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Choosing Between Two Dry Bag Sizes

The decision is about your least compressible item

Most buyers compare two sizes against their average load and end up wrong, because an average hides the one object that dictates the bag. A load of clothing, a towel and food compresses beautifully; a -5 °C sleeping bag, a 3/2 mm wetsuit, a climbing rope or a camera body with two lenses does not. Identify that single object, measure its packed volume once, and the choice between two adjacent sizes usually becomes obvious within a minute.

Measuring is simpler than it sounds. Fill a bucket or a large measuring jug to a known level, submerge the item pressed flat, and read the displaced volume in litres; a kitchen scale and a 10 litre bucket give you everything you need. Do this for the three bulkiest things in your kit and note the largest figure. That number, not the total of everything else, is what sizes the bag, because the rest will fill the gaps around it.

There is a useful corollary: if the bulkiest single item takes more than 60 percent of the smaller size, go up. Below about 60 percent the smaller bag will still close cleanly; above it you will be fighting the fabric on every roll, and a closure that cannot be rolled three times cleanly is a closure that leaks under pressure, not one that merely looks untidy.

Every roll-top size gives capacity back to the closure

A capacity figure printed on a dry bag describes the volume of the tube before it is closed. Three folds consume the top 100 to 140 mm of that tube depending on the diameter of the bag, which is why the usable number is always lower. Across most of the size range from 5L to 90L the loss is between 12 and 18 percent of nominal volume, and it is slightly worse in percentage terms on small bags because the fold height is a bigger fraction of total height.

Two further reductions follow. Anything you pack that cannot be squeezed to fill corners leaves air gaps, typically 5 to 8 percent of the load, and most people want some slack so the roll can be made with cold hands. Put together, the working rule is simple and reliable: multiply your measured packed volume by 1.25 to get the nominal bag size you should buy. Our capacity loss notes show where each litre goes.

How much headroom is actually enough

Headroom is not wasted space; it is the working margin that lets the film meet itself cleanly. Fifteen percent is the minimum for a bag closed once and left alone, and twenty percent is the practical figure for anything opened several times a day, because compressible items fluff back up and the closure gets harder to make as the trip goes on. Below ten percent you will be pressing the roll against solid contents, which is how closures fail silently in the field.

Too much headroom has its own cost, and it surprises people: a half-empty large bag sloshes, the load shifts, and the roll becomes floppy, which lets the fold creep open under vibration. A 30L bag carrying 12 litres is worse to pack and worse to carry than a 20L bag carrying the same load, because the excess film has to go somewhere and usually ends up in a crease that becomes a permanent weak line.

Reading the two sizes against a real packing list

Write the list before you look at either bag, and give every line a volume and a weight. A typical summer weekend load looks like this: a 900 g sleeping bag that packs to 6 litres, a 320 g insulated jacket at 2 litres, two base layers at 1.6 litres, a wash kit at 0.8 litres, food at 2.2 litres and 1.4 kg, plus a 1.1 kg camera in 1.8 litres. That is 14.4 litres packed, which after the 1.25 multiplier needs about 18 litres nominal, so the answer is a 20L rather than a 15L.

Run the same list for winter and the answer flips again. A -5 °C bag packs to 9 or 10 litres on its own, a heavier jacket adds 3 litres, and mittens, a hat and a thermal layer add another 1.5. Now the total is around 20 litres packed and the nominal requirement is 25 litres, which puts the buyer into a 30L for anything more than one night. This is the mechanism behind most returns: the same person legitimately needs different sizes in July and January.

Weight matters as much as volume in this comparison. Using our weekend trip sizing numbers as a reference, a 20L bag packed to 16 litres typically weighs 5.5 to 7 kg, which is fine to carry 400 m from a car but tiring over 5 km. If your access involves a long walk, buy the smaller bag and split the load into two, even though two bags cost more than one.

