Home / Blog / Capacity & Sizing

What Fits in a 40L Dry Bag

What forty litres looks like in the hand

Forty litres is abstract until you convert it into something you can picture. It is forty one-litre bottles, a cube about 34 cm on each side, or a cylinder 30 cm in diameter filled to 62 cm. A soft bag is none of those shapes, which is exactly why capacity claims mislead: the fabric bulges at the middle, tapers at the rolled mouth and squares off only where a flat base is welded in. Plan against the cylinder and you will rarely be wrong by more than two litres.

Real packing loses volume to geometry before it loses volume to the closure. Soft items such as a down jacket, a sleeping bag in a compression sack and a dry towel mould into the corners and recover most of the nominal figure. Rigid items do not. A stove, a pot set, a water filter and a camera insert leave air pockets around them, and on a mixed load that waste runs to roughly 15% of the stated volume. A 40L bag packed with half rigid kit behaves like a 34L bag.

Dimensions, empty weight and what the spec sheet hides

A typical 40L roll-top measures 62 to 68 cm tall when unrolled, 28 to 32 cm across the body, and has a welded flat base around 26 by 22 cm so it will stand on a rock or a boat deck. Rolled three times and buckled, the height drops to roughly 56 cm. That matters in a sea kayak hatch with a 42 cm opening: the bag goes in soft and sideways, not rigid and upright, so check the opening against the relaxed circumference rather than the height.

Empty weight separates the serious builds from the light ones. A 40L bag uses close to 0.9 square metres of laminated fabric plus welds, straps and hardware, so film thickness is felt immediately in the hand. On 0.4 mm TPU laminated to a 420D face the bag lands near 620 g; move to 0.5 mm TPU on a 500D face and you are near 780 g. The heavier build costs 160 g and buys roughly a third more abrasion life at the base corners, where a loaded bag gets dragged.

A two-night camping kit, item by item

The table below is a genuine two-night load for one person in shoulder-season conditions, packed as it would be carried rather than vacuum-compressed. Volumes are measured for each item on its own and then summed, which slightly overstates the total because soft goods share space; the correction is applied in the final row. Weights include the stuff sack or pouch each item lives in.

The sum is 34.8 litres against roughly 34 litres of usable space, which tells you something useful: a two-night camping kit sits right on the edge of a single 40L bag. In practice the soft items overlap and you gain two to three litres back, so the load fits. But there is no headroom for wet hands on the second morning, a damp tent that went up in rain, or a second water bottle added at the last minute.

The clean fix is to move the tent, poles and all, into a second 10L roll-top carried outside the main bag. That takes the main load down to about 27 litres and 7.8 kg, gives you a dry bag that closes with a comfortable fourth fold if the weather turns, and splits the risk: if one seal ever fails, you lose the tent or the sleeping bag, never both. Campers who pack this way consistently report faster camp setup as well, because the shelter is the first thing they unpack.

On the water: 40L as the group bag

Forty litres is the size at which a bag stops being personal and starts being communal. On a raft, in a canoe or across the hatches of two sea kayaks, one 40L bag will carry the lunch, the shared first-aid pouch, the repair kit, a throwline and the dry clothing for four people on a day trip. The same bag on a canoe camping weekend becomes one person's entire kit, which is why a party of two typically travels with three.

Split the count by trip length rather than by headcount, because food and shelter scale with days and clothing does not. A day trip needs one bag regardless of whether there are two or six people. Add roughly one bag per person per two camping nights. That rule has held up across raft support rigs and sailing charters we have supplied, and it keeps the total under the weight a single vehicle or a single boat can absorb.

Distribution matters as much as count on a boat. Four 40L bags stacked in one stern hatch of a sea kayak put 30 kg aft and lift the bow, which makes the boat broach in a following sea. Spread them forward and aft and the same load is barely noticeable. Before a multi-day paddle, load the boat, float it in waist-deep water and sight down the waterline; if the stern sits more than 3 cm low, move one bag forward a compartment.

Weather windows decide how much of this you can afford to get wrong. A five-day plan built around a 24-hour forecast gap is a different packing problem from one built around a settled high, and the difference belongs in the food and clothing lines rather than the bag count. Check marine forecasts through NOAA before you commit to a load plan, and remember that a 40L bag packed to the limit is slower to re-pack on a beach in wind than three smaller ones.

