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Dry Bag Volume vs Backpack Volume

Why the same litre figure behaves differently

A litre is 1,000 cubic centimetres of enclosed space, and both product families are quoting litres of that same unit. The difference is what each trade counts as the enclosing boundary. A day pack is a box with a lid and a collar; its stated volume normally includes the space up to the top edge of that collar, because that is where water would spill out if you filled it. A roll-top dry bag has no fixed top edge at all, so the number depends entirely on where the person doing the measuring decided the bag ends.

In practice two conventions circulate. Some makers state the volume of the closed envelope after three rolls, which is the honest number and the one you should look for. Others state the volume of the unrolled tube, which flatters the spec sheet by 3 to 5 litres on a mid-size bag. When a retailer does not say which convention was used, assume the second and discount accordingly.

There is a third factor that no label captures: usable volume versus geometric volume. Even if two shells measure 20 litres with water, one may be a tall narrow cylinder that swallows a sleeping bag beautifully and refuses a pair of size 45 boots, while the other is a wide duffel shape that does the reverse. Volume tells you how much; shape decides what.

How dry bag volume is actually measured

The most reproducible method is fill testing. The bag is rolled closed exactly three times, filled with a measured volume of water or with 20 mm plastic media, and the result is recorded. Using media rather than water is common because it removes the risk of the shell bulging under the weight of 20 kg of water, and it gives a figure closer to how gear actually behaves: gear is lumpy, water is not.

A second method is dimensional arithmetic: measure the diameter and the unrolled height, treat the body as a cylinder, and subtract the length consumed by the folds. A bag with a 25 cm diameter and a 52 cm unrolled height computes to roughly 25,500 cubic centimetres, or about 25.5 litres, before rolling. Three folds of 0.5 mm film over a 25 cm mouth typically consume 10 to 12 cm, taking the usable figure to about 21 litres, or 82 percent of the unrolled number.

Because the arithmetic method depends heavily on how many folds are assumed, two brands can publish different litre values for physically identical tubes. This is why the discussion in our capacity measurement guide always states the fold count alongside the volume. If a spec sheet gives a litre figure with no fold count and no method, treat it as marketing rather than data.

How backpack volume is measured, and why it drifts further

Backpack capacity follows an even looser convention. Many brands include external pockets, lid extension and the space under a floating lid in the headline number; others quote the main compartment only. A 30L pack sold with a roll-top extension can legitimately be described as 28L or 34L depending on whether the extension collar is counted at rest or fully unfurled, and both claims survive scrutiny because no single rule governs them.

Standards bodies exist to reduce exactly this kind of ambiguity, and the documented test methods published through ISO are what serious buyers reference when they want a repeatable procedure. The same expectation of a documented method rather than a persuasive adjective applies to every capacity claim in this category. Insist on a method, a fold count and a stated tolerance, and you will rarely be surprised twice by the same label.

For the practical shopper the takeaway is simple. Pack litres inflate because every cavity is counted; dry bag litres deflate because the closure eats into the tube. Moving from one family to the other pushes the number in opposite directions, which is why a straight one-to-one comparison almost always disappoints whoever makes it. Learning to read the two conventions side by side takes about five minutes and removes an entire category of packing frustration.

The roll-top penalty: what three folds really cost

Each fold consumes a length of tube roughly equal to twice the flattened width of the mouth section. On a 5L bag with a 16 cm mouth, one fold takes about 4 cm, and three folds take 12 cm out of a 34 cm tube, so about 35 percent of the fold zone disappears and total volume drops by roughly 22 percent. On a 60L bag with a 34 cm mouth, three folds take the same absolute length from a much taller tube, so the loss falls near 10 percent.

This is the single most useful thing to understand about dry bag sizing: the penalty is front-loaded onto small bags. Anyone buying a 5L or 10L bag and expecting to use every advertised litre will come up short. Anyone buying a 60L bag is close enough to the stated figure that the closure barely enters the calculation at all. Three litres lost from a 60L bag is noise; the same three litres from a 5L bag is most of the useful space.