Six common size pairs and the honest answer for each

Most ranges are spaced so that adjacent sizes overlap by roughly 30 percent of their usable volume, which is why the choice feels arbitrary. It is not: each pairing has a natural breakpoint determined by what people actually carry. The table below gives usable volume after three folds, the empty weight difference, and the load at which each pair should be resolved.

Read that table top to bottom and one pattern jumps out: the empty weight penalty between adjacent sizes is small, usually under 100 g, while the usable volume gained is large, typically 30 to 50 percent. On pure cost-benefit grounds the larger size wins far more often than people assume, and the real constraints are packed diameter, how far the load must be carried, and whether the bag has to fit inside something else.

Those constraints are specific. A bag that must drop into a kayak hatch with a 200 mm opening cannot go up a size even if the volume fits, and a bag carried inside a 40 litre rucksack steals capacity from the rucksack itself. Check the physical limit first, then decide, and compare against our full capacity guide rather than trusting the size printed on the swing tag.

What the larger size really costs you

Empty weight is only the start. A larger tube has more surface area, so it weighs more at the same fabric specification, it dries more slowly because there is more film to dry, it takes longer to find anything inside because contents settle further, and it presents a bigger wind and snag profile on a deck or a bike. Against that, it closes more easily, tolerates a bulky addition, and rarely needs repacking.

Quantified over a season the differences look like this. A 40L bag takes about 25 percent longer to dry fully in shade than a 30L of the same construction, simply because the mouth area and internal surface scale up. It also accumulates creases in a different place, nearer the middle of the tube, which is worth knowing when a bag is stored rolled rather than flat.

Carrying comfort changes faster than volume does

Volume scales roughly with the cube and perceived effort does not. Going from 20L to 30L adds 50 percent of nominal volume but usually pushes a comfortable carry distance down noticeably, because the bag gets wider as well as taller and that changes how it sits against your back or how far it protrudes into a boat. For any load carried more than 20 minutes on foot, the harness, not litres, becomes the deciding factor.

This is where format can solve a size argument. A 20L roll-top is comfortable as a backpack but awkward as a shoulder carry; a 25 to 30 litre dry duffel with a detachable harness carries the same load further with less fatigue, at the cost of a longer opening that is slower to use on a rocking deck. Our backpack versus duffel comparison sets out the trade-offs in detail.

Three cases where the smaller size is right

The first is a physical constraint: a hatch opening, a pannier frame, a rucksack lid or a bar cradle with a fixed envelope. If the larger bag cannot physically enter the space it will be used in, there is no decision to make, and this single check resolves perhaps a third of all size questions. Measure the opening at its narrowest diagonal, not its nominal diameter, because hatches are rarely circular.

The second is one-handed access in a moving environment, where a bag you must rummage through is a liability. The third is load management: two smaller bags let you separate wet from dry, food from clothing, and clean from dirty, and they provide redundancy if one closure ever fails. A split also buys you the option of leaving one bag behind on short trips, which a single oversized bag never does.

• A hatch, rack or frame opening with a fixed envelope smaller than the bigger bag.
• Regular one-handed packing where a deep bag means digging for everything.
• Wet and dry separation, or a need for redundancy across two sealed volumes.
• Carry distances over 20 minutes where a slimmer bag stays out of your stride.
• Kit that must be split between two people or two craft.

Three cases where the larger size is right

Buy larger when the load is dominated by something bulky and incompressible. Sleeping bags are the classic case: a synthetic winter bag that packs to 9 litres will consume most of a 15L bag and leave almost nothing else, whereas in a 20L it sits with room for a jacket and a wash kit. Wetsuits, climbing ropes, dive fins, camera bodies and group cooking gear behave the same way.

Buy larger when access is awkward and repacking is expensive, such as a bag buried in a boat hatch or in the bottom of an expedition load, where you want the closure to be easy every single time. And buy larger when you expect the kit list to grow: a rental fleet, a family bag that has to absorb a spare jacket for a cold child, or a travel bag that must swallow an unplanned purchase on the road.

