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What Size Dry Bag Fits a Sleeping Bag?

Measure the packed bag, not the temperature rating

Temperature ratings tell you nothing about volume. Two bags rated to 0 °C can differ by a factor of two in packed size depending on fill power, shell fabric and how aggressively the owner compresses them. What matters is the volume your actual bag occupies when it is packed the way you pack it, which you can measure at home in two minutes with a bucket and a measuring jug, or by displacement in a bath.

Fill the bucket with a known volume of water, push the packed bag in until it is just submerged, and note how much water spills or how far the level rises. That is your number. Most people are surprised: a bag that looks enormous in the loft often packs to 8 litres, and a modest-looking synthetic bag may not go below 16. Write the figure down, because it is the only input the rest of this guide needs.

Down and synthetic compress differently

Down compresses hard and recovers, which is why a high-fill-power bag can be squeezed into a fraction of its lofted volume and then shaken back to full height in a minute. Synthetic insulation is a continuous filament batt: it compresses to a floor set by the fibre itself and does not recover fully from repeated hard compression. In practice a synthetic bag has a hard minimum volume that no amount of strapping will beat, and packing it smaller simply damages it.

That difference decides both the bag size and how you use it. A down bag can be compressed into a 10L sack for the afternoon and released in camp with no long-term penalty. A synthetic bag should be packed once at a sensible volume and left there, because the same aggressive compression applied every day for a month measurably reduces loft. If you own synthetic, size the dry bag to the comfortable packed volume rather than to the tightest possible one.

Why the stated litres are not the litres you get

Stated dry bag volume is measured with the mouth open and the bag filled to the brim. Three folds consume the top of the bag, and the consumption is not linear with size: a 10L loses about 1.5 litres, a 20L about 3 litres, and a 40L about 5 litres. Add the fact that a sleeping bag is a soft but springy cylinder that does not fill corners, and the working rule is to buy 25 to 30% above the packed figure rather than 15%.

The tolerance matters in one direction only. Too large a dry bag is a minor nuisance: you roll further, the bag is slightly heavier, and it takes more space in a hatch. Too small is a failure: the roll will not close properly, the fold sits under tension, and a closure under tension is a closure that leaks. When in doubt, take the larger size. Our capacity loss note shows the measurement across a full size run.

Shape: mummy, rectangular and quilt

Shape changes how easily a bag packs into a cylindrical dry bag. A mummy bag with a tapered foot packs into a rough cone, which leaves a void at the bottom corners of a cylinder. A rectangular bag folds and rolls into a rectangular block, which wastes more space in a round bag. A quilt packs flattest of all and is the easiest item to match to a dry bag, provided you roll rather than stuff it.

The practical consequence is that a rectangular bag usually wants a duffel rather than a cylinder. A 30L dry duffel with a flat base swallows a folded rectangular bag that fights a 30L roll-top, because the base geometry matches the packed geometry. If you carry a rectangular synthetic bag on canoe trips, this is the single change that removes the most frustration at camp.

Compression sack, dry bag, or both?

A compression sack makes the bag smaller; a dry bag makes it dry. They do different jobs and the best answer for most people is both: compress in the sack, then drop the sack into a welded roll-top one size larger. The combination gives you the smallest packed volume and a genuinely waterproof outer, and it means the compression straps never have to be tightened against the film of the dry bag, which is what eventually wears it.

Using a dry bag alone is perfectly workable and lighter by about 120 g, but you lose the compression. A sleeping bag stuffed loosely into a 30L roll-top occupies 20 litres rather than the 13 it would occupy in a sack, and that difference matters in a kayak hatch or a bikepacking seat pack. Using a compression sack alone is the mistake: most sacks are water-resistant at best, and a sack that soaks through in a capsize is no protection at all.

Choosing between the four sizes that cover almost every case comes down to a short sequence.

1. Measure the packed bag by water displacement and write the figure down.
2. Add 25 to 30% for fold loss and voids around the packed shape.
3. Pick the next size up in the range rather than the nearest one.
4. Check the closed bag: three full folds with the buckle seated, no tension on the fold.
5. If it only closes with two folds, go up one size before the trip.

