Scuba Gas Planner: Rock Bottom, Usable Gas and Turn Pressure

Enter your depth, how the dive ends and each diver’s cylinder and start pressure. The planner works out rock bottom, the gas two divers need to reach the surface in an emergency, then each diver’s usable gas and turn pressure. Every step of the math is shown, so you can check your own.

By Jeremiah Gardner · Free. Runs in your browser. Nothing you enter is saved or sent.

Plan the dive

Maximum depth (ft)

Average depth (ft) optional

Working breathing rate (cu ft/min) per diver

How the dive ends

The team

Diver

Cylinder

Start pressure (psi)

Diver

Cylinder

Start pressure (psi)

Rock bottom assumptions

One minute to solve the problem at depth, an ascent at 30 feet a minute to half the maximum depth, then one-minute stops every 10 feet to the surface. Change these only if your training tells you to.

Stressed breathing rate (cu ft/min) per diver

Time to solve the problem (min)

Divers breathing one cylinder

Check my math. Hide the answers until I’ve worked them out.

Turn at 1,900 psi

Rock bottom: 20.0 cu ftfor the team
Gas to spend, then turn: 28.4 cu fteach diver
Time to turn: About 13 minat 60 ft, 0.75 cu ft/min each

Diver	Cylinder	Turn at (psi)	Start (psi)	Rock bottom (psi)	Usable (psi)	Tank factor
Diver 1	AL80	1,900	3,000	800	2,200	2.58
Diver 2	AL80	1,900	3,000	800	2,200	2.58

Take it to the boat

Print this plan with your team’s numbers and the work filled in.

The work

Rock bottom ascent5 min, including 1 min to solve the problem, averaging 2.00 ATA
Rock bottom gas, for the team5 min × 1.0 cu ft/min × 2 divers × 2.00 ATA = 20.0 cu ft
Rock bottom, each diver20.0 cu ft ÷ 2.58 × 100 = 775 psi → round up: 800 psi
Usable gas, each diver3,000 − 800 = 2,200 psi, or 2,200 ÷ 100 × 2.58 = 56.8 cu ft
Gas to spend56.8 cu ft ÷ 2 (halves) = 28.4 cu ft each
Turn pressure, each diver3,000 − (28.4 ÷ 2.58 × 100) = 1,900 psi
Time to turn28.4 cu ft ÷ (0.75 cu ft/min × 2.82 ATA) = 13.4 min → about 13 min

Rock bottom ascent, segment by segment

Segment	Minutes	ATA	Gas (cu ft)
Solve the problem at depth	1	2.82	5.6
Ascend to 30 ft	1	2.36	4.7
Stop at 30 ft	1	1.91	3.8
Stop at 20 ft	1	1.61	3.2
Stop at 10 ft	1	1.30	2.6

A training aid, not a substitute for training or your team’s plan. Imperial units, salt water.

How does the planner work out rock bottom?

Rock bottom is the gas two divers need if one has a problem at the worst moment: at maximum depth, at the end of the dive. The planner allows one minute to solve the problem, an ascent to half your maximum depth, then one-minute stops every 10 feet to the surface. Two divers breathe 1.0 cubic foot a minute each, a stressed rate, at the average pressure of that ascent.

That gives a volume. Each diver converts it to their own pressure with their tank factor, rounded up to the next 100 psi. For the full walkthrough, read Rock Bottom Calculator.

How is turn pressure set?

Everything above rock bottom is usable gas. How much of it you spend before turning depends on how the dive ends.

All usable for a drift dive or a live-boat pickup. You don’t have to swim back.
Halves for a shore dive or an anchored boat. Half out, half back.
Thirds when you must return to the start. A third out, a third back, a third in reserve. See the rule of thirds.

What about mismatched cylinders?

Same pressure, different gas. 1,500 psi in an AL80 isn’t 1,500 psi in double HP100s. So the planner works in cubic feet. The diver with the least usable gas sets how much everyone can spend. Each diver turns after spending that same volume, which reads as a different number on each gauge. Tank Factors explains the conversion.

Why show the work?

Because the planner shouldn’t be the one who knows your turn pressure. You should. Work it out by hand, switch on Check my math and see how close you got. The goal is to need this page a little less every time you use it. That’s the idea behind Instruments Should Confirm What You Already Know.

Frequently asked questions

What breathing rate does rock bottom use?

1.0 cubic foot per minute per diver, a stressed rate, for two divers breathing from one cylinder. It covers one minute to solve the problem at depth and the ascent to the surface.

Why is rock bottom rounded up?

So the reserve never comes up short. Rounding up to the next 100 psi keeps the error on the safe side, and it matches how you read a gauge.

How do mismatched cylinders change the plan?

Plan in cubic feet. The diver with the least usable gas sets how much everyone can spend before turning. Each diver converts that same volume into their own turn pressure.

Can I use this for decompression dives?

No. It plans no-stop dives with a direct ascent to the surface. On decompression dives, rock bottom covers the ascent to the next gas source and the team’s decompression, which is part of technical training.

Does the planner save what I enter?

No. It runs in your browser, and nothing you enter is stored or sent. Print your plan if you want to keep it.