Diving guide
Turn Pressure and the Rule of Thirds
Calculate a usable turn pressure, distinguish thirds from minimum gas, and choose the earlier limit for direct-ascent and return-to-exit dives.
A reserve says what must still be available for trouble. A turn pressure says when normal exploration ends so the team can finish the route without spending that reserve. They are connected, but they are not interchangeable.
Reserve comes before turn pressure
First calculate or select the reserve required by the actual exit: direct ascent, a swim back to the anchor, an overhead, or decompression. Gas above that reserve is the usable planning budget. The rock-bottom guide calculates a two-diver emergency ascent and explains why the commonly taught fixed 500 psi reserve can be too small.
Then divide usable gas according to the route. A direct-ascent drift dive, an out-and-back reef, and a cave penetration do not justify the same split. The gas planner models the planned profile; it does not grant overhead-diving training.
The classical rule of thirds
The rule of thirds comes from cave diving, where a direct ascent is impossible and the team must swim back the way it came. It is popularly credited to cave explorer Sheck Exley, who wrote it into the National Speleological Society Cave Diving Section's Basic Cave Diving: A Blueprint for Survival and framed it as a minimum rather than a target.
For a return-to-exit profile, classical thirds allocates one third out, one third back, and one third in reserve. With 3,000 psi at the start, one third is 1,000 psi: the outbound leg turns at 2,000 psi, the return consumes the second 1,000 psi, and 1,000 psi remains.
Thirds is not universal
The rule assumes a suitable team, route, cylinders, and exit strategy. It splits each diver's own gas, so it does not by itself protect a teammate who carries less gas or breathes faster; unequal cylinders require volume matching. It also assumes the return costs no more than the outbound leg, yet a strong current or a stressed, faster-breathing exit can make it cost more. Overhead environments require specific training and more detailed gas rules.
A worked minimum-gas turn
When minimum gas is the reserve and the outbound and return legs are expected to cost the same gas, the turn sits halfway between the starting pressure and the minimum:
Pstart is the gauge pressure at the start of the dive, Pmin the calculated minimum gas expressed as gauge pressure in the same cylinder, and Pturn the pressure at which the outbound leg ends. All three use the same unit, bar or psi. Round the turn up, so the team turns earlier rather than later.
In a different profile, classical thirds may be the earlier limit.
The earliest team limit controls
- Agree starting pressure, minimum gas, turn pressure, route, and who leads before descent.
- Compare actual gas volume when team cylinders differ, not gauge fractions. If a teammate carries less gas or breathes faster, base the reserve on that diver's needs, for example by keeping at least one third of the teammate's supply in reserve; this moves your turn earlier. The RMV converter puts each diver's consumption in volume units so the comparison is possible.
- The first diver to reach their planned limit calls the turn; the other diver's surplus does not cancel it.
- Recompute after a deeper profile, delay, current change, gas loss, or route change.
This is educational gas-allocation arithmetic. Penetration, decompression, mixed teams, and stage cylinders require procedures taught and practiced in the relevant training.
Sources
- Lessons in Gas Management · Divers Alert Network
- Estimating Your Air Consumption · Divers Alert Network
- Safe Diving Guide: Gas · British Sub-Aqua Club — Says reserves depend on cylinder capacity, dive plan, and both buddies' breathing rates; a stressed diver can breathe 50 L/min or more at the start of an incident; and a reserve should not be used to extend the dive.
- Rule of thirds (diving) · Wikipedia — Secondary summary citing Exley's NSS-CDS Basic Cave Diving: A Blueprint for Survival; describes the overhead-environment origin and, for buddies with different gas volumes or breathing rates, setting the reserve from the buddy's supply so the turn comes earlier.
- Was Sheck Wrong? · InDEPTH Magazine — Stratis Kas (2025): the rule of thirds is popularly credited to cave diver Sheck Exley, who framed it as a minimum, and it does not account for stress raising consumption on the exit.
Always use current training materials and operational procedures for dive planning.