35–50 days, FOB Xiamen.

Measuring a waterproof backpack panel during specification — Two bags can hold the same litres and pack completely differently.

Waterproof tote with handle drop being checked against the shoulder — Handle drop is a dimension, and it is the one most often left to chance.

Roll-top dry bag with three rolls closed — A dry bag has two capacities: rolled and unrolled. Quote both or argue later.

Volume is not a dimension: why litres mislead

Most custom bag dimensions start life as a capacity target — "we need a twenty-five litre dry bag" — and that single sentence is the origin of most sizing disputes. Capacity tells you nothing about proportion, nothing about opening size, and nothing about how the volume is distributed. A tall, narrow twenty-five litre bag carries a camera body and a jacket badly, tips over when set down, and cannot be packed through a narrow mouth. A wide, flat one of the same volume packs flat items beautifully and bulky items not at all. In waterproof bag sizing the proportion is the decision and the litre figure is the consequence.

The second problem is that there is no single agreed way to measure it. Fill a bag with water and you get one number. Fill it with twenty millimetre plastic balls and you get a smaller number, because the balls do not pack into corners. Compute it from external geometry and you get a larger number, because that includes the space occupied by fabric, foam, seams and internal dividers. The spread between methods on the same bag is routinely ten to fifteen per cent, which is larger than most of the differences buyers argue about. If the capacity claim has commercial value on a listing, the measurement method has to be written into the specification alongside it.

The third problem is usable versus geometric volume, and it is worse than the first two. Geometric volume is what the envelope encloses. Usable volume is what you can actually get into the bag and then close. A stiff shell does not conform around contents, so air pockets remain. A roll-top closure consumes height. Internal pockets and dividers consume volume and then do not compress. A bag with a padded base loses several centimetres of usable height to the padding. The gap between the two figures is not a rounding error — on a roll-top dry bag it can be a quarter of the stated capacity.

State capacity as geometric and usable, with the closure method that defines usable.
State the measurement method: water displacement, ball fill, or dimensional calculation.
State whether the figure is taken empty, packed flat, or packed with a named load.
Never let a capacity claim be the only dimensional requirement on a tech pack.

A practical framing that resolves most of this: capacity is a marketing number and dimensions are an engineering number. Market the capacity you can defend, engineer to the dimensions the load requires, and make sure the two are reconciled before sampling rather than afterwards. Our separate dry bag sizing guide approaches the same question from the retail buyer’s side of the counter.

The three carry geometries that set the real dimensions

Bags are carried in three fundamentally different ways, and each one imposes its own dimensional logic. Copying a dimension across categories is the most common specification error, and it is why a tote sized like a backpack feels wrong and a backpack sized like a duffel carries badly.

Carry type	Dimension that actually governs fit	Typical range	What happens if it is wrong
Backpack	Torso length, not overall height; then hip belt wrap	Adult torso 380–530 mm; belt wrap 800–1,300 mm	Load sits on the shoulders; strap geometry pulls the bag away from the back
Shoulder tote or messenger	Handle drop, then strap length range	Hand carry 100–150 mm; shoulder 250–350 mm	Handle too short to shoulder; too long and the bag swings and hits the hip
Dry bag or duffel	Circumference and mouth diameter, then length	Mouth 180–400 mm; length 400–900 mm	Contents will not go in, or the roll cannot be closed around the load
Waist or sling	Strap length range and body contact footprint	Adjustable range 600–1,100 mm	Cannot be tightened enough to stabilise, or dangles

For backpacks the torso measurement deserves emphasis because it is the number buyers never supply. Torso length is measured from the seventh cervical vertebra to the iliac crest, not from the top of the bag to the bottom, and it is what determines whether a load transfers to the hips. A bag whose panel is longer than the wearer’s torso puts weight above the shoulders and pulls backwards; one that is too short leaves the hip belt riding on the waist rather than on the pelvis. Sizing a backpack around body height instead of torso length is the reason "it fits but it hurts" returns happen.

