Minimum gas explained: how to work out your rock bottom

What it is

You are at the deepest point of the dive and your buddy runs out of gas. You share, you breathe hard, and you take a minute to sort it out before you ascend together. Minimum gas, also called rock bottom, is the gas that whole plan needs, taken from one cylinder. If you reach that pressure, the dive is over.

The sum

The plan has four phases, and each uses gas at the pressure of the depth it happens at:

1. The problem. One minute at the deepest point.
2. The ascent. From the deepest point to the stop, at your ascent rate.
3. The stop. Three minutes at 5 m.
4. The last metres. From 5 m to the surface.

Two divers breathe, and both breathe harder than usual: team rate = 2 × SAC × stress factor. With a SAC of 20 litres per minute and a stress factor of 2, that is 80 litres per minute at the surface.

At 30 m, the pressure is 4 bar, so the problem minute takes 80 × 1 × 4 = 320 litres. The ascent from 30 m to 5 m at 9 m per minute takes 2.8 minutes at an average of 2.75 bar: 611 litres. The stop takes 80 × 3 × 1.5 = 360 litres, and the last metres 56 litres.

The total is 320 + 611 + 360 + 56 = 1,347 litres. A 12 litre cylinder holds 112 bar of that, and the calculator rounds up to the next 10 bar: 120 bar.

Try it with your numbers

The calculator below starts on this example. Change the depth, the cylinder or your SAC and watch the reserve move. Without the 3 minute stop, the same dive needs 90 bar.

What goes into the number

• Your SAC. Take it from your harder dives, not your best. The SAC guide shows how to work it out.
• The stress factor. Between 1.5 and 3. Two is a common choice; your training may say otherwise.
• The cylinder. The same litres of gas are fewer bar in a bigger cylinder.
• The depth. Gas use grows with pressure, so the same dive 10 m deeper needs far more.

What the number does not tell you

• It is the reserve, not the gas for the whole dive. Plan the dive on top of it.
• It does not cover decompression dives or overhead environments, which need their own plans.
• It does not know your agency's rule. Where it differs from yours, use yours.

These calculations are planning aids, not a dive computer. Your training and your computer decide the dive.