Gas Management for Recreational Divers: Rock Bottom, Turn Pressure and Thirds

Gas management means planning, before the dive, how much gas you can use and how much you must keep in reserve. That reserve, called rock bottom, lets two divers sharing one supply reach the surface, or the next gas source, from the worst point of the dive. Everything above it is usable gas.

By Jeremiah Gardner · Last reviewed October 15, 2026

Most recreational divers learn one rule: be back on the boat with 500 psi. That rule says nothing about depth, about your buddy, or about what happens if something goes wrong at the far end of the dive. A real gas plan does.

What is rock bottom?

Rock bottom is the minimum gas you need to handle the worst realistic emergency: your teammate runs out of gas at the deepest point of the dive, and you share your gas while the two of you make a controlled ascent.

That's why rock bottom is calculated for two divers, not one. It's also why it changes with depth. At 100 feet, every breath uses about four times as much gas as at the surface.

In UTD's system, rock bottom gas management applies to every dive and every class. It's one of UTD's ten covenants.

How to calculate rock bottom

UTD's dive-planning method calculates reserve gas from the emergency profile:

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

Emergency profile time includes one minute to solve the problem at depth plus the planned ascent. The worked examples below model travel to half maximum depth and one-minute stop intervals. Travel between stops is included in the next stop minute. A different ascent obligation requires its own gas calculation.
Stressed breathing rate is 1.0 cubic foot per minute per diver. UTD plans with 0.5 at rest, 0.75 while swimming and 1.0 under stress.
Average depth is the time-weighted average of the modeled emergency profile, including the problem-solving minute, converted to atmospheres absolute (ATA). In salt water, ATA is approximately depth in feet ÷ 33 + 1.

Worked example: 60 feet on an aluminum 80

Ascent time: 1 minute to solve the problem, 1 minute to ascend from 60 to 30 feet, then one-minute stops at 30, 20 and 10 feet = 5 minutes.
Average depth: 33 feet, or 2.0 ATA.
Gas needed: 5 × 1.0 × 2 divers × 2.0 = 20 cubic feet.
Convert to psi: an aluminum 80 holds 77.4 cubic feet at 3,000 psi. 20 ÷ 77.4 × 3,000 = 775 psi. Round up: rock bottom is 800 psi at 60 feet.

For calculated examples by depth and cylinder, see the Rock Bottom Calculator. These pressures depend on the modeled profile, breathing rate and cylinder rating.

Max depth	Rock bottom, aluminum 80
40 ft	600 psi
60 ft	800 psi
80 ft	1,300 psi
100 ft	1,600 psi

At 100 feet, the "500 psi" habit leaves two divers about 1,100 psi short.

What is usable gas?

Usable gas is everything above rock bottom. Start with a full aluminum 80 at 3,000 psi on a 60-foot dive, and your usable gas is 3,000 − 800 = 2,200 psi.

How much of that you spend before turning around depends on the dive.

How to set your turn pressure

UTD uses three rules, chosen by whether you need to get back to where you started.

Dive type	Rule	Usable gas spent before turning	Turn pressure (60 ft, AL80)
Drift dive or live-boat pickup	All usable	All of it; start the ascent at rock bottom	Ascend at 800 psi
Shore dive or anchored boat dive	Halves	Half	Turn at 1,900 psi
Must return to the start, such as a cave	Thirds	One third	Turn at 2,300 psi

All usable: when navigation doesn't matter, like a drift dive with a live boat, all the gas above rock bottom is usable. Start the ascent when you reach rock bottom.
Halves: when it's good to return to the start but not required, like a shore dive or an anchored boat dive, spend half your usable gas going out. Turn at 3,000 − 1,100 = 1,900 psi.
Thirds: when you must return to the start, UTD divides usable gas into thirds after deducting rock bottom. The arithmetic example gives 3,000 − 733 = 2,267 psi, rounded up to 2,300. This is an illustration, not a cave or wreck plan: emergency return travel while sharing gas, delays, gas matching and overhead training must all be accounted for.

Thirds come from cave diving, where the exit is the only way out. In UTD planning, thirds are always taken from usable gas, after rock bottom is set aside. Thirds without rock bottom don't hold up in the ocean.

UTD Min Deco (NDL): time and ascent are separate limits

Rock bottom answers a gas question. Min Deco answers a time-and-ascent question. Having gas left does not extend the Min Deco limit, and staying within that limit does not replace a gas plan.