Handle drop, the second row, is the dimension most often left entirely to chance. It is simply the distance from the top of the handle to the top of the bag, and it decides whether the bag goes over a shoulder, over a shoulder with a coat on, or nowhere at all. Hundred to one hundred and fifty millimetres is hand carry. Two hundred and fifty to three hundred and fifty millimetres is shoulder carry over light clothing, and more is needed for winter outerwear. Retail buyers reject bags on this alone, and it costs nothing to get right.

Strap and handle dimensions interact with load in ways that matter for durability as well as comfort, and the geometry is worth checking before tooling. The load path work behind this is covered in our article on load stress testing for straps and handles.

Dimensioning from the load, not from the competitor

The most reliable way to size a product is to start with a physical list of what must fit, give each item a measured envelope, and add ease. This is unglamorous and it takes an afternoon, and it prevents the most expensive class of sampling failure — the bag that is dimensionally correct and functionally useless.

List the contents with actual measured dimensions, not nominal ones. A "fifteen inch laptop" is not 381 mm; measure the specific model with its corners.
Add ease of roughly 10 to 15 mm per side for soft goods, less for a close-fitting sleeve, more for anything rigid or bulky.
Organise the list into a packing arrangement rather than a sum of volumes. Volume adds; arrangement constrains.
Identify the single largest rigid item, because it sets the minimum opening and usually the minimum internal depth.
Decide whether the load is packed in cubes or loose. Cubes impose their own grid on the internal dimensions and should be specified together with the bag.

Copying a competitor’s dimensions fails for a specific reason worth understanding. That bag was dimensioned around that brand’s contents, that brand’s fabric thickness, that brand’s foam density and that brand’s closure hardware. Change any of those four and the same external envelope yields a different internal volume. A bag copied at external dimensions from a thin-shell product and then built with ten millimetre closed-cell foam throughout loses a substantial fraction of its usable space, and the buyer discovers it only when the product is compared side by side in a showroom.

There is one legitimate use for competitor dimensions, which is as a sanity check on proportion and on retail expectations. Shoppers have learned what a thirty litre pack looks like, and a product that departs wildly from the category norm photographs oddly on a shelf even if it is functionally better. Use the reference for proportion, then derive the actual numbers from your own load list.

Tolerance stacking: where custom sizes actually go wrong

This is the part of sizing that separates a specification from a drawing. Every operation in the sequence contributes its own variation, and those contributions add. A buyer who specifies 450 mm with no tolerance and a factory that holds 450 mm plus or minus ten has not disagreed about anything — neither of them wrote down what was actually meant, and the argument only surfaces when the bag no longer fits the thing it was designed to hold.

Contributor	Typical variation	Worst-case sum	Root sum square
Panel cutting	± 2.0 mm	± 2.0 mm	± 2.0 mm
Hem or turn-under	± 1.5 mm	± 3.5 mm	± 2.5 mm
Weld or seam position	± 1.2 mm	± 4.7 mm	± 2.8 mm
Foam or padding thickness	± 3.0 mm	± 7.7 mm	± 4.1 mm
Base insert or stiffener	± 2.0 mm	± 9.7 mm	± 4.6 mm
Closure and roll variance	± 2.5 mm	± 12.2 mm	± 5.2 mm

The two columns on the right are the whole point. Worst-case stacking assumes every contributor happens to be at its limit in the same direction simultaneously, which is conservative, rarely occurs, and if written into a specification will be quoted as a much more expensive product. Root sum square, the square root of the sum of the squares, assumes the variations are independent, which they mostly are, and gives a figure that matches reality far better. On this example the difference is 12.2 mm against 5.2 mm — and that difference is the entire argument.

The correct approach is to split the specification by function rather than to apply one tolerance everywhere. Dimensions that have to mate with something else — a laptop sleeve, a lid that has to close over a load, a zipper that has to meet a stop, hardware that has to align, a panel that has to fit a moulded frame — need a tight stated tolerance and a defined measurement method. Dimensions that only affect appearance or packed volume can carry a generous one. A tech pack that says "plus or minus five millimetres on all dimensions" is both too tight on the cosmetic and too loose on the critical.