That sequence resolves nearly every case. A summer down bag that packs to 7 litres goes into a 15L; a three-season bag at 12 litres goes into a 20L; a winter bag at 18 litres goes into a 30L; an expedition bag at 25 litres goes into a 40L. Those four sizes cover the entire range of sleeping bags in normal use, and they are also the four sizes that pack best together in a canoe barrel or a raft dry box.

The remaining question is whether to put anything else in with the bag. Do not. Sleeping bags should travel alone: a stove, a fuel bottle or a set of tent poles pressed against the film risks puncturing the dry bag, and once the outer is breached the bag inside gets wet. Clothing is the only acceptable companion, wrapped around the sack as padding, and only when the volume allows three full folds afterwards.

How to pack without damaging the loft

Stuff, do not roll, if you own down. Rolling a down bag repeatedly compresses the same chambers along the same lines and creates cold spots over time, while stuffing distributes the compression randomly and the loft recovers fully. Push the foot end in first, then feed the rest in a series of handfuls rather than folding it, and finish by pressing down once before you close the sack.

Roll if you own synthetic or a quilt. Continuous filament insulation tolerates rolling better than random stuffing, because the batt is designed to flex along its length, and a quilt rolls flat with no chamber structure to damage. Either way, alternate the fold direction between trips on a long expedition so the same crease does not work repeatedly. Alternating is free and it extends the life of the insulation noticeably over a season of continuous use.

Keeping the bag dry on water and in sustained rain

On water, the sleeping bag is the classic victim of a canoe capsize or a kayak hatch that was not quite seated. Treat it as a two-layer problem. The inner layer is the compression sack or a light liner; the outer is a welded roll-top rated IPX7, which covers 30 minutes at one metre and therefore covers a capsize, a swim and the ten minutes it takes to right a boat. Two folds is not enough here; use three and check the roll before you launch.

In sustained rain on foot or on a bike, the risk is different and slower: condensation and strap pressure rather than immersion. Water vapour from a damp bag sealed inside a dry bag has nowhere to go, so dry the bag as far as you can before packing it, and if the trip is longer than three days, open the dry bag in the middle of the day to air the contents. Condensation inside a sealed bag is a real phenomenon and it is not a leak; the explanation is in our condensation note.

What else goes in the same dry bag?

Ideally nothing. A sleeping bag travelling alone is the safest arrangement, because every additional object is a puncture risk against a film that is already under pressure from the springy load inside. If you must combine, the only items that belong with it are soft: a fleece, a down jacket, a pillow or an inflatable mat in its own stuff sack. All of them are compressible, all of them pad the film rather than pressing into it, and none of them has a corner.

The items that must never share the bag are easy to list: stoves, fuel bottles, tent poles, cook sets, camera bodies and anything with a buckle or a metal edge. Each of those has a hard corner that will find the same spot on the film every time the bag is moved, and a dry bag that carries a stove and a sleeping bag together is a dry bag with a puncture waiting to happen. Put hardware in its own bag, ideally a separate roll-top in the 5L to 10L range.

Between trips: store lofted, not compressed

Long-term compression is the thing that kills a sleeping bag, not use. A bag left in its compression sack for six months loses measurable loft, and a down bag stored compressed for a whole off-season can lose 10 to 15% of its height permanently. Store it loose in a large breathable cotton sack or hanging in a cupboard, and keep the dry bag empty and open for the winter.

The sealed dry bag row has a second problem beyond loft: moisture. A bag packed slightly damp and sealed for six months will mildew, and mildew in down is difficult and expensive to deal with. If you must store inside a welded bag for space reasons, make certain the bag is bone dry first, add a small moisture absorber, and open it every two months to check. Our off-season storage guide covers the same question for the bags themselves.

