Scuba Tank Duration Calculator

Find out how many minutes your tank gives you at a given depth before the gauge drops to your reserve.

Results update as you type.

Quick examples

Load a full example, then change any value.

SAC rate (L/min)

Litres per minute at the surface. Many recreational divers fall between 15 and 25.

Measure your SAC from a logged dive

Tank volume (L)

Water capacity of the tank. An aluminium 80 (AL80) holds 11.1 L.

Quick picks

Start pressure (bar)

What the gauge shows before you descend.

Reserve to keep

FixedRule of thirdsRock bottom

The pressure you don't spend at depth. It is left for the ascent, the safety stop and your buddy.

Reserve (bar)

50 bar is a common minimum.

Depth (m)

The depth you stay at. For a multilevel dive, use the average depth.

Time at 18 m

Within limits

At 18 m you can stay about 32 minutes before the gauge drops to your 50 bar reserve.

Usable gas: 1800 L (150 bar)
Reserve kept: 50 bar
Gas use at 18 m: 56 L/min
Gauge drop at 18 m: 4.67 bar/min

Minutes by depth

Minutes at each depth with SAC 20 L/min, a 12 L tank at 200 bar and a 50 bar reserve. Rounded down to whole minutes.
Depth	Pressure (bar)	Gas use (L/min)	Time (min)
5 m	1.5	30	60
10 m	2	40	45
15 m	2.5	50	36
18 mYour depth	2.8	56	32
20 m	3	60	30
25 m	3.5	70	25
30 m	4	80	22
35 m	4.5	90	20
40 m	5	100	18

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These tools are educational planning aids; they are not a substitute for dive training, current printed tables or a dive computer. Verify results against your training and always plan within the limits of your certification.

About tank duration

Tank duration is how long your gas lasts at a given depth. Two things set it: the gas you can spend, which is the start pressure minus the reserve, and how fast you use it there. At 10 m you use twice as much gas per minute as at the surface, and at 30 m four times as much.

The reserve stays in the tank for the ascent, the safety stop and your buddy. You can keep a fixed amount such as 50 bar, a third of the fill, or rock bottom: the gas two divers need to ascend together from that depth while sharing one tank.

With your numbers

Enter valid values to see the calculation worked through with your numbers.

Formula

Time at depth, rounded down to a whole minute

t = ((Pstart − Pres) × V) ÷ (SAC × P)

Absolute pressure

P = (d) ÷ (10) + 1

Rule of thirds reserve

Pres = (Pstart) ÷ (3)

Where

Pstart: start pressure (bar); Pres: reserve (bar)
V: tank water volume (L)
SAC: surface air consumption (L/min)
d: depth (m); P: absolute pressure at that depth (bar)
Rock bottom: the gas planner's figure for two divers at 2 × SAC each, never below 50 bar

How it's calculated

Usable gas (L) = (start pressure − reserve) × tank volume.
Gas use at depth (L/min) = SAC × (depth ÷ 10 + 1). Gauge drop (bar/min) = gas use at depth ÷ tank volume.
Time at depth (min) = usable gas ÷ gas use at depth, rounded down to a whole minute.
Rule of thirds reserve = start pressure ÷ 3, rounded to a whole bar.
Rock bottom reserve: the gas planner's calculation for the chosen depth with two divers at 2 × your SAC each (1 minute at depth, ascent to 5 m at 9 m/min, 3 minutes at 5 m, then the last 5 m to the surface). It is rounded up to 5 bar and never below 50 bar.

Good to know

The minutes are time spent at that one depth until the gauge reads your reserve. The descent, the ascent and the safety stop use gas too. With a fixed reserve they come out of it, and rock bottom is sized to cover them for two divers.
Stress, cold water, current and hard finning raise your SAC, sometimes a lot. Plan with a SAC from dives like the one ahead, rounded up.
Plan a buddy pair on the diver who breathes more. Both divers turn and ascend together.
Like most dive planning, this treats air as an ideal gas. A full tank at high pressure holds a little less than bar × litres, so read the result as a ceiling.

How to use the tank duration calculator

1
Enter your SAC rate in litres per minute. If you don't know it, work it out from a logged dive with the SAC calculator.
2
Set the tank volume and start pressure, then choose the reserve: a fixed amount, a third of the fill, or rock bottom for the depth.
3
Enter the depth. The result shows the minutes at that depth, and the table shows the same tank at other depths.

What the results mean

The result is the time you can spend at that depth before the gauge drops to your reserve. With a 12 L tank at 200 bar, a 50 bar reserve and a SAC of 20 L/min, that is 45 minutes at 10 m, 32 minutes at 18 m and 22 minutes at 30 m.

It assumes the whole time is spent at one depth. A multilevel dive that spends most of its time shallower lasts longer, so use the average depth for it. Hard work, cold and current shorten it because they raise your SAC.

The reserve changes the answer a lot on deeper dives. The rule of thirds keeps a third of the fill and leaves two thirds for the dive, out and back. Rock bottom grows with depth: at 30 m with a SAC of 20 L/min it is 115 bar in a 12 L tank, so the time at 30 m drops to 12 minutes, against 22 with a 50 bar reserve.

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