General tolerancing practice for manufactured parts is codified in the ISO general tolerances series, which is worth reading once if you specify dimensions regularly. The transferable idea is the one above: tolerance is assigned by function, and unstated dimensions inherit a default.

The measurement method nobody writes down

Every dimensional dispute that reaches a factory floor is, underneath, a disagreement about how the measurement was taken. Two people measuring the same bag in good faith will report different numbers if one measures it empty and flat and the other measures it stuffed; if one measures seam to seam and the other measures at the widest bulge; if one lays it on a table and the other measures it hanging; if one pulls the tape tight and the other drapes it. None of them is wrong. The specification simply failed to say.

State the condition: empty and flat, lightly filled, or filled with a defined load.
State the reference points: seam to seam, edge to edge, or outermost to outermost including hardware.
State whether the tape is pulled taut or laid relaxed, because on a curved panel the difference is several millimetres.
State the units and the rounding, and whether the figure is a target or a limit.
Mark the measurement points on a dimensioned diagram, not in prose. A diagram with numbered points removes most ambiguity in one step.

A numbered dimension diagram with a tolerance beside each number is perhaps the highest-value half-hour in the whole development process. It also becomes the inspection sheet, so the same document that prevents the argument is later used to settle it. What to include in the file set, and how to supply it so it survives translation into a cutting pattern, is covered in our guide to submitting effective design files.

What disappears into the seam

External dimensions are not internal dimensions, and the difference is not small. Every joint consumes material, and a bag specified purely at external envelope will come back with less usable space than expected. The allowances are predictable, and they should be subtracted before the external size is fixed rather than discovered afterwards.

Stitched seams typically consume 10 to 15 mm per side in seam allowance, plus whatever the fold takes.
Welded seams consume the weld width, commonly 10 to 20 mm, plus 3 to 5 mm of material outside the weld that carries no load.
Bound or taped seams add the tape width plus the fold, and on curved panels the binding pulls the edge inwards slightly.
Boxed corners convert width and depth into each other, and the corner geometry consumes depth from the base panel.
A curved or domed lid measures shorter flat than it does assembled, which trips up anyone specifying a lid from a flat pattern.

Circumference conversions are the specific place this bites. A tubular body made from a flat panel needs a panel width equal to the circumference plus the joint allowance. A dry bag with a 250 mm diameter base has a circumference of roughly 785 mm; build it with a single welded seam and the flat blank has to be around 815 mm wide, not 785. Scale that across a body panel and it changes how many panels nest across the fabric roll width, which changes material consumption, which changes price. The joining routes and what each consumes are compared in our article on stitched, welded and bonded construction.

One more consequence worth planning for: welding equipment has a maximum effective seam length per pass, set by the platen or the horn, commonly in the region of 600 to 800 millimetres. A body seam longer than that has to be made in two or more passes, and the overlap between passes leaves a visible step and a locally thicker joint. It is not a defect, but it is a design constraint that argues for keeping long seams within a single pass where the product allows.

Closures and the usable-volume trap

The closure is where the gap between geometric and usable capacity is largest, and it is the single most common source of a capacity complaint. Take a roll-top dry bag. Roll it closed properly — three full rolls, which is what any competent user does — and you consume roughly 100 to 150 millimetres of height, depending on fabric thickness and how tightly it is rolled. On a bag of 250 millimetres diameter, 130 millimetres of height is about 6.4 litres. A bag sold as twenty-five litres unrolled is a bag that holds roughly eighteen and a half litres as a customer will actually close it.

That is not dishonesty; it is an unspecified measurement condition. The fix is simply to state both figures and to be clear which one is on the label. Several reputable brands now quote rolled and unrolled capacity precisely because the confusion was generating returns.