Mistakes that make the bag wet anyway

• Buying to the stated volume instead of the usable volume, so the roll only closes twice.
• Packing the sleeping bag with a stove or tent poles that press a corner into the film.
• Storing the bag compressed all winter and then wondering why it is cold in spring.
• Sealing a damp bag into a welded dry bag for weeks, which produces mildew rather than leaks.
• Assuming a water-resistant compression sack is waterproof because it has a coated base.
• Closing with two folds because three feels tight, which is exactly the case that leaks.
• Choosing a cylinder for a rectangular synthetic bag instead of a flat-base duffel.

The second-to-last item is worth emphasising. A roll that feels tight is the signal that the bag is correctly sized for the load: three folds with the fabric under light tension is a well-matched bag, and the tension is what makes the seal. A roll that closes easily with slack usually means the bag is too large, which is safe but wasteful; a roll that cannot complete three folds means the bag is too small and needs to go up a size.

Quick selector: pick your size in four questions

1. What is the packed volume of your bag by water displacement?
2. Is it down, synthetic or a quilt, and do you compress it hard?
3. Does it travel on water, where IPX7 matters, or on land, where IPX4 is enough?
4. Is the packed shape a cone, a flat roll or a rectangular block?

Answer those four and the size follows: add 25 to 30% to the packed figure, step up for synthetic or rectangular shapes, and choose a duffel rather than a cylinder when the packed form is a block. On water the rating is non-negotiable; on land a lighter film and a smaller bag are entirely reasonable. The full comparison of bag formats is in our volume comparison, and compression behaviour is unpacked in the compression capacity guide.

Field examples: three real loads

A three-season down bag rated to 0 °C, 700 fill, packed in a compression sack to 11 litres with a mass of 1.05 kg, travels in a 20L welded roll-top on a canoe trip. Three folds close with light tension, the bag fits a standard canoe barrel alongside a 20L for clothing and a 10L for food, and the whole load stays dry through a capsize. Add a silk liner and the packed volume rises to 12.5 litres, which still fits.

A rectangular synthetic bag rated to 0 °C packs to 16 litres and fights every cylinder it meets. In a 20L roll-top it closes with two folds only, which is not acceptable on water. In a 30L flat-base duffel it closes with three folds and 4 litres of slack, which is fine; the duffel also doubles as a camp seat and packs flat under a canoe thwart. This is the clearest example of shape deciding format rather than volume deciding size.

An expedition down bag rated to minus 25 °C, 2.4 kg, packs to 24 litres and goes into a 40L dry duffel with room for a down jacket and an inflatable mat. In cold conditions the packing decision has a safety dimension: a wet expedition bag at minus 20 °C is not an inconvenience, it is an emergency. Sleeping bag temperature ratings are standardised under ISO 23537 by the ISO, and the fabric and film test methods behind a waterproof claim are published by ASTM; both are worth reading before you trust a number on a label.

A packing routine that protects the loft and the seal

1. Dry the bag fully in the morning sun or inside a tent vestibule before packing.
2. Stuff a down bag foot-first in handfuls; roll a synthetic bag or a quilt.
3. Close the compression sack to its marked line, never beyond, and check the straps are even.
4. Drop the sack into the dry bag, add soft clothing around it, and press the air out.
5. Roll three times away from the direction of travel and seat the buckle.
6. Store the bag loose and the dry bag open for the whole off-season.

That routine takes about six minutes in the morning and removes almost every failure mode described above. The one step people skip is the first: packing a bag that is damp from condensation. Down can absorb a surprising amount of water before it feels wet, and that water then sits inside a sealed dry bag for the rest of the trip, quietly undoing the loft you paid for.

If you are kitting out a group, a club or a rental fleet, standardise on two sizes and a colour code: 20L for three-season bags and 40L for winter bags, with the sleeping bag always in the same colour so it can be found in the dark. We have produced waterproof bags in these configurations since 2014 in an SGS-verified 4,950 m² facility, with a 500-piece minimum and sampling in 6 to 10 working days.Sizing across a full size run is also covered in our sleeping bag sizing note and the capacity measurement guide, with the whole size range shown on the product line page.