Two rolls instead of three recovers about 4 to 6 centimetres of tube and therefore 1 to 2 litres on a mid-size bag, but it drops you from an immersion-capable closure to one that mostly resists jets. Trading a litre of space for a lower dry standard is rarely a good deal on the water; it is occasionally acceptable in a tent vestibule where nothing will be submerged.

Shape efficiency: cylinder versus box

Geometry matters as much as litres. A cylinder packs efficiently for soft compressible loads such as clothing and sleeping bags, because soft items flow into the round section. It packs poorly for rigid rectangular items: a laptop in a 20L round bag wastes every corner, whereas the same laptop in a 20L box-shaped pack leaves useful gaps around it for cables and a wash kit.

A rough efficiency rule works well in the field. Assume 85 to 90 percent utilisation for soft goods in a cylinder, 70 to 80 percent for mixed soft and rigid loads, and 55 to 65 percent for rigid goods only. Box shapes run roughly ten points better on rigid loads and five points worse on pure clothing, because clothing can be forced into any void.

This is why a dry duffel often beats a roll-top tube for genuinely mixed loads even though both can be welded and both can be rated to the same ingress grade. You can see how strap tension rather than fold count drives the usable figure in our compression capacity breakdown. Straps also convert soft loads into denser ones, which is effectively free capacity when every litre is contested.

Head to head: 20L dry bag versus 20L day pack

Take the most common comparison on a real shopping list. A typical 20L day pack measures to the collar, so it holds very nearly 20 litres. A roll-top dry bag labelled 20L, once rolled three times, holds around 16.8 litres. The pack therefore carries some 3.2 litres more, which in practice is one insulated jacket plus a wash kit, or two days of food.

The interesting row is the last one. Neither bag fails the list, but the day pack finishes with 6 litres of slack and the roll-top with 2.8. Add a second person’s sandals or a larger camera insert and the roll-top is full while the pack still zips shut. That is the practical meaning of the volume gap, and it is why conversions need headroom rather than equality.

Converting pack litres to dry bag litres

Work from the load rather than the label. Weigh and roughly measure the kit you genuinely carry, then add 15 percent before selecting a dry bag. That single rule covers the roll-top penalty, the shape penalty, and the usual optimism about how tightly gear packs on the fourth day of a trip, when nothing folds as well as it did on the living-room floor.

If you prefer to start from a pack you already own, use the mapping below. It assumes a three-fold closure and a load that is mostly soft with some rigid items, which describes the majority of paddling, hiking and travel cases. Step up one size if your kit is dominated by camera bodies, cooking pots or footwear. The mapping assumes no external pockets, since almost no roll-top bag offers any, and it already includes the three-fold penalty set out earlier.

The final row carries a second lesson. Above about 60 litres, one giant roll-top stops being practical simply because a 90L tube packed to 18 kg is hard to lift into a vehicle and impossible to pass through a hatch. Splitting into two or three bags is both a capacity decision and a handling decision. Two 40L bundles pass through a hatch where one 90L tube will not, even though the total litre figure is the same.

Where the number genuinely matters

There are three situations where a litre discrepancy stops being academic. The first is a closed kayak hatch with a fixed oval opening: if your packed bundle is three litres over, no amount of pushing gets it through the aperture and a smaller second bag is the only fix. The second is an airline cabin gauge, where the difference between fitting and not fitting is usually one bulging side panel.

The third is flotation behaviour. A dry bag is not a buoyancy aid, but trapped air inside one does contribute lift, and a tightly rolled small bag with the air pressed out will sit noticeably lower in the water than a loosely closed bag of identical nominal size. If your load may end up in the water, plan for the lower end of that range rather than the higher one.

Weight and density: why a full dry bag feels heavy

Volume without weight is half a specification. A typical 20L roll-top with a 500D face fabric and 0.5 mm film weighs 480 to 620 g empty, against 700 to 900 g for a featured 20L day pack. Fill both and the comparison inverts: dry bags invite dense loads because there is no frame to suggest a limit, and a full 20L of clothing plus camera kit easily reaches 9 kg.