Closure type	What it consumes	Effect on usable capacity	Dimensional consequence
Roll-top, three rolls	100–150 mm of height	Loses 15–25 per cent of stated volume	Body must be taller than the target by the roll allowance
Horizontal zipper	Zip length minus corner stops	Opening narrower than the panel by 60–100 mm	Minimum internal dimension must exceed the largest rigid item
Lid with buckles	Lid depth plus compression	Loses the lid volume unless the lid floats	Lid has to be sized over the loaded body, not the empty one
Drawcord or cinch	Gather allowance	Small, but the mouth never closes fully waterproof	Mouth circumference sets the minimum, not the body

The zipper row contains the second most common packing failure: an opening that is narrower than the item that has to go through it. A zip that stops 40 mm short of each corner loses 80 mm of usable opening, and because the fabric around a zip is stiffened by the tape, the effective opening is narrower still. If a helmet, a camera body or a shoe has to go in, size the opening from that object with ease, then design the zip around it. Closure engineering is treated in more depth in our review of zipper and roll-top closures and the dedicated piece on roll-top closure design.

Packaging, carton and freight ceilings

Maximum size is rarely set by what can be manufactured. It is set by what can be shipped economically, and that constraint arrives late in the process with an unwelcome price attached. Three ceilings matter, and they are worth checking before the dimensions are frozen rather than after the first shipment is quoted.

Ceiling	Typical limit	Why it bites	What to do
Air volumetric weight	Chargeable at roughly 167 kg per cubic metre	A light bulky bag bills as if it weighed many times its actual weight	Design to ship flat or rolled; state the packed dimension as a requirement
Parcel and courier tiers	Oversize thresholds around 1.0–1.2 m on the longest side	Crossing a tier adds a surcharge per unit	Keep the longest packed side under the tier boundary
Marketplace size tier	Standard tiers around 450 × 350 × 200 mm in some fulfilment programmes	A dimension change can move a product into an oversize fee band	Check the fee tier before finalising, not after listing
Pallet and container	Pallet footprint 1,200 × 800 or 1,200 × 1,000 mm; air pallet height around 1.6 m	Awkward footprints waste pallet volume and raise cost per unit	Design cartons that stack to a clean pallet layer
Retail shelf and peg hook	Shelf depth commonly 300–400 mm; hook spacing fixed	A bag deeper than the shelf cannot be faced normally	Ask the retailer for the planogram limits early

The air freight row deserves a worked example because the numbers are startling the first time you see them. A rolled dry bag occupying 25 litres is 0.025 cubic metres. At a volumetric divisor of 6,000 it bills as roughly 4.2 kilograms, against an actual weight that might be 0.4 kilograms. You pay for ten times the weight you are shipping. Ship the same bag flat-packed in a carton at 0.008 cubic metres and the chargeable weight drops to about 1.3 kilograms. That difference, multiplied across a container of product, is frequently larger than the entire material cost difference between two fabric options — which is why packed dimensions belong on the tech pack as a hard requirement, not as an afterthought for the logistics team.

Carton sizing has its own minimum. An odd carton size is a custom carton, and custom cartons carry their own production minimum, often in the hundreds of pieces, plus a lead time. Changing the bag dimensions after the carton has been ordered can mean paying for cartons twice. Our articles on custom packaging options and international shipping logistics cover the downstream decisions in detail, and GS1 maintains the standards most retail logistics systems expect cartons and labels to follow.

Tolerance against manufacturing reality

A tolerance on paper has to be one the process can actually hold, and the honest limits differ by operation. Cutting with a steel die or an automated cutter holds about a millimetre. Manual cutting with a template holds two to three on a good day, and less on a thick or slippery coated material. Laser cutting seals the edge, which is useful for fraying but slightly reduces the dimension and stiffens the edge. Welding holds position well but the weld width varies with material thickness and with how the operator lays the panel.