Depth	Air	Nitrox 32 or 25/25	Total ascent time
10 ft / 3 m	100 min	110 min	1 min
20 ft / 6 m	90 min	100 min	2 min
30 ft / 10 m	80 min	90 min	2 min
40 ft / 12 m	70 min	80 min	3 min
50 ft / 15 m	60 min	70 min	4 min
60 ft / 18 m	50 min	60 min	4 min
70 ft / 21 m	35 min	45 min	5 min
80 ft / 24 m	30 min	40 min	6 min
90 ft / 27 m	25 min	35 min	7 min
100 ft / 30 m	20 min	30 min	7 min
110 ft / 33 m	—	25 min*	7 min
120 ft / 36 m	—	20 min*	8 min
130 ft / 39 m	—	15 min*	8 min

*25/25 only. The entries deeper than 100 feet/30 meters do not authorize air or nitrox 32 at those depths. Gas choice, depth and time must remain within your training and the team's plan.

UTD's recreational ascent strategy begins at 50% of average depth, or 50% of maximum depth when you are deeper than average at the end of the dive. The initial ascent is approximately 30 feet/10 meters per minute. From the first stop, use one-minute intervals every 10 feet/3 meters; travel to the next stop is included in that next minute. Use the trained profile and the table together rather than deriving a new schedule from the total ascent-time column alone.

These are Min Deco limits, not planned staged-decompression schedules. Repetitive dives and accidentally exceeding a limit require the specific procedures taught in training. Emergency Ratio Deco is a contingency for an accidental exceedance, not a way to plan a longer recreational dive.

Planning as a team

Gas planning is a team activity. Before every dive, UTD teams run through a short plan review, GLADDDDD:

Goal: what the dive is for.
Leader: who leads the dive, leads the ascent and deploys the SMB.
Air/gas: the gas, the turn pressure and rock bottom.
Depth: maximum and average depth.
Duration: maximum and expected dive time.
Deco: the ascent plan.
Direction: headings and route.
Distance: limits on how far the team goes.

Everyone states their numbers, and the team turns when the first diver reaches their turn pressure. The ascent starts on or before rock bottom. If teammates have different tanks or breathe at very different rates, the plan follows the most limiting diver.

SAC rate: the number behind the plan

UTD's planning material uses surface consumption rate (SCR). For volume-based planning, express it in cubic feet or liters per minute at the surface; this is also commonly called RMV. If your log or computer reports SAC in psi or bar per minute, convert using the cylinder rating before comparing different cylinders. Measure gas used, elapsed time and average depth on representative dives, then normalize to surface pressure.

Knowing your SAC turns gas planning from guesswork into arithmetic. It also shows you when something is off: a dive where you breathe far more than usual is a signal to slow down and ask why.

Common gas planning mistakes

Planning for one diver. Rock bottom must cover two.
Using the same reserve at every depth. Deeper dives need more.
Using thirds without rock bottom. Set rock bottom aside first, then divide what's left.
Not stating numbers before the dive. If the team doesn't know each other's turn pressures, nobody does.
Checking your gauge too rarely. Check it often enough that it never surprises you.

Learn to plan it, then live it

Rock bottom gas planning, gas sharing with a long hose and team awareness are all part of Essentials of Recreational Diving. Planning to go deeper? Essentials of Tech introduces more advanced gas planning and ascent strategies.

Frequently asked questions

Is 500 psi enough to end a recreational dive?

As a surface arrival target, it can be. As a reserve at depth, it usually isn't. The worked 60-foot example calculates 800 psi in an aluminum 80 under the stated emergency-profile assumptions. It is not a universal reserve or a guarantee.

What is the difference between rock bottom and minimum gas?

They mean the same thing. Different agencies and instructors use different names, including minimum gas and minimum gas reserve.

When should I use halves instead of thirds?

Use halves when returning to your starting point is preferred but not required, like most shore and anchored boat dives. Use thirds of usable gas when you must return to the start, as in an overhead environment; the plan must also cover emergency exit travel.

Does rock bottom change with a steel tank?

Yes. The gas needed is the same, but each psi holds a different amount of gas in different cylinders, so the pressure changes. The calculator article shows both aluminum 80 and HP100 values.