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Can a Dry Bag Sink With Air Inside?

The short answer and the number behind it

Buoyancy is a comparison between two numbers: the mass of water the sealed bag pushes aside, and the total mass of the bag plus everything in it. One litre of fresh water weighs one kilogram, so a 20 litre sealed volume displaces 20 kg. If the bag and contents together weigh 5 kg, there is 15 kg of spare lift and the bag sits high in the water with roughly three quarters of its volume above the surface.

Air itself weighs almost nothing here. A litre of air at sea level is about 1.2 grams, so 8 litres of trapped air adds 10 grams of mass while contributing 8 litres, or 8 kilograms, of displacement. That is the entire reason a dry bag floats: the air is not lifting the bag, it is keeping the bag inflated so the shape displaces water it would otherwise collapse out of.

So the question is not whether there is air inside, but whether the average density of the sealed system is above or below the surrounding water. Above 1.0 kg per litre in fresh water and it sinks; below and it floats. Everything else in this guide is detail on how to work out that average density for a real load.

Packed density: the figure that decides everything

Packed density is not the density of the material, it is the mass of the packed item divided by the space it occupies including the air trapped between fibres and around odd shapes. A down sleeping bag compresses to about 0.12 kg per litre because loft is mostly air. Synthetic clothing sits near 0.25. Dry pasta and rice are around 0.6, canned food around 1.1, and hand tools or steel spares run to 1.8 or more.

This is why two bags of identical volume behave differently. Twenty litres of clothing weighs roughly 5 kg and floats with 15 kg to spare; twenty litres of canned food and tools weighs 22 kg and sinks. The air pocket is the same in both cases, and it is irrelevant to the outcome, because the outcome is set by the average.

Why a half-full bag floats and a packed one does not

A bag packed to 85%, which is the practical maximum if you want a three-fold closure, still contains 15% air by volume. On a 20L bag that is 3 litres of air, worth 3 kg of displacement, and the contents weigh maybe 5 kg. Total 5.64 kg against 20 kg of displacement: it floats, and it floats with most of its body out of the water.

The failure case is not packing it full, it is packing it dense. A 20L bag holding 12 cans, 2 litres of water and 3 kg of ice weighs about 14 kg in a 20 litre volume, which floats but with only 6 litres of reserve. Add a wet towel, which contributes mass while displacing almost nothing extra because it is already inside the sealed volume, and the reserve drops further. That is the bag that floats in the morning and sinks by the afternoon once the ice melts and the water sloshes.

Wet items are the hidden variable in this calculation. A dry towel is 210 g; the same towel saturated holds 400 to 600 g of water, all of which counts as mass and none of which adds volume if it is inside the sealed bag. Pack wet gear in a separate bag and the arithmetic of the main bag stays predictable.

What happens as the bag is pulled deeper

Pressure rises by roughly one atmosphere for every 10 metres of depth, and a flexible bag responds to that pressure by shrinking. Boyle’s law is the relevant relationship: double the absolute pressure and the gas volume halves. An 8 litre air pocket at the surface is 4 litres at 10 metres, 2.7 litres at 20 metres and about 2 litres at 30 metres.

Each litre lost is a kilogram of lift lost. Take a 20L bag carrying 16 kg of gear that occupies 12 litres, leaving 8 litres of air. At the surface it displaces 20 kg against 16.6 kg of total mass, so it floats with about 4 litres of freeboard. At 10 metres the air has halved to 4 litres and the bag volume is 16 litres, giving 16 kg of displacement against 16.6 kg of mass: it is now negatively buoyant and will keep descending.

That is the runaway. Once a bag crosses into negative buoyancy, it accelerates downward, pressure rises faster, the air shrinks further and the descent speeds up. In practice this matters for anything tethered to a sinking object, anything towed behind a boat at speed, and anything dropped into deep water during a rescue. It is also the reason a bag recovered from 15 metres often comes up looking half empty.

Where the air actually goes

In the field, bags lose air far more often through the closure than through the film. A roll-top with three folds holds air well at the surface, but under sustained pressure the folds can burp: a small amount of air escapes each time the bag is squeezed or rolled by a wave, and over an hour of being towed that adds up to litres. A bag that started with 8 litres of air can end the afternoon with 2.

Purge valves make this deliberate. A valve lets you squeeze air out so a bag packs flat in a hatch, and the same valve lets water pressure push remaining air out if the bag is dragged. If you want maximum flotation, leave the valve closed and do not compress the bag; if you want a flat, stable load in a kayak hatch, purge it and accept that the bag will not float.

Slow film leaks are rare on welded TPU but real on cheap coated fabric. A pinhole at the corner of a weld lets water in and air out simultaneously, and the exchange is roughly equal by volume, so displacement stays constant while mass climbs. That is the worst possible failure mode: the bag looks fine, floats lower each hour, and eventually submerges without any visible damage.

Salt water buys you about 2.5% more lift

Seawater is denser than fresh water, typically around 1.025 kg per litre against 1.0, because of dissolved salt. That gives a sealed bag about 2.5% more displacement in the sea than in a lake, which on a 20L bag is a difference of half a kilogram. It is a small margin, but it is the margin that decides the outcome for a bag loaded near neutral buoyancy.

Temperature moves the number in the same direction but by less. Cold fresh water is at its densest near 4 °C at about 1.000 kg per litre and drops to roughly 0.997 at 25 °C, a swing of three tenths of a percent that is negligible next to the effect of contents density. Salinity varies much more than temperature does, and estuaries where fresh and salt water mix can sit anywhere between the two values; salinity and density data are published by NOAA.