Foam and padding are the loosest contributors by a wide margin. Closed-cell foam sheet thickness commonly varies by ten to fifteen per cent, and it compresses permanently under sustained load, which means a dimension measured on a fresh sample is not the dimension after three months of use. Where a padded compartment has to hold a specific device, specify the internal dimension at a stated compression and test it after conditioning rather than on the day the sample arrives.

Filling state matters more than most buyers expect. A soft bag measured empty and laid flat, then measured lightly stuffed, differs by five to twelve per cent in its external dimensions, and the stuffed figure is the one the customer sees. This is why the measurement condition has to be stated twice: once for approval of the sample and once for inspection of the shipment, and they should be the same condition.

Agree which dimensions are checked at pre-shipment inspection, and on how many pieces. Checking every dimension on every unit is unaffordable; checking nothing is how shipments drift.
Set an acceptance rule: a tolerance band, a sample size, and an action if the band is exceeded. This is the same logic as any attribute inspection plan.
Record the dimension report against the golden sample rather than against the drawing. The drawing is interpreted; the sample is not.

The acceptance-rule thinking above is the same structure used elsewhere in the industry and is described in our buyer’s guide to AQL sampling and the companion pre-shipment inspection checklist, both of which assume the dimensional requirements were written down first.

Hardware and closures have to be dimensioned with the bag

Hardware is bought in stock sizes and stock lengths, and it quietly constrains the design. Webbing and zippers in particular are sold in increments, and a bag dimensioned without reference to those increments either wastes material or forces a custom component with its own minimum quantity.

Zipper length: stock chain is supplied in increments, often around 50 mm. Custom lengths usually mean a higher unit cost and a component minimum.
Webbing: strap length has to cover the body dimension plus the adjustment range. A useful adjustment range is 200 to 300 mm; less than that and the bag cannot be tuned across clothing layers.
Buckle and ladderlock placement: determined by panel width. Moving a panel dimension by 20 mm can move every attachment point and invalidate the reinforcement layout.
Handle drop and grab handle position: these set the balance point, and a bag that is dimensionally correct can still carry badly if the handles are in the wrong place.
D-rings and attachment points: their position is dimensioned from seams, not from the silhouette, because the seam is the only repeatable reference on a soft product.

That last point is a useful general rule for soft goods: dimension from a seam or a join, never from a silhouette edge, because the silhouette moves and the seam does not. It costs nothing to write the reference into the diagram and it removes an entire category of ambiguity. Hardware selection itself is covered separately in our guide to custom hardware selection.

The physical sample is the contract

Drawings do not settle dimensional disputes; a physical reference does. The reason is simple: a drawing has to be interpreted, and two people interpreting the same two-dimensional representation of a three-dimensional soft object will interpret it differently. A sealed sample does not need interpreting. The practical rule is that every custom programme should produce two identical approved references from the same production run — one held by the buyer, one held at the production site — and every subsequent shipment is judged against the sample rather than against the drawing.

Approve the golden sample with a signed dimension report, not with a photograph.
Photograph the sample next to a scale and next to the largest item it must contain.
Fit-test with the actual contents rather than with a stand-in. A laptop sleeve tested with a book is not a laptop sleeve test.
Keep the sample in controlled conditions; foam and coated fabrics move with heat and humidity, and a sample stored badly becomes a bad reference.
Re-approve after any material, foam or tooling change, because those change dimensions even when the drawing does not.

Fit-testing with real contents is the step most often skipped, and it is the cheapest way to catch the failure that costs the most. A bag that measures correctly and will not accept the product it was designed for is a total loss on the run, and it happens regularly in categories where the contents are bulky and rigid: camera bags, helmet bags, lunch and cooler bags, tool bags. Put the object in the sample, close it, and carry it before approving anything.

What to expect during that approval stage, and how much iteration is normal, is set out in our guide to custom waterproof bag sampling. The wider development sequence is described in design from concept to production.

