How long does a 12 L tank last?

There’s no single number — it depends on how deep you go and how much you breathe. But the maths is simple, and once you know your own consumption you can plan any dive on the back of a slate.

7 min readDive time & consumption

Key takeaways

A 12 L at 232 bar holds about 2,784 L of free gas.
Depth multiplies consumption: at 20 m you breathe ~3× the surface rate.
For an average diver, expect roughly 35–45 minutes at 20 m with a reserve.
Your personal consumption (SAC/RMV) is the figure that matters — log it.

The two numbers that decide it

How long your gas lasts comes down to how much you carry and how fast you use it. Carry is easy: capacity × pressure. A 12-litre cylinder at 232 bar holds about 2,784 litres of free gas at the surface. Usage is personal — it’s your breathing rate, and depth makes it worse.

Your breathing rate (SAC / RMV)

Your surface air consumption — sometimes called SAC or RMV — is how much gas you’d use per minute at the surface. A relaxed, efficient diver might sit around 15 L/min; an average diver around 20 L/min; someone new, cold or working hard, well above that. It’s the single biggest variable, and the good news is it improves with experience, fitness and good trim.

Why depth changes everything

The deeper you go, the denser the gas you breathe, so each breath uses more of your supply. At 10 m you’re at 2 bar absolute (twice the surface), at 20 m you’re at 3 bar, at 30 m at 4 bar. So a 20 L/min surface rate becomes 40 L/min at 10 m, 60 L/min at 20 m, 80 L/min at 30 m.

A 12 L at 232 bar, by depth

Assuming a 20 L/min surface consumption, here’s roughly what a 12 L holds — shown both at the theoretical limit and with a sensible 50-bar reserve held back.

Depth	Consumption	To empty (ideal)	With 50 bar reserve
10 m	~40 L/min	~70 min	~55 min
20 m	~60 L/min	~46 min	~36 min
30 m	~80 L/min	~35 min	~27 min
40 m	~100 L/min	~28 min	~22 min

These are estimates The figures assume a 20 L/min surface rate and ideal conditions. Cold, current, exertion and stress all raise consumption. Treat the table as a starting point, then replace 20 L/min with your own logged figure for real planning. Note too that at recreational depths your no-decompression limit may end the dive before your gas does.