The Rule of Thirds: Where It Comes From and When It Isn't Enough

The rule of thirds comes from cave diving: use a third of your gas going in, a third coming out and keep a third in reserve. On its own, it can leave too little gas when teammates carry different cylinders or breathe at different rates. UTD sets rock bottom aside first, then divides only the usable gas into thirds.

By Jeremiah Gardner · Last reviewed October 27, 2026

The rule of thirds is the best-known gas rule in diving: a third of your gas to go in, a third to come out, and a third for emergencies. It's simple and memorable, and on its own it can let you down.

Where does the rule of thirds come from?

From cave diving, where the exit is the only way out. In a cave, you can't swim straight up. Whatever happens, you have to get back to where you started, and you might have to do it while sharing gas with a teammate who has none. Keeping a third in reserve was meant to cover that return.

That's still the case for using thirds today: dives where you must return to your starting point.

How does basic thirds work?

Take your starting gas, divide it by three, and turn the dive once you've used one third.

Example: start with 3,000 psi. A third is 1,000 psi. Turn at 2,000 psi. In theory, you've got 1,000 psi to get back and 1,000 psi in reserve.

Why isn't it enough?

1. Mismatched teams. The reserve is a third of your gas. If your teammate's cylinder is smaller, or they breathe faster, their third may run out first, and your reserve has to carry two divers. A third of a small cylinder isn't a third of a large one.

2. Stress changes everything. UTD's planning material points out the assumptions hiding in thirds: a breathing rate under stress, time lost to delays and confusion, and slower travel when divers are in touch contact or sharing gas. The trip out is never as easy as the trip in.

3. It ignores the ascent. In open water, the emergency often isn't the swim back. It's the ascent from the deepest point while sharing gas. Plain thirds doesn't size the reserve for that.

How does UTD fix it?

By reserving rock bottom first. Rock bottom is the gas two divers need to reach the surface or the next gas source while sharing one supply, from the worst point of the dive. Set it aside, then divide only what's left, the usable gas.

Here's the arithmetic: a 100-foot dive where the team must return to the start, in double HP100s filled to 3,400 psi.

Step	Calculation	Result
Rock bottom	From the dive's emergency profile	700 psi
Usable gas	3,400 − 700	2,700 psi
One third of usable gas	2,700 ÷ 3	900 psi
Turn pressure	3,400 − 900	2,500 psi

The team turns at 2,500 psi, with two thirds of the usable gas plus rock bottom still in the cylinders.

That's the arithmetic, not a complete plan. Mismatched cylinders, delays, travel while sharing gas and any decompression all change the numbers, and planning a real return-to-start dive is part of training. Check your own math with the gas planner.

When should you use thirds, halves or all usable?

UTD chooses the rule by whether you need to return to the start:

Situation	Rule	Example
Navigation doesn't matter, such as a drift dive with a live boat	All usable	Start the ascent at rock bottom
Returning is nice but not required, such as a shore or anchored boat dive	Halves	Spend half the usable gas going out
You must return to the start, such as a cave	Thirds	Spend a third of the usable gas going out

In every case, rock bottom comes off the top first. See Gas Management for Recreational Divers.

How do you plan for a mismatched team?

Plan around the most limiting diver: the smallest supply or the highest consumption.
Everyone states their numbers before the dive: starting pressure, rock bottom and turn pressure.
The team turns when the first diver reaches their turn pressure.

Learn to plan for the real dive

Essentials of Tech introduces more advanced gas planning and ascent strategies. For the numbers behind your own planning, see How to Calculate Your SAC Rate and RMV and the Rock Bottom Calculator.

Frequently asked questions

Does the rule of thirds work for open-water diving?

Not on its own. UTD's planning material puts it bluntly: thirds without rock bottom doesn't work for ocean diving. Thirds are for dives where you must return to the start; most open-water dives use all usable gas or halves.

What happens with thirds when teammates have different cylinders?

A third of a large cylinder can be more gas than a third of a small one. If the diver with the smaller supply runs out at the far point, their teammate's reserve may not cover two divers getting out. Plan around the most limiting diver.

Is the rule of thirds enough for a cave dive?

The arithmetic is only a starting point. Overhead diving takes specific training and a plan that allows for stress, delays and slower travel while sharing gas.

When does the team turn?

When the first diver reaches their turn pressure. Everyone states their turn pressure before the dive.