What a dimension change actually triggers

A small dimensional change looks free and rarely is. Because the geometry propagates into tooling, material consumption and packaging, changing one number can trigger a cascade. The table below is the honest list, and it is the reason dimensions should be frozen once and changed reluctantly.

Change	What it triggers	Typical consequence
A few millimetres within existing tooling	Updated cutting pattern only	Low cost, quick; usually absorbed
A change that crosses a marker or roll-width boundary	New nesting; material consumption rises	Unit cost can move 8–15 per cent with no visible design change
New panel size outside existing dies	New cutting die or template	Tooling charge plus a few days of lead time
New seam length beyond one weld pass	New jig or multi-pass procedure	Longer cycle time and a visible overlap step
New packed or carton dimension	New carton specification	Carton minimum quantity, often in the hundreds, plus lead time
Crossing a freight or fulfilment size tier	Higher per-unit freight or fulfilment fee	Landed cost changes after the retail price was set

The second row is the one that surprises people. Fabric is bought by the metre and cut from a marker, and marker efficiency is a step function of panel width. Nudge a panel from 740 to 760 millimetres and you may go from three panels across a roll to two, which raises consumption per bag by a third on that panel. There is no way to see this from the bag itself, which is why a dimensional change should always be quoted rather than assumed.

Where this money appears in the finished quotation is laid out in our custom waterproof bag cost breakdown, and the packaging-specific consequences are treated in our article on packaging design for retail. For sellers listing on marketplaces, the fee-tier arithmetic is covered in our piece on building a profitable marketplace listing.

A worked example: dimensioning a 25 litre roll-top dry bag

Numbers make the method concrete. Suppose the product is a roll-top dry bag that must hold a synthetic sleeping bag, a change of clothes and a wash kit, and must be sold as twenty-five litres. Work backwards from the load rather than forwards from the label.

Set the base diameter from the load: a compressed synthetic sleeping bag sits comfortably in a 250 mm diameter, so choose 250 mm. Circumference is roughly 785 mm.
Compute unrolled height for the geometric volume: a cylinder of 250 mm diameter holds 4.91 litres per 100 mm of height, so 25 litres geometric needs about 510 mm.
Add the roll allowance: three rolls consume roughly 130 mm. If the label must be honest when closed, the body needs about 640 mm of unrolled height, giving roughly 31 litres geometric and about 25 litres usable when rolled.
Size the blank: a single welded seam needs the circumference plus the joint allowance, so the body panel is about 815 mm wide by 640 mm tall, plus the base weld allowance.
Check the weld: a 640 mm vertical seam is inside a single platen pass on most equipment, so the seam is continuous and clean.
Check the roll: 130 mm of roll across an 815 mm width has to be achievable without buckling the stiffened top edge, which argues for a soft coating in that region.
Check the carton: rolled flat the blank is roughly 815 by 640, which sets the carton footprint and therefore the pallet layer count and the volumetric weight.
Then check the load: pack the actual sleeping bag, clothes and wash kit into a sample, roll it three times, and confirm it closes. Adjust from there, not from the arithmetic.

Two things usually come out of doing this. First, the honest usable capacity is often smaller than the marketing number, and it is far cheaper to change the label than to change the bag. Second, the largest single lever is usually the base diameter rather than the height, because volume scales with the square of the radius — going from 250 to 280 mm diameter adds about twenty-five per cent of volume for no extra height and no extra roll allowance. That is a genuinely useful piece of arithmetic and it is invisible if you start from a litre figure.

The same backwards method applies to every category. Start from the load envelope, choose the proportion from the carry geometry, add what the closure and the seams consume, check the result against the freight and packaging ceilings, and only then write the number down. It takes longer than copying a competitor’s dimensions and it removes most of the reasons a custom size programme fails.

If you want this worked through on your own style, send the contents list, the carry mode and the target capacity, and let the dimensions be derived rather than assumed. Every style starts at 500 pieces minimum, with samples in 6–10 working days and bulk in 35–50 days, quoted FOB Xiamen.

