Rule of thirds or minimum gas?
Two ways to decide how much gas to keep in reserve. One is simple and proven in caves. The other is calculated for the dive. Here's how each works, with a worked example.
Read this first Information, not instruction. Nothing here replaces training with a qualified instructor.
Every technical dive needs a turn pressure: the point at which you stop going further and start coming back. The two most common ways of setting it start from different assumptions about what goes wrong.
Gas planning
How much gas is enough?
Both approaches set a turn pressure before the dive. They differ in what they assume about the emergency.
Rule of thirds
Use one third going in, one third coming out, and keep one third for a team member's emergency.
Why divers choose itSimple, fast to apply underwater, and backed by decades of cave diving. It can be tightened to quarters or sixths for siphons and high-flow sites.
Watch out forIt breaks down with mismatched cylinders and assumes both divers breathe at the same rate.
Minimum gas (rock bottom)
Calculate the gas two stressed divers need to solve a problem and ascend from the deepest or furthest point.
Why divers choose itScales to the actual dive, cylinders and decompression. Turns the reserve into a number you can check.
Watch out forOnly as good as the stress breathing rate you assume. More arithmetic to get wrong.
How thirds works
Take a twinset of two 12-litre cylinders filled to 230 bar. A third of 230 is about 77, so round the usable third down to 75 bar. You turn the dive at 155 bar, leaving 75 bar for the exit and 80 bar in reserve.
The reserve third exists for one scenario: your buddy loses their gas at the turn point and needs to share yours all the way out. Because you both breathed a third to get there, you have exactly enough for both of you to breathe for the same time on the way out.
Where thirds goes wrong
The classic problem is mismatched cylinders. Imagine one diver has twin 12s and the other has twin 7s. If both use a third of their own gas going in, the diver with the big twins has breathed far more litres. If that diver loses gas at the turn, the other diver's reserve third of a much smaller volume can't cover the exit. The fix is to set both divers' turn pressures from the smaller supply.
Thirds also assumes calm, matched breathing. A diver sharing gas in an emergency will usually breathe much harder than on the way in.
How minimum gas works
Minimum gas asks a different question: how much gas do two divers need to get from the worst point of this dive to safety, if one of them has no gas and both are stressed?
Here's an illustration for an open-water dive to 30 m. We assume each diver breathes 20 litres a minute under stress, so 40 litres a minute for the pair, and an ascent rate of 9 m a minute.
| Phase | Time | Average pressure | Gas for two divers |
|---|---|---|---|
| Sort the problem at 30 m | 2 min | 4.0 bar | 320 L |
| Ascend 30 m to 6 m | 3 min | 2.8 bar | 336 L |
| Stop at 6 m | 3 min | 1.6 bar | 192 L |
| Ascend 6 m to surface | 1 min | 1.3 bar | 52 L |
| Total | 900 L |
In a 24-litre twinset, 900 litres is 37.5 bar. Round up to 40 bar. That's the minimum pressure you should ever have on this dive, and the point at which you start your ascent regardless of plan.
In this example, thirds is actually more conservative, keeping 80 bar in reserve. On a deep dive with a long decompression obligation, minimum gas can easily exceed a third, and it tells you so. That's its main strength: it reacts to the dive rather than to the cylinder.
Many divers use both
In practice many teams calculate minimum gas and use thirds as well, then take whichever reserve is larger. Cave divers often use thirds for penetration gas and add a minimum-gas check for depth. Divers in high-flow caves or siphons, where the exit is against the current, often tighten thirds to quarters or sixths.
The number that matters most
Every method depends on your breathing rate, and most divers don't know theirs under stress. Measure your surface air consumption (SAC) on real dives, then add a generous margin for stress. A figure you've measured beats a textbook number.