A bathtub protocol for measuring your own bag

You can measure the buoyancy of any bag in fifteen minutes with a bath, a spring balance and a marker pen. Fill the bath, load the bag exactly as you would for a trip, seal it with the closure you actually use, and lower it in. Mark the waterline on the outside of the bag with the pen, then read off roughly how much of the volume is above the surface.

1. Weigh the packed bag on a spring balance and note the mass in kilograms.
2. Lower the bag into a full bath and let it settle without pushing it under.
3. Mark the waterline on the film, then remove the bag and measure the submerged height.
4. Estimate submerged volume from the height and the bag footprint; compare it with the mass.
5. Repeat at a deeper forced immersion: push the bag under 60 cm and note how much it shrinks.
6. If submerged volume at rest is above 90% of total, treat the bag as sinkable and tether it.

The forced immersion step is the one people skip and the one that predicts real behaviour. Pushing the bag under 60 cm raises the pressure by about 6%, which compresses the air slightly and shows whether the bag is near the tipping point. A bag that barely comes back up is a bag that will sink if it is ever towed or dragged deeper, whatever it did in the bath.

Floating is not flotation

A dry bag that floats is not a buoyancy aid, and the gap between the two is measured in certification rather than in litres. A personal flotation device is built to turn an unconscious person face up, to keep the airway clear in rough water, and to hold its buoyancy after prolonged immersion, puncture and use. A dry bag does none of those things, and no amount of trapped air changes that.

The practical risk is behavioural. A bag that floats confidently invites people to rely on it: to hold onto it in the water, to strap it on for a swim, or to leave a life jacket in the boat because the bag looks buoyant enough. That is the failure path that causes real incidents. The performance classes and test methods for actual buoyancy aids are set out in the ISO 12402 series published by ISO, and nothing in a dry bag specification overlaps with them.

If the bag goes over the side: the recovery window

Recovery is a timing problem, not a swimming problem. A bag floating with high freeboard is visible for 200 to 400 metres in calm water and maybe 80 metres in a chop, and it drifts with the wind faster than a swimmer can paddle because so much of it sits above the surface. Every minute in the water roughly doubles the search area.

Act in the first thirty seconds. Note the bearing relative to a fixed object on shore before the boat or board moves, because the reference disappears the moment you turn around. If you are in a group, one person points continuously while the other moves; a bag that is being pointed at is recovered far more often than one that is being looked for.

Colour matters more than any other choice at this moment. A black bag in a 0.5 metre chop is effectively invisible beyond 60 metres, while an orange, yellow or mint bag stays visible at 150 metres or more. If you paddle or sail anywhere with a real risk of losing gear over the side, pick the colourway for visibility and add a reflective patch for low light.

Packing a bag so it floats on purpose

Sometimes you want guaranteed flotation: a throw bag for a group, a marker for a dive site, or simply insurance for a bag you cannot afford to lose. The method is to reserve air deliberately rather than incidentally. Pack the bag to no more than 60% with low-density items, roll it with three folds without squeezing the air out, and leave the purge valve closed. On a 20L bag that leaves 8 litres of air and gives roughly 12 kg of reserve lift, which will support the bag plus a wet towel plus a small amount of water ingress.

Two additions make the margin robust. A closed-cell foam block of 2 litres, weighing about 60 g, provides lift that cannot be compressed away at depth and cannot leak out; it is the cheapest flotation insurance available. And a bright webbing loop or a 20 mm reflective band on the outside turns a floating bag into a visible one, which matters more than another litre of air once the bag is 100 metres away.

Reading a buoyancy claim on a product page

Be sceptical of any listing that says a dry bag floats without saying how much it carries. The honest version states a volume, a load and a water type: a 20L bag with 5 kg of clothing floats in fresh water with about 15 kg of reserve. The dishonest version shows a photograph of an empty bag on a lake, which proves nothing about the bag you will actually pack.

Look for three specific pieces of information. The sealed volume after the closure is rolled, which is not the nominal capacity. The empty weight of the bag, which is mass you have to lift. And whether the closure holds air under pressure, which is the property that decides whether the bag keeps its buoyancy for an hour or loses it in ten minutes. If a listing gives only litres and a photograph, treat flotation as unproven.

The floating dry bag range is built around that third point: closure geometry that holds air while being towed, closed-cell assistance rather than reliance on trapped air alone, and colourways chosen for recovery. Compare the sealed-volume figures against your own packed load rather than against the nominal size, and you will know the answer before you get on the water.

The rules that never bend

Three rules cover every case in this guide. First, a sealed bag floats only while its average density stays below the water around it, so packed density is the number to watch rather than the presence of air. Second, buoyancy falls with depth, so a bag that floats at the surface can sink at ten metres and will accelerate once it starts. Third, nothing about a floating dry bag makes it a safety device, and the only equipment certified to keep a person afloat is a personal flotation device worn on the body.

Tether anything you cannot afford to lose. A 6 mm cord and a screw-gate carabiner clipped to a welded D-ring weigh 60 g and convert a lost kit into a recovered one in almost every scenario, including the ones where the bag sinks, because a tethered bag can be hauled up from far deeper than a swimmer can dive. Our line has produced 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.

For the related cases, read the full-bag flotation answer for the simpler version of this question, and the buoyancy science guide for the underlying physics in more depth. If the bag is being relied on for safety in any way, stop and read when a dry bag is not a life jacket first. Sealing behaviour under sustained immersion is covered in the waterproof claim guide, and practical depth effects in the submersion guide. Our academy section carries the rest of the technical background.