Frequently Asked Questions

Q1. Why do two bags with the same litre rating pack differently?

Because litres describe volume and packing is governed by proportion, opening size and how the volume is distributed. A tall narrow bag and a wide flat one of identical capacity behave completely differently, and the opening size usually matters more than the total.

Q2. How is bag capacity actually measured?

Three common ways, and they disagree by ten to fifteen per cent: water displacement, filling with standard balls, and calculation from external dimensions. Always state which method the capacity claim refers to.

Q3. What is the difference between geometric and usable capacity?

Geometric is what the envelope encloses. Usable is what fits and still closes. Roll-top closures, stiff shells, padding and internal dividers all consume usable volume, and the gap is often a fifth to a quarter of the stated figure.

Q4. How much capacity does a roll-top closure consume?

Three full rolls typically take 100 to 150 millimetres of height. On a 250 mm diameter bag that is about six litres, so a bag sold as twenty-five litres unrolled holds roughly eighteen and a half when closed as a customer closes it.

Q5. What tolerance should I specify on bag dimensions?

Assign tolerance by function rather than applying one figure everywhere. Anything that mates with hardware, a device or a closure needs a tight stated band; purely cosmetic dimensions can carry a generous one.

Q6. What is tolerance stacking?

The accumulation of variation from every operation: cutting, hemming, welding, foam thickness, stiffeners and closure. Worst-case summing is very conservative; root sum square matches reality far better and is what should be written down.

Q7. Why do two people measure the same bag differently?

Because the measurement condition was never specified. Empty or filled, seam to seam or outermost, tape taut or relaxed, flat or hanging all give different answers. Specify the condition and mark the points on a diagram.

Q8. How much material do seams and welds consume?

Stitched seams take 10 to 15 mm per side, welded seams take the weld width of 10 to 20 mm plus a few millimetres outside it, and bound edges add the tape width. External dimensions are always larger than usable ones.

Q9. Should I dimension from the silhouette or from the seams?

Always from the seams or joins. The silhouette of a soft product moves with filling and handling; the seam is the only repeatable reference on the pattern.

Q10. What is handle drop and why does it matter?

The distance from the top of the handle to the top of the bag. Around 100 to 150 mm is hand carry, 250 to 350 mm is shoulder carry over light clothing. Getting it wrong is a common retail rejection and it costs nothing to get right.

Q11. What is torso length and why is it not the same as height?

Torso length runs from the seventh cervical vertebra to the iliac crest and determines whether a backpack transfers load to the hips, typically 380 to 530 mm in adults. Sizing by body height instead is the main cause of packs that measure right but carry badly.

Q12. How do I stop a bag being expensive to ship?

Design the packed dimension, not just the product dimension. Air freight bills on volume at roughly 167 kg per cubic metre, so a light bulky bag can be charged as if it weighed ten times its actual weight.

Q13. Can I change the dimensions after sampling?

Yes, but quote it rather than assuming it is free. A small change can cross a marker width and raise fabric consumption by eight to fifteen per cent, or trigger a new die, a new carton minimum or a different freight tier.

Q14. What is a golden sample and why do I need two?

An approved physical reference from production that later shipments are judged against. Keep one with the buyer and one at the production site, because a drawing has to be interpreted and a sample does not.

Q15. How many units should be dimensionally checked before shipment?

Define which dimensions matter, a tolerance band, a sample size and an action if the band is exceeded. Checking every dimension on every unit is unaffordable, and checking nothing is how shipments drift.

Q16. Does foam change the dimensions over time?

Yes. Closed-cell sheet thickness varies ten to fifteen per cent as supplied and compresses permanently under sustained load, so a padded compartment measured fresh will not measure the same after months of use.

Q17. What is the fastest way to make a bag bigger without making it taller?

Increase the base diameter. Volume scales with the square of the radius, so moving from 250 to 280 mm diameter adds roughly twenty-five per cent of volume with no extra height and no additional roll allowance.

Size and Dimension Customization for Waterproof Bags