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Dry Bag Size for a Sleeping Bag

Why a sleeping bag is the hardest item to size for

A sleeping bag is the single most compressible, least predictable object in a kit list. Unlike a stove or a first-aid pouch, its packed volume depends on how hard you compress it, how dry it is, and how much loft the fill has lost over its service life. Two bags with the same published rating can differ by 40% in packed volume, which is the difference between a comfortable fit and a closure that never seals properly.

The second problem is that the bag is usually the last thing packed and the first thing pulled out, so it ends up in whatever space is left. On a canoe trip that means the sleeping bag absorbs the shape of the hull; on a bike it takes the shape of the pannier. Sizing it properly means deciding in advance whether you will compress it hard for transport or pack it loosely to protect loft, because those two choices need different dry bag sizes.

Damp is the third variable. A bag that has been slept in for four nights in 85% humidity holds moisture in the fill and packs 10 to 15% larger than it did on day one. If you size to the dry-day volume, the closure will be strained by the end of a week, and a strained roll-top is the most common cause of water ingress in the field. Always size to the worst case, not the best case.

Packed volume by temperature rating and fill type

The table below uses real packed volumes measured after a standard compression routine: three shakes to settle the fill, then rolling from the foot end with body weight applied once. These figures assume bags in good condition; a bag with three seasons of use will sit in the upper half of each range. Use the higher number whenever you expect wet weather or a long trip.

Notice how synthetic fill jumps in volume: a synthetic -10 °C bag needs roughly twice the space of a down bag rated 15 degrees warmer. That is because synthetic insulation cannot be compressed beyond a certain point without permanently deforming the fibre, so the published compressed volume is closer to its natural lofted volume. If you carry synthetic for wet-climate reasons, accept the volume penalty and buy the bigger dry bag.

What compression actually does to loft

Compression is a loan against loft. Down recovers well from short compression: a bag squeezed into 60% of its natural volume for eight hours a day and released at camp will regain 92 to 97% of its loft. Leave it compressed for three weeks of storage and recovery drops to around 80%, and the loss is cumulative over years. Synthetic fibre is worse, losing roughly 1 to 2% of loft per season of hard compression.

This matters for dry bag sizing because the temptation is to buy the smallest bag that the sleeping bag will physically fit into. That is exactly wrong. A dry bag sized so the fill is permanently squeezed turns into a long-term storage compression sack, which is the fastest way to destroy the insulation you paid for. Size the dry bag so the sleeping bag goes in with one hand of slack around it.

How much volume a roll-top closure costs you

A roll-top dry bag does not deliver its nominal volume. Closing the mouth with three folds consumes fabric at the top of the bag, and the buckle pull tightens the walls inward. On a 20L platform the practical loss is 2.5 to 4 litres, on a 40L platform around 4 to 6 litres, and the loss is proportionally larger on tall narrow bags than on wide ones because there is more fabric circumference to fold.

Work in usable litres, not marketing litres. If your sleeping bag and liner pack to 14 litres, a 15L dry bag will not close; you need a 20L. This is the single most common sizing mistake we see, and it is why the recommendation in the table above carries one size of headroom. Detailed geometry is covered in our guide to how much capacity a roll-top costs.

Compression sack plus dry bag, or one dry bag?

There are two workable systems. The first is a compression sack inside a dry bag: you squeeze the sleeping bag hard for transport, then release the straps at camp. The second is a single dry bag sized generously, with the sleeping bag packed loosely and the air pressed out by hand before you roll the closure. The first saves volume, the second saves loft and time.

On a canoe or sea kayak trip where hull space is fixed and the bag stays packed for six hours a day, the two-part system wins: a compression sack takes the bag to 60% volume and the dry bag keeps it dry. On a bike or a walking trip where you unpack every evening and the load is carried on your back, the single dry bag wins, because a hard-compressed cylinder is uncomfortable against a spine and adds pointless packing time twice a day.

If you use the two-part system, the compression sack must be shorter than the dry bag is deep, otherwise the straps and cords press against the welded corners and abrade them from the inside. Leave at least 4 cm of slack at each end, and think about how compression changes real capacity before you commit to a size.

Sizing the whole sleep system, not just the bag

The sleeping bag rarely travels alone. A liner adds 0.8 to 1.5 litres, an inflatable sleeping pad adds 2 to 4 litres when rolled, a pillow adds 0.6 litres, and a bivvy sack or one-person shelter adds 3 to 6 litres. Packing these items separately spreads weight better but costs you two extra closures; packing them together saves closures and makes one very large awkward cylinder.

Our recommendation is a two-bag sleep system: one dry bag holds the sleeping bag plus the liner, sized from the table above with headroom; a second smaller bag holds the pad, pillow and any sleep clothing. The second bag is usually a 10L or 15L and doubles as a pillow case when stuffed with a jacket. It also means a single seal failure cannot ruin both your insulation and your ground comfort.

Shape: mummy, rectangular and quilt footprints

Shape changes how a bag packs even when the volume is identical. A mummy bag rolls from the foot and produces a tight cylinder that drops into a dry bag cleanly. A rectangular bag folds and produces a flat brick that leaves triangular voids in a cylindrical dry bag, so a rectangular bag with the same litre rating often needs one dry bag size larger than a mummy bag.

Quilts are the awkward case. A quilt has no back insulation, packs smaller for its warmth, but has no fixed shape at all, so it tends to spread to fill whatever container you give it. That is useful in a soft dry bag and a problem in a structured pack: the quilt will bulge into harness corners and make the load lumpy. A wide-mouth duffel-style dry bag suits quilts better than a tall roll-top.

