Rock Bottom Calculator: Minimum Gas by Depth, With the Formula

Rock bottom is the gas you keep in reserve so two divers sharing one supply can reach the surface, or the next gas source, from the worst point of the dive. UTD multiplies ascent time, stressed breathing rate, two divers and average depth in atmospheres: 800 psi at 60 feet and 1,600 psi at 100 feet in an aluminum 80.

By Jeremiah Gardner · Last reviewed October 27, 2026

Rock bottom is the foundation of a gas plan. Every UTD class uses it. For how to turn it into a turn pressure, see Gas Management for Recreational Divers.

Rock bottom calculator

Illustrates UTD’s reserve-gas formula with a simplified emergency profile: one minute at depth, travel to half maximum depth rounded to a deeper 10-foot stop, then one-minute stop intervals including travel, for two divers breathing 1.0 cubic foot per minute each. Salt water; pressure rounded up to the next 100 psi. This is not a Min Deco or decompression schedule.

Cylinder

Maximum depth (ft)

Divers sharing gas

Stressed surface breathing rate per diver (cu ft/min)

Problem-solving time (min)

Ascent rate to half depth (ft/min)

Stop interval (ft)

Minutes per stop interval (including travel)

Rock bottom: 1,600 psi / 110.3 bar

Salt-water segment breakdown
Segment	Minutes	ATA	Surface-equivalent min	Gas (cu ft)
Solve the problem at depth	1	4.03	4.03	8.06
Ascend to 50 ft	2	3.27	6.55	13.09
Stop at 50 ft	1	2.52	2.52	5.03
Stop at 40 ft	1	2.21	2.21	4.42
Stop at 30 ft	1	1.91	1.91	3.82
Stop at 20 ft	1	1.61	1.61	3.21
Stop at 10 ft	1	1.30	1.30	2.61

Ascent time: 8 min at an average of 2.52 ATA. Gas needed: 40.2 cu ft. Unrounded pressure: 1560 psi.

This is a planning aid, not a substitute for training. Review your gas plan with your instructor.

Rock bottom by depth

Max depth	Ascent time	Average depth	Gas needed (cu ft)	Aluminum 80 (77.4 cu ft at 3,000 psi)	HP100 (100 cu ft at 3,442 psi)
40 ft	4 min	25 ft (1.76 ATA)	14.1	600 psi	500 psi
60 ft	5 min	33 ft (2.00 ATA)	20.0	800 psi	700 psi
80 ft	7 min	43 ft (2.30 ATA)	32.2	1,300 psi	1,200 psi
100 ft	8 min	50 ft (2.52 ATA)	40.2	1,600 psi	1,400 psi

Pressures are rounded up to the next 100 psi, in salt water. UTD's own 100-foot example rounds the average to 2.5 ATA; the calculator uses the exact figure, about 2.52.

The table stops at 100 feet on purpose. UTD keeps the equivalent narcotic depth at 100 feet or less and doesn't use air or nitrox deeper than that; beyond 100 feet, UTD divers breathe helium mixes, and gas planning is part of that training.

The assumptions

Every rock bottom number rests on assumptions. This calculator uses UTD's rock bottom method:

Two divers breathing from one gas supply.
A stressed breathing rate of 1.0 cubic foot per minute per diver, UTD's emergency planning rate. If your team breathes harder than that, use the higher number.
One minute at depth to recognize the problem and start sharing gas.
An ascent from maximum depth to half that depth, rounded to a deeper 10-foot stop, then one minute at each stop to the surface, with the travel included in each minute.
The average depth of that whole emergency, including the minute at depth, in atmospheres absolute (ATA).

If your team breathes more heavily, dives in colder water or plans longer stops, your rock bottom goes up.

The formula

UTD's rock bottom formula is:

Rock bottom = ascent time × stressed breathing rate × 2 divers × average depth (ATA)

Ascent time. Add one minute for solving the problem at depth to the time it takes to ascend, including the stops.
Average depth. Work out the average depth of that ascent and convert it to ATA: depth in feet ÷ 33 + 1.
Gas needed. Multiply ascent time × 1.0 cubic foot per minute × 2 divers × ATA.
Convert to psi. Divide the gas needed by your cylinder's rated volume, then multiply by its rated pressure.
Round up to the next 100 psi.

A worked example: 100 feet on an aluminum 80

This is UTD's own dive-planning example: an eight-minute emergency and 40 cubic feet of gas. Here it's converted to pressure in an aluminum 80.

Ascent segment	Minutes
Solve the problem at 100 ft	1
Ascend from 100 ft to 50 ft	2
One-minute stops at 50, 40, 30, 20 and 10 ft	5
Total ascent time	8

Average depth of the ascent: 50 feet, about 2.5 ATA
Gas needed: 8 minutes × 1.0 cu ft/min × 2 divers × 2.5 ATA = 40 cubic feet
Aluminum 80: 40 ÷ 77.4 × 3,000 = 1,550 psi, rounded up to 1,600 psi

Start that dive with 3,000 psi and only 1,400 psi is usable. That's why a single aluminum 80 runs out of usable gas quickly at 100 feet.

How to adjust for your team

Different cylinder: divide the gas needed by that cylinder's rated volume, then multiply by its rated pressure.
Different depth: rebuild the ascent profile, add up the minutes and find the new average depth.
Mismatched tanks: plan around the diver whose cylinder reaches rock bottom first.
Technical dives: rock bottom gets you to the next available gas source, such as a decompression bottle, rather than to the surface. That planning is part of technical training.

To plan the whole team at once, including mismatched cylinders, use the gas planner. It shows every step, so you can check your own math.

Know your own breathing rate. Measure it on a few relaxed dives, and you'll see how much margin the stressed rate builds in.

Common rock bottom mistakes

Calculating for one diver. The whole point is to cover a teammate who has no gas.
Using relaxed breathing rates. A calm practice drill does not justify reducing the emergency planning rate.
Using one reserve at all depths. The table shows how fast it grows.
Rounding down. Always round up.
Not sharing numbers before the dive. Everyone on the team should know everyone's rock bottom, and the team ascends on or before it.

Learn to plan it in the water

Gas planning, rock bottom and gas sharing with a long hose are part of Essentials of Recreational Diving. For more advanced gas planning and ascent strategies, see Essentials of Tech.

Frequently asked questions

Is rock bottom the same as minimum gas?

Yes. Different agencies and instructors also call it minimum gas or minimum gas reserve.

Why is rock bottom lower in an HP100 than an aluminum 80?

The gas needed is the same. An HP100 holds more gas for each psi, so the same volume shows up as a lower number on the gauge.

Why does this calculator use 1.0 cubic foot per minute?

Because 1.0 cubic foot a minute is UTD's stressed, emergency planning rate. A diver in trouble breathes hard. If your team breathes harder than that, plan with the higher number.

Does rock bottom include my ascent stops?

Yes. The ascent time includes the one-minute stops every 10 feet from half your maximum depth.