Travel, car boots and the cabin-limit question

As a travel bag, 40L sits in an awkward but useful place. It is well above the free under-seat allowance on most low-cost carriers, which tends to be a 40 x 20 x 25 cm box of about 20 litres, and it is just under the volume of a maximum-size cabin trolley at 55 x 40 x 20 cm. A soft 40L roll-top that is not over-packed will usually compress into that sizer, but a rigidly packed one will not, and a bag that fails the sizer at the gate is an expensive lesson.

Where the 40L genuinely shines is the car. Three of them fill a hatchback boot with 120 litres of sorted, waterproof, colour-coded luggage: one for clothing, one for food and cooking, one for wet and sandy kit that you do not want near anything else. On a motorcycle the same bag strapped across the pillion seat is stable up to about 110 km/h if it is lashed with two cam straps rather than one bungee, because a single strap lets the load rotate.

For documents and electronics there is a better answer than a 40L. A large bag encourages loose packing, and a laptop sliding inside 40 litres of clothing takes corner impacts it would never see in a padded sleeve. Use a tote or a dedicated insert inside the bag, or carry electronics in a small roll-top of their own. Our laptop and document sizing guide covers the sleeve dimensions that work inside a larger dry bag.

Winter and bulky loads: where 40L runs out

Cold-weather kit is a volume problem, not a weight problem, and 40 litres is the size where that becomes obvious. A synthetic bag rated to -5 °C packs to around 11 litres rather than the 8 litres of a 3-season bag. Add a belay jacket at 3.5 litres, a pair of insulated trousers at 2.5 litres, a foam pad that will not compress, and a thermos, and the clothing and sleep system alone reach 20 litres before food, stove or shelter are counted.

The practical rule is seasonal: 40L covers a three-season weekend for one person, and shoulder-season trips down to about freezing if you are disciplined about clothing. Once the forecast low drops below -5 °C, or once the trip runs past two nights in those temperatures, move to 50L or split into a 40L plus a 20L. Trying to force a winter kit into 40 litres always ends the same way, with the bag closed on two folds instead of three because there was no height left for a proper seal.

• Below freezing, budget 11 to 13 litres for the sleep system rather than 8.
• Insulated outer layers and a foam pad cannot be compressed, so they cost full volume.
• A wide-mouth stove and a thermos are the two rigid items that waste the most space.
• Below -5 °C or past two nights, move to 50L or add a 20L bag rather than forcing the closure.

Food, cooking kit and using 40L as a cooler

Empty, a 40L bag swallows 24 half-litre bottles or around 18 cans standing upright with the tops below the roll line. Filled with ice for a beach day, it holds roughly 16 cans plus 3 kg of ice, and that ice will be water in four to six hours at 28 °C because a single-wall roll-top has no insulation at all. Treat it as a drinks tub with a lid, not as a cooler, and add a closed-cell liner if you need the ice to last a full afternoon.

For food transport the more useful habit is separation. Raw meat, oil and anything that can leak belongs in a sealed container inside the bag, never loose against the film, because a spill inside a welded bag means washing the whole bag rather than one tub. Cooking kit should travel as one nested unit: stove, pot, fuel and lighter in a single pouch so the sharp edges of a canister cannot work against the film during a portage. Our food and cooking kit sizing guide gives volume figures for stoves, pots and multi-day rations.

How much capacity the roll-top costs you

Every fold you make removes height and therefore volume, and on a bag this size the loss is measured in whole litres rather than fractions. Two folds hold paddle spray and nothing more. Three folds are the standard for a reason: three contact lines, each of which water must cross sideways under pressure, and a seal that survives 30 minutes at one metre. Four folds buy a little more margin and cost a meaningful slice of usable space on a bag that is already full.

Read the third row against the camping total and the picture is clear. A 40L bag rolled four times holds about 33.5 litres, and a two-night kit measures 34.8 litres before overlap. That is why the tent moves to a second bag: it is not a packing preference, it is arithmetic. If you know you will need four folds because the trip is rough, buy the 50L and roll it three times.

Weight, lifting and carrying a loaded 40L

A full 40L camping load weighs 8 to 11 kg, which is more than most people should carry one-shouldered for any distance. Webbing grab handles welded to the body are fine for lifting the bag in and out of a boat or a boot; they are not comfortable for a 2 km walk. If the bag will be carried any real distance, use one with a removable harness rather than a single strap, and check that the strap attachment is welded to the body rather than stitched to a seam allowance.

Lashing is the other half of the weight question. A 10 kg bag that shifts in a canoe in rough water can destabilise a boat faster than the water can, so the attachment points need to be rated well above the static load. Welded D-rings on a 0.5 mm TPU body take a 25 mm cam strap comfortably; the weak point is almost never the ring but the strap itself or the deck fitting it clips to. Our strap load rating guide sets out the working limits for common webbing widths.