Where the sleeping bag rides: canoe, bike and backpack

Position affects both the size you choose and the shape you pack. In a canoe, the sleeping bag belongs in a mid-ship dry bag that can be pushed under a thwart, which favours a lower-profile 20L over a tall 20L. On a bike, a 20L dry bag strapped across the rear rack or stuffed into a pannier works best if it is soft enough to conform; a stiff welded bag wastes 15% of a pannier volume.

On foot, the sleeping bag is the lowest, most central item in the pack, and its dry bag should be the softest item in the load so it fills the gaps around harder objects. A dry bag with a purge valve is genuinely useful here: you can push the air out after the pack is closed and recover 2 to 3 litres of pack space. See our comparison of dry bag volume against backpack volume for the conversion maths.

Measuring your own sleeping bag at home

1. Loosen the bag fully and let the fill recover for at least two hours at room temperature.
2. Pack it exactly as you would on a trip, including the liner and any sleep clothing.
3. Lower the packed bag into a 20-litre bucket of water placed inside a larger tub and measure the displaced overflow in litres.
4. If you have no bucket, measure length, width and height in centimetres, multiply, then divide by 1,000 and subtract 15% for rounded corners.
5. Add 1.5 litres for a damp trip condition, then divide the result by 0.83 to convert usable litres into nominal dry bag litres.

The displacement method is more accurate than the box method because a cylindrical bag leaves fewer air gaps than a rectangular approximation assumes. If you measure 11 litres by displacement and expect wet weather, the calculation is 11 + 1.5 = 12.5 usable litres, divided by 0.83, which gives 15 nominal litres: buy a 20L. This is the calculation behind every size in the tables above, and it is the same method explained in our capacity measurement guide.

Choosing between two sizes when the bag sits on the boundary

Boundary cases are common: your bag measures 15 usable litres, which sits between a 20L and a 25L. Choose the larger size when the trip is longer than four days, when the bag is synthetic, when you expect sustained rain, or when the dry bag must also swallow a bivvy or a bulky jacket. Choose the smaller size when the load is carried on your back for long days or when hull or pannier space is genuinely fixed.

There is also a handling argument for the larger size. A dry bag that is 85% full is easy to close with cold hands in the dark; one that is 98% full requires two hands, a knee and a bad temper. Since the closure is the only thing keeping your insulation dry, favour the size that lets you roll three clean folds without fighting the fabric. More on that trade-off is in our guide to choosing between two dry bag sizes.

Keeping a sleeping bag dry across a multi-day trip

A sleeping bag that gets wet loses most of its insulating value within minutes, because water fills the air spaces that provide the loft. Field tests put the loss at 70 to 90% of warmth for a soaked bag, which on a 0 °C night is a genuine hypothermia risk rather than a comfort problem. This is why the dry bag is not optional equipment; it is part of the sleep system.

Double-bag anything critical. A 20L dry bag holding the sleeping bag, with the whole thing slipped inside a 40L dry bag at night or in heavy weather, costs about 180 g and removes most of the risk from a single seal failure. On a canoe trip the outer bag can be the one that lives in the hull; the inner bag is the one you carry to the tent. Rinse and dry the inner bag every morning if it has taken spray.

Loft loss over time changes the size you need

A sleeping bag does not keep its packed volume forever. Down loses loft through oil depletion and contamination; synthetic loses it through fibre fatigue. After roughly 60 nights of use plus normal storage, expect packed volume to rise 8 to 12% because the fill no longer compresses as tightly and the shell fabric has relaxed. That means a bag that fitted a 20L dry bag comfortably in year one may need a 25L by year four.

Store the bag loose, in a large cotton or mesh sack, in a dry cupboard away from direct sun. Never store it inside its dry bag with the closure rolled: a sealed environment traps residual moisture and accelerates fill degradation. If you must store it packed, leave the closure open and check the inside of the dry bag for condensation once a month. Guidance on long-term care sits in our off-season storage guide.

Rapid size selection checklist

• Measure packed volume by displacement, not by guesswork, then add 1.5 L for damp conditions.
• Convert usable litres to nominal litres by dividing by 0.83 for a three-fold roll-top.
• Size to the sleeping bag plus liner, and pack the pad and pillow in a second smaller bag.
• Choose the larger of two sizes when the trip exceeds four days or the fill is synthetic.
• Avoid permanent compression: one hand of slack around the bag protects loft.
• Prefer a lower-profile bag for canoes and a softer bag for panniers and backpacks.
• Double-bag the sleeping system on any trip where a wet bag would be dangerous.

Temperature ratings themselves are worth checking against a standard rather than a brand claim. Sleeping bag temperature ratings are defined by ISO 23537, which sets the test manikin and the limits of comfort, lower limit and extreme rating, so two bags quoting the same number are at least comparable. The same discipline applies to ASTM test methods used for fabric and film properties in the outdoor industry.

For group and rental programmes the sizing logic scales directly: a fleet of identical 20L bags standardises a three-season sleep system, while a 40L covers synthetic or winter kit. Sampling takes 6 to 10 working days and inspection follows AQL 2.5, which is how a mixed-size programme stays consistent across a whole order.

Once the sleep system is sized, the rest of the kit list follows the same method. Our weekend trip sizing guide covers the two-day case, the 7-day expedition guide scales it to a week, and the capacity guide from 2L to 90L maps every size in the range. You can compare the finished specification against the full product line before you commit.