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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

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)

  • Ascent time is one minute to solve the problem at depth, the ascent to half your maximum depth, then one-minute stops every 10 feet to the surface.
  • Stressed breathing rate is 1.0 cubic foot per minute per diver, UTD's emergency planning rate. UTD plans with 0.5 at rest and 0.75 while swimming, but rock bottom plans for the emergency.
  • Average depth is the average of that ascent, in atmospheres absolute (ATA): depth in feet ÷ 33 + 1.

Worked example: 60 feet on an aluminum 80

  1. 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.
  2. Average depth: 33 feet, or 2.0 ATA.
  3. Gas needed: 5 × 1.0 × 2 divers × 2.0 = 20 cubic feet.
  4. 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 more depths and cylinders, see the Rock Bottom Calculator, or run your own numbers in the gas planner.

Max depthRock bottom, aluminum 80
40 ft600 psi
60 ft800 psi
80 ft1,300 psi
100 ft1,600 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 typeRuleUsable gas spent before turningTurn pressure (60 ft, AL80)
Drift dive or live-boat pickupAll usableAll of it; start the ascent at rock bottomAscend at 800 psi
Shore dive or anchored boat diveHalvesHalfTurn at 1,900 psi
Must return to the start, such as a caveThirdsOne thirdTurn 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 setting rock bottom aside. Here, 3,000 − 733 = 2,267 psi, rounded up to 2,300.

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.

DepthAirNitrox 32 or 25/25
10 ft / 3 m100 min110 min
20 ft / 6 m90 min100 min
30 ft / 10 m80 min90 min
40 ft / 12 m70 min80 min
50 ft / 15 m60 min70 min
60 ft / 18 m50 min60 min
70 ft / 21 m35 min45 min
80 ft / 24 m30 min40 min
90 ft / 27 m25 min35 min
100 ft / 30 m20 min30 min
110 ft / 33 m—25 min*
120 ft / 36 m—20 min*
130 ft / 39 m—15 min*

*25/25 only. Air and nitrox 32 stop at 100 feet.

UTD's recreational ascent strategy goes to 50% of your average depth, or of your maximum depth if you're deeper than average when the dive ends, at about 30 feet a minute. Then it's one-minute stops every 10 feet to the surface, with the travel between stops included in each minute.

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.

The plan starts with the exit, not the dive. See How to Plan a Dive: Start With the Exit.

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.

The gas planner runs rock bottom, usable gas and turn pressure for the whole team, including mismatched cylinders.

SAC rate: the number behind the plan

UTD calls it surface consumption rate (SCR): the gas you breathe per minute at the surface, in cubic feet or liters. Measure it from real dives: gas used, time and average depth. The SAC rate calculator does the arithmetic and shows the work.

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.

Go deeper: Tank Factors, Instruments Should Confirm What You Already Know, Ratio Deco Explained and Diving in Gauge Mode.

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 number to surface with, maybe. As a reserve at depth, usually not. At 60 feet in an aluminum 80, rock bottom is 800 psi, and it climbs as you go deeper.

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.

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.

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