Film, face fabric and seam choice at this size

Forty litres is where film thickness stops being optional. A 5L deck bag on 0.3 mm film will survive a season of casual use, but a bag carrying 10 kg of camping kit gets dragged across gravel, slid into hatches and stacked under other loads. At that size we specify 0.5 mm TPU laminated to a 500D face as the default, with radio-frequency welded seams rather than stitched-and-taped construction, because a needle hole is a leak waiting for pressure.

Look closely at the base corners of any 40L bag you are considering. That is where the film is folded and welded, and it is the first place a puncture appears after two seasons of use. A reinforced corner patch adds about 20 g and doubles the life of the bag in rental service. The same film and seam logic runs through our TPU waterproof backpack range, and the thickness trade-off is set out in the film thickness comparison.

When 40L is the wrong answer

There are three situations where a 40L bag is too big, and buying one anyway creates its own problems. A day paddle needs 10 to 15 litres, and a 40L bag half empty is a floppy, buoyant nuisance on a deck. A carry-on-only trip is bounded by the sizer rather than by your gear, and a 40L bag tempts you to pack past the limit. And a bag that will live in a small hatch may simply not fit once rolled, whatever the litre figure says.

It is equally wrong when it is too small, and the failure is more annoying because it appears on the second day rather than in the shop. Multi-day trips past three nights, any trip with a winter sleep system, and any group of more than two sharing communal gear all push past 40 litres. The symptom is a bag closed on two folds with the buckle fighting the fabric, which is a seal waiting to fail in the first capsize.

• Buy 30L for day trips, short paddles and carry-on-only travel.
• Choose 40L for two-night camps, group day kit and shoulder-season travel.
• Step up to 50L for three nights, winter sleep systems or shared group food.
• Use two 20L bags instead of one 40L when access and redundancy matter more than efficiency.
• Check the rolled circumference against your hatch opening before trusting the litre figure.

Packing a 40L bag in nine steps

Sequence matters more than it does in a smaller bag, because a 40L load is deep enough that whatever goes in first is genuinely inaccessible until you unpack on shore. Work from the heaviest, least-needed items at the bottom to the lightest, most-needed items at the top, and keep the closure line clean throughout. Lay out everything on a dry surface first; a bag packed from a sorted pile closes faster and fuller than one packed from memory.

1. Lay the bag flat and square the welded base so it stands without support on the ground.
2. Load the sleep system first, pressed down firmly into the base corners.
3. Add the tent or the heaviest shelter item along one side, poles running vertically.
4. Pack clothing in a compression sack and push it into the gaps around the shelter.
5. Nest the stove, pot and fuel as a single unit and place it against a flat wall, not the base.
6. Slide food in a sealed dry liner above the cooking kit so spills stay contained.
7. Put the items you need first, such as a shell and a head torch, at the mouth.
8. Press down on the load to expel air, then wipe the fold line with a dry cloth.
9. Roll three times evenly, clip the buckle, and squeeze the bag to listen for escaping air.

Finish by closing the bag on a surface you can see, not on a boat deck in the dark. Press down on the load to expel air, check the fold line with a thumb for sand or grit, then roll three times and clip. A bag packed in this order opens like a filing cabinet on the first night of a trip, which is the whole point of carrying one large bag instead of five small ones.

40L against 30L, 35L and 50L

The step from 35L to 40L is the cheapest capacity you will ever buy: about 50 g of empty weight and four extra litres, which is the difference between a tent inside the bag and a tent on the outside of it. The step from 40L to 50L costs more, in money and in packed weight, and it is only worth taking when the sleep system itself is bulky. If you own exactly one dry bag, make it a 40L.

If you are specifying bags for a rental fleet, a club or a retail range, the same table becomes the buying brief. Volume, film thickness, closure design, attachment hardware and the test standard behind any waterproof claim are the five lines that decide whether a bag survives a season. Our production line has built waterproof bags since 2014 in an SGS-verified 4,950 m2 facility, with a 500-piece minimum and sampling in 6 to 10 working days.Cold exposure is the one risk a bag cannot manage for you, and the guidance published by the CDC on hypothermia is worth reading before any shoulder-season trip.

Use the full capacity guide to place 40L inside the wider range, then cross-check against the 35L and 30L breakdowns if your load is lighter than you thought. Multi-day planners should read the weekend trip sizing notes, and anyone comparing platforms should start from the product line and the roll-top range, where every size shares the same closure geometry so a fleet can standardise on one system.