A 10L Dry Bag Is Not 10 Litres: What the Roll Takes Back

A 10L dry bag does not hold ten litres of your things, and nobody is lying to you. The litre figure on a roll-top sack describes the open cylinder, measured to the top of the body. Before it is a dry bag at all you have to give three folds of that cylinder back to the closure — and the volume those folds consume does not scale with capacity. On a large sack it is a modest deduction. On a small pouch it is a third of the bag. That is the single most useful thing to understand before you choose a size for a few hundred recipients.

This is a sizing article about roll-tops specifically, because a rolled litre and a packed litre are different quantities. It covers where the number comes from, the arithmetic of what the roll takes back, the second constraint that catches people out even when the volume works, and what each of our size bands realistically carries. If you are choosing between dry bags from 2L to 20L for a programme, start here rather than with the product photographs.

Where the litre figure on a dry bag comes from

A roll-top sack is a cylinder with a welded base and an open mouth. Its stated capacity is the volume of that cylinder from the base to the top of the body panel, before anything is folded. It is a real measurement of a real shape, and it is also, unavoidably, a measurement of the bag in the one state it is never used in — open.

Two things happen the moment you close it correctly. The height drops, because three folds of the top band consume band depth plus the material each turn takes to make the turn. And the shape changes, because a sealed roll-top is not a cylinder any more. It is a flattened wedge: rectangular and compressed at the top where the roll is, round and full at the base. Volume you could have used in an open cylinder is simply not available in a wedge.

The roll tax, and why it is unfair to small bags

Here is the part that is rarely spelled out. The depth of the top band is set by what a human hand can fold, not by the capacity of the bag. A band has to be deep enough to take three even folds and still leave the buckle halves meeting. That is a broadly similar depth whether the bag is a small pouch or a large sack. So the roll consumes a slab of the cylinder that is roughly the same height on every size — and the same slab is a far bigger share of a short bag than of a tall one.

You can work out the deduction on any sample in about a minute, and we would rather you did that than take a figure from us.

1. Measure the depth of the top band on the sample, from the top edge down to where the band meets the body.
2. Multiply by roughly 3.3. Three folds plus the material consumed in making each turn is a little more than three times the band depth.
3. That result is the height of the cylinder you no longer have. Compare it against the total height of the bag to get the deduction as a percentage.
4. Then take off a little more for the wedge shape at the top, which is usable for soft items only.

Run that on a small pouch and a large sack from the same range and the asymmetry is obvious immediately. It is also why a small dry bag feels disproportionately small in the hand compared with its stated capacity, and why recipients who over-pack a small bag end up folding it once, which takes you straight back to closing a roll-top properly.

The practical rule we give buyers: choose the size that fits your contents with the bag closed and folded three times, then go one band up. The bag that is exactly big enough will be closed badly, and a badly closed dry bag is not a dry bag.

The constraint that catches people out: diameter, not volume

Volume is only half the sizing problem. A roll-top sack has a fixed circumference, so it has a fixed maximum diameter and a fixed maximum flat width. A rigid item longer than the bag’s usable height will not go in no matter how much spare volume there is, and a rigid item wider than the flat width will not lie down.

This is why briefs written purely in litres go wrong. A tablet, a rolled towel, a pair of fins, a document folder, a tripod, a water bottle stood upright — each of these is a dimension problem before it is a volume problem. Our first question on any sizing brief is not “how many litres” but “what is the longest rigid thing that has to go in, and does it have to stand up or can it lie flat”.

What each size band actually carries

These are the loads we see people put in each band in practice, described in objects rather than litres. The right-hand column is the honest caveat.

If a single band has to serve an entire mixed group, the larger of the two candidates is almost always the right answer. Recipients under-fill a large bag happily. They over-fill a small one and then close it badly.

Divided bags never give you the full number in either half

A wet and dry bag splits the interior so damp kit can be isolated from everything else. It is a genuinely useful format, and it comes with an arithmetic consequence worth stating plainly: a divider does not create volume, it allocates it. A divided bag of a given nominal capacity gives you two compartments whose combined usable volume is slightly less than the undivided version, because the divider itself occupies space and because neither half can borrow from the other.

It is still the right choice where the whole point is separation — swimwear away from a phone, wet shoes away from dry clothes. Just size it as two small compartments rather than as one large bag, and check the closure arrangement carefully, since a divided bag may have two closures to get right instead of one.

The pack-out card

This is the framework we use on sizing calls and the one we suggest you circulate internally before signing off a size. It takes three lines and it prevents nearly every sizing complaint we have seen.

1. List the contents. Not categories — actual objects. “Towel, T-shirt, shorts, phone, wallet, sunblock” beats “a change of clothes”.
2. Find the longest rigid item and measure it. If it is longer than the usable height of your candidate size, the size is wrong regardless of volume.
3. Add the roll allowance. Take the band-depth arithmetic above, then choose the band that still closes with three folds when the listed contents are inside.

Once the size is settled, the remaining decisions are construction and exposure — welded seams and the film underneath on one side, and splash, downpour and immersion on the other. Sizing first, though, because the size changes both of them.

What size does to your carton count

One consequence of sizing that only appears at the shipping stage: dry bags ship flat, and a larger band and stiffer film pack thicker. A jump of one size band is not a proportional jump in carton volume, it is a step, and it interacts with how many bags fit per carton and therefore with your shipping and storage. If your programme has a fixed store room or a fixed handover space, ask for the carton dimensions and the pack quantity at quotation stage rather than after despatch. It is a two-minute question that has saved several of our buyers an awkward conversation with a facilities team.

Ordering by size

Each capacity is a separate design, so each size in a mixed order needs to meet the minimum on its own. Our minimum order is 300 pcs per design. Production runs 20–30 working days after artwork approval depending on quantity, plus roughly 15 days shipping, with an express route of 7 working days for limited styles and quantities. A digital mockup normally comes back within one business day, and invoicing is SST-compliant with ePerolehan-ready documentation for government and statutory buyers.

If your programme genuinely needs two capacities, it is usually better to commit to one main size for everybody and treat the second as a separate line item with its own timeline, rather than splitting a single run and finding neither size reaches the quantity you needed.

Sizing questions we get asked

What size dry bag should I choose for a corporate gift?

For general island, beach and water-sports gifting, the 10L band is the most widely useful. It takes a towel, a change of clothes and personal items, it is still comfortable to carry, and it has enough band to roll properly when full.

Is a 10L dry bag the same as a 10L backpack?

No. The figures are arrived at differently and used differently. A roll-top sack loses height to the closure and changes shape when sealed, so its usable volume is meaningfully less than the number on the label.

Can I fit a laptop in a dry bag?

Sometimes, but check the dimension rather than the capacity. The usable height and flat width of the sealed bag decide it, and a device that only just fits will prevent the bag from rolling three times.

Should I order one size for everyone or several?

One size for everyone, unless the use cases genuinely differ. Mixed sizes multiply artwork approvals, packing lists and handover errors, and each size has to meet the order minimum separately.

What is the minimum order for custom dry bags in Malaysia?

300 pcs per design, and each capacity counts as its own design. Tell us the contents rather than the litres and we will recommend the band.