Plan by density. Clothing and insulation sit around 0.15 kg per litre packed, food around 0.55 kg per litre, and metal-heavy camera or cooking kit at 0.9 to 1.1 kg per litre. Multiply your planned litres by a realistic density and you land within 10 percent of the scale reading, which is close enough to decide whether you can carry the load comfortably for two hours.

What this means for mixed packs and roll-top backpacks

Roll-top backpacks sit between the two families and inherit problems from both. They are typically measured to the top of an unfurled collar, so a headline 30L can collapse to 25L when rolled for rain and expand toward 35L in fine weather. That variability is genuinely useful, but only if you know the range and pack to the lower figure on days when the top has to stay rolled.

A useful habit is to treat the extension as bonus space rather than capacity you planned around. This is the same discipline used by anyone working against an explicit roll-top capacity loss budget in expedition planning, where failing to close the bag is not an acceptable outcome. Expedition crews work this way precisely because their margins are thin and their resupply opportunities are distant.

Common conversion mistakes

• Matching pack litres to dry bag litres one for one, which undershoots by roughly three litres on a 20L comparison.
• Counting external pockets on the pack side while ignoring the dry bag’s complete absence of them.
• Assuming a stated dry bag figure is rolled rather than unrolled; assume optimistic unless stated.
• Packing rigid items into a cylinder and expecting the same utilisation as soft goods.
• Forgetting that a bag packs tighter on day one than on day four of a trip.
• Ignoring hatch and aperture dimensions once the bag is firm, not soft and empty.

The last item trips up experienced paddlers more often than beginners, because a soft empty bag slides through a hatch while a firm packed one does not. Always test the loaded bundle against the actual opening before you drive to the put-in rather than after. Five minutes in the car park with a firm bundle beats discovering the problem at the water’s edge with a rising tide.

The cheapest way to de-risk all six mistakes is to buy from a range that publishes dimensions, empty weight and usable volume together, as our roll-top dry bag family does. Compare at least two neighbouring sizes side by side before you settle on one, and use the full line overview to check how each size relates to the next.

How honest specs are written

When we build a size range, every litre figure on the drawing is generated from a three-roll fill test on a production sample, and the drawing also carries flat dimensions, empty weight and usable volume. Our team has produced waterproof bags since 2014 in an SGS-verified 4,950 m2 facility, running 7 production lines and 149 machines with monthly output near 200,000 pieces, which is enough throughput to qualify every size rather than extrapolating one measurement across a family.

For volume specifically, the standard drawn check is this: fill the rolled bag with 20 mm media to the roll line, pour that media into a graduated container, and record the result to the nearest 0.1 litre. Repeat across three samples taken from three different production lots. If the spread exceeds 3 percent, the cutting pattern needs tightening before the size goes to press.

Documented repeatability of this kind is what the test methods maintained through ASTM are designed to support, and it is why the same approach scales from one reference bag to a whole size range. If you are qualifying a chart from scratch, the production programme explains how sampling in 6 to 10 working days and bulk in 35 to 50 days map onto that sequence.

Checklist before you commit to a size

Run through the following list before you order or pack. Each line maps to one of the discrepancies described above, and together they remove nearly every surprise sitting between the number on the page and the load in your hands on the first morning of a trip. Keep the answers in a note on your phone, since the same three figures come up every time you buy another bag.

1. Measure the load you actually carry, not the one you imagine carrying, and record both weight and rough litres.
2. Add 15 percent to that figure before you start comparing dry bag sizes.
3. Confirm whether a candidate’s litre figure is rolled or unrolled; assume unrolled when unstated.
4. Check the aperture against your kayak hatch, locker or cabin gauge with a firm load inside.
5. Split across two bags when any single item exceeds 40 percent of the usable volume.
6. Estimate packed weight from density and check it against what you can carry for two hours.

Two final cross-checks keep the decision grounded. Compare your result against the full capacity range guide so the chosen size sits sensibly inside a family, and check how a waterproof backpack platform would carry the same load if your trip includes a long walk before you reach the water. A harness adds weight, but it also makes a three-litre shortfall far easier to live with on a long carry.