Trip length against each side of the pair

Duration is the last filter, and it interacts with climate. In warm conditions overnight kit is compact and forgiving; in cold or wet conditions the same trip needs substantially more volume for exactly the same activity. The table gives typical figures for one person with mid-range kit, packed rather than compressed.

Notice how often the answer lands on a pair rather than a single bag. Above about three days the split option stops being an indulgence and becomes the practical answer, because no single dry bag carries well when it is both bulky and heavy. If your planning lands you exactly between two sizes on this table, take the smaller size and add a second bag, rather than taking the larger one and hoping.

Testing both sizes at home in twenty minutes

You do not have to guess. Put the actual kit on the floor, pack it as tightly as you would on the trip, then stack it or drop it into a cardboard box of known volume to get a real figure. Boxes are easier to find than buckets in the sizes involved: a 20 litre box measures roughly 300 x 250 x 270 mm inside if you want to improvise with a tape measure.

1. Lay out the exact kit list you would take, including food and water in the volumes you carry.
2. Pack the bulkiest item first, then fill around it the way you would on the trip.
3. Measure the packed volume in a box or bucket of known capacity.
4. Multiply by 1.25 to reach the nominal size you need.
5. Compare that figure to 0.76 x nominal for each candidate size.
6. Check the winning size against any physical opening it has to pass through.
7. Load it to 85 percent, roll three folds, and confirm you can do it without straining.

Finish with a closure test using the real load rather than a pile of rags. Load the bag to 85 percent, close it, then squeeze it and listen: a good seal holds pressure and hisses slightly at the buckle, whereas an over-filled bag will not hold pressure at all. That single test, done with your own kit, resolves most two-size arguments far faster than any chart, and it is described in full in our home testing guide.

Choosing between two dry bag sizes for a club, fleet or retail range

Groups buying in volume face a different version of the same question, and the answer is usually to stock both sizes in a sensible ratio rather than to pick a winner. Rental operations typically find that two-thirds of bookings want the middle size and most of the remainder want either the smallest or the largest, so a 60/20/20 split across three adjacent volumes covers demand better than any single compromise.

Standardising the closure geometry across those sizes is the part that pays. Every bag in the range should close the same way with the same number of folds, because tuition given once then applies to every size in the store room, and staff do not have to relearn the seal each time they swap a visitor from a 10L to a 20L. That is also why our roll-top line uses one fold geometry from the smallest to the largest size.

For programmes specified from scratch, the sizing question becomes a procurement line: nominal capacity, usable capacity after three rolls, target packed weight, and the standard used to verify the claim.

The one-minute decision rule

• Measure your least compressible item; if it exceeds 60 percent of the smaller bag, size up.
• Multiply your measured packed volume by 1.25 to get the nominal size to buy.
• Treat 0.76 x nominal as the honest usable figure for any roll-top.
• Check the physical opening first: a hatch diameter beats any volume argument.
• Buy the smaller size when you must carry it more than 20 minutes on foot.
• Buy the larger size when a bulky item dominates or access is awkward.
• Above three days, split into two bags instead of buying one bigger one.
• Check the method behind capacity claims: there is no single compulsory measurement rule for soft goods, which is why two bags both sold as 20L can differ by 1.5 litres. The volume and dimensional standards published by the ISO and the associated test methods from ASTM are what a serious spec sheet references.
• Compare your shortlist in the full size range before ordering.

If you take one thing from this guide, take the 1.25 multiplier. It converts a real, measured load into a nominal bag size in one step, it accounts for closure loss and packing gaps together, and it removes nearly all of the guesswork from comparing two figures printed on a label. It is also easy to check afterwards: close the bag you bought with your actual kit inside and see whether three folds land cleanly. If they do, the number was right.

Everything else is refinement. Style, colour, harness format and closure hardware change how pleasant a bag is to live with, but they do not change whether your sleeping bag fits, and that is the decision being made here. Our 20L versus 30L breakdown and the 5L versus 10L comparison apply this same rule to the two pairs where buyers hesitate most, and the figures are worth reading together before you commit.