What is minimum gas and how do I work it out?

Rustam Ismailov, 19 October 2026

Minimum gas is the gas you must still have in your cylinder, at any moment of the dive, so that two divers can get from where they are to the surface when one of them has none. Every agency has its own name for it and its own way of working it out. The idea is shared and the details are not.

It is not a turn pressure. You reach a turn pressure on purpose, with gas to spare. You reach minimum gas only when the plan has run out or something has gone wrong, and then the dive is over and the ascent starts.

The idea

Assume the worst happens at the worst moment. At the deepest point of the dive one diver has no gas, the buddy shares, and both of them breathe from one cylinder, stressed, all the way up with whatever stops you would do. What that takes is the reserve. You work it out from the deepest point because that is where the ascent costs the most.

What goes into it

Depth, first: the deepest point, because pressure multiplies everything else. Then a minute or two at depth to sort the problem out, find each other and start moving. One minute is a common allowance; some courses give two. Then the ascent at a rate you will actually manage, no faster than 9 metres a minute, and if you will really go slower, plan slower: a slower ascent needs more gas. Then the stops: a safety stop if you would do one, or the decompression you owe.

Then the part people argue about, the breathing rate. Two divers in trouble do not breathe at the rate they have on a quiet day. Courses use anything from one and a half to three times your normal rate, or a flat 20 to 30 litres a minute each, and I will not pick for you. Last, there are two of you, so the rate is doubled.

A worked example, not a recommendation

These figures are an illustration of the arithmetic, not numbers for your dive. Take 30 metres, 9 metres a minute up, one minute to sort it out at the bottom, a three minute stop at 5 metres, and each diver breathing 20 litres a minute, so 40 together. Pressure is 4 bar at 30 metres and 1.5 bar at 5 metres. Multiply minutes by average pressure:

16.8 bar-minutes at 40 litres a minute is about 670 litres of gas at surface pressure. Divide by the size of the cylinder: roughly 45 bar from a 15 litre steel, roughly 61 bar from an aluminium 80, which holds about 11 litres. Same dive, same two divers, 16 bar apart. Start up from 30 m on 50 bar and the steel covers it with 5 bar to spare; the aluminium 80 does not. Breathe harder, ascend slower or go deeper and every figure rises.

A reserve, not a turn point

Minimum gas is a floor under the plan, and you do not plan to touch it. The turn pressure comes from the gas above the floor. Take what is left once the minimum gas is set aside, the usable gas, and decide how much of it you spend on the way out. With thirds you spend a third going out, a third coming back and keep a third. With halves you spend half each way. Some teams plan on all of it. In this method the fraction applies to the usable gas; the older cave rule of thirds takes a third of the whole fill. If either diver’s gauge reads minimum gas, whatever the plan said, the dive ends and the ascent starts.

On a decompression dive the ascent includes the stops you owe, so the sum is longer and larger, and that is a job for a planner rather than a pencil.

Turn Pressure does this arithmetic for the dive I am actually planning: the depth, the cylinder, a consumption rate that you type in or work out from the numbers of one real dive, and thirds, halves or all usable gas. It prices both divers at your rate, raised if you say there is current, never below 40 bar; if you want stress in the number, enter a stressed rate; the help text in the app says the same. It is a no-stop recreational planner, to 40 metres, for the boat or the night before, not for use underwater.