How long does the air in a scuba tank last?

How long the air in a scuba tank lasts is, without a doubt, one of the most common questions among people who are just starting to dive. There's no single answer in minutes, because it depends directly on the depth of the dive and each person's breathing rate — but the logic behind it is easy to understand, and you can make pretty realistic estimates.

The starting point is pressure. Water pressure increases by 1 ATA (atmosphere) for every 10 meters of depth. That means that at the surface you're already under 1 ATA of pressure (from the atmosphere itself), and for every 10 meters you descend, you add another 1 ATA. At 20 meters, for example, the total pressure is 3 ATA: 1 ATA from the atmosphere at the surface, plus 2 ATA from the 20 meters of water above the diver.

That pressure directly affects air consumption, because every breath draws in a volume of compressed air proportional to the ambient pressure. A diver who breathes, say, about 15 liters per minute at the surface will use roughly 3 times as much at 20 meters — around 45 liters per minute — simply because each breath at that depth pulls more compressed air out of the tank to fill the lungs.

That's exactly why the same tank lasts so much longer on a shallow dive than on a deep one: at the surface, that 15-liters-per-minute rate would stretch a tank far longer than the 45 liters per minute used at 20 meters with that same tank.

In practice, tank duration always comes down to a simple relationship: usable air volume divided by the consumption rate at that specific depth. That 'usable volume,' though, is never the full tank — every dive plan sets aside a portion of the air as a safety reserve for emergencies and a comfortable ascent, so real duration estimates already subtract that reserve.

This is also why depth is one of the most decisive factors in dive planning: two divers with the same tank, breathing at the same rate, will have noticeably different bottom times if one is at 10 meters and the other at 30 meters — the deeper diver will burn through their air visibly faster.

It's worth repeating a basic safety principle: no dive should ever be planned around 'using up the whole tank.' The rule is always to surface with a reserve of air, respecting the limits shown on your dive computer and the plan agreed with your buddy and instructor before entering the water.

Understanding this relationship between depth and air consumption helps divers plan each dive better and avoid being caught off guard when the same tank 'runs out faster' on a deeper day. At Búzios Divers, this concept is explained calmly right from the first dives of any course, so nobody has to guess anything when it matters.