Air Consumption Calculator: How Long Will My Tank Last?

Gas consumption at depth is your surface breathing rate (SRMV) multiplied by the ambient pressure and the time: at 20 m you breathe about three times your surface rate. This calculator turns that into the pressure left in your cylinder (or tank), the turn pressure and the reserve for the rule you dive, and how long you can stay at that depth before reaching the reserve.

The formula

V_{used} = SRMV \times \frac{P_{amb}}{1.01325} \times t

SRMV in surface litres a minute, P_amb the ambient pressure in bar at the depth (1.01325 bar of air plus the water column for your water type), t the minutes. The pressure left is the gauge reading at which the cylinder still delivers its full content minus V_used, from real-gas tables.

p_{res} = p_{fill} \times \frac{N - 2}{N} V_{turn} = \frac{V (p_{fill}) + V (p_{res})}{2}

N is 3 for thirds, 4 for fourths, 5 for fifths and 6 for sixths; a fixed reserve is the pressure you set, capped at the fill. V(p) is the surface gas the cylinder delivers from gauge pressure p, and the turn pressure is the gauge reading at V_turn.

t_{reserve} = \frac{V (p_{fill}) - V (p_{res})}{SRMV \times P_{amb} / 1.01325}

The usable gas divided by the litres a minute at depth, rounded down to whole minutes.

Worked example

A diver plans 20 minutes at 20 m on one AL80 (11.1 L water volume) filled to 207 bar (3000 psi), breathing 20 L/min at the surface, with the rule of thirds.

1At 20 m in EN 13319 water the ambient pressure is 2.97 times the surface, so the level uses 20 × 2.97 × 20 = 1,190 L of surface gas.
2Full, the cylinder delivers 2,202 L before the gauge reads zero (real gas, 15 °C). After 1,190 L the gauge reads 91 bar.
3Thirds keeps 69 bar (766 L) in reserve. Half the usable 1,436 L is gone at 135 bar, the turn pressure.
4At 20 m and 20 L/min the gauge reaches the reserve after 24 minutes, and the cylinder is empty after 37 minutes.

In imperial: an HP100 at 3442 psi, 60 ft for 30 minutes at 0.6 ft³/min uses 50 ft³ and ends with 1630 psi. Thirds keeps 1147 psi, the turn pressure is 2221 psi, and the reserve is reached after 38 minutes at that depth.

Reserve and turn pressure by rule

Each rule spends 1/N of the fill going out and 1/N coming back, and keeps the rest. The turn pressure is halfway through the usable gas, halved in real litres, so it sits slightly below the plain average of fill and reserve.

Rule	Reserve, 200 bar	Turn, 200 bar	Reserve, 3000 psi	Turn, 3000 psi
Thirds	67 bar	131 bar	1000 psi	1954 psi
Fourths	100 bar	148 bar	1500 psi	2218 psi
Fifths	120 bar	159 bar	1800 psi	2379 psi
Sixths	133 bar	166 bar	2000 psi	2486 psi

Steel 12 L at 200 bar, and an AL80 at 3000 psi. Single cylinder; a twin set gives the same pressures.

Minutes to reserve by depth

Depth	Gas a minute	To reserve	To empty
10 m (33 ft)	40 L	38 min	58 min
15 m (49 ft)	50 L	30 min	46 min
20 m (66 ft)	59 L	25 min	38 min
25 m (82 ft)	69 L	21 min	33 min
30 m (98 ft)	79 L	19 min	29 min
35 m (115 ft)	89 L	16 min	25 min
40 m (131 ft)	99 L	15 min	23 min

Steel 12 L at 200 bar, SRMV 20 L/min, rule of thirds, EN 13319 water. Time spent at that depth only: descent, ascent and stops come out of the same gas.

How Dive Kit computes this

Dive Kit does not treat pressure as volume. It reads how many surface litres the cylinder delivers at the fill from real-gas (compressibility) tables for air at 15 °C, subtracts the litres the level uses, and converts what is left back to a gauge reading. Near a full fill, air packs less tightly than the ideal-gas rule assumes, so the pressure left reads a little lower than the simple pressure-times-volume sum.

The reserve is a pressure fraction of the fill, as the rules are taught: thirds keeps a third, fourths half, fifths three fifths, sixths two thirds. The turn pressure halves the usable gas in real litres rather than in bar, so it sits a few bar below the plain average of fill and reserve.

The status follows the app's three tiers. Above reserve: the level ends at or above the reserve. Below reserve: it ends under it. Not enough gas: it ends under 50 bar (725 psi), a fixed floor whatever the rule, or runs the cylinder empty. A twin set doubles the gas but leaves every pressure the same.

This page plans one level. The app's Gas Usage tool chains as many steps as the dive has (descent, bottom, a shallower level, a safety stop), each with its own SRMV, and marks the step where you pass the turn pressure. Measure your SRMV first with the SAC and RMV calculator.

The app screen for this tool is documented in the user guide: Gas Usage in the Dive Kit guide.

How long will my tank last at depth?

Divide the gas your cylinder delivers by your SRMV times the ambient pressure. The ambient pressure grows by about one bar every 10 m, so the same cylinder lasts roughly half as long at 30 m as at 10 m. The table above lists minutes to reserve and to empty for a 12 L cylinder at seven depths; enter your own cylinder and SRMV in the calculator.

What is the rule of thirds in scuba diving?

A third of the gas takes you out, a third brings you back and a third stays in the cylinder for problems, such as sharing gas with a buddy. Fourths, fifths and sixths keep more in the cylinder, and a fixed reserve keeps the pressure you set. BSAC's Safe Diving guide gives a third left at the surface as a rule of thumb and advises planning a larger reserve when a third might not be enough.

What is turn pressure?

The gauge reading at which you turn the dive, so the same usable gas is left to get back. It is halfway between the fill and the reserve. Dive Kit halves the usable gas in litres rather than in bar, which puts the turn a few bar below the simple average.

Why does it say Not enough gas when I am above my reserve?

Under 50 bar (725 psi) the calculator always shows Not enough gas, whatever rule you pick: it is too little to ascend with. A fixed reserve set below 50 bar does not move that floor.

Is this a rock bottom calculator?

Not directly. Rock bottom, which GUE now calls minimum gas, is the gas two divers need to reach the surface from the deepest or farthest point while sharing one supply; it depends on your ascent plan and your stressed breathing rate. This calculator does not work that figure out for you. If you have it as a pressure, pick the Fixed reserve rule and enter it: the turn pressure, the time to reserve and the status then follow that reserve.

Which SRMV should I enter?

A rate measured on a dive like the one you plan, not your calmest dive. Cold, current, work and stress all raise it. If you have never measured it, measure it from a logged dive. The page starts at 20 L/min, the app's default.

Sources

BSAC Safe Diving (2020), Gas reserves : A third of the cylinder kept as a reserve as a rule of thumb, a larger reserve when a third might not be enough, and gas consumption varying with cold, exertion and depth.
GUE, Gas Management: Minimum Gas Nomenclature (2005) : The definition of rock bottom, renamed minimum gas: enough gas to surface from the maximum depth or distance while sharing with an out-of-gas diver.
DAN Alert Diver, Lessons in Gas Management : Building a breathing-gas limit into the dive plan, and turning back when a set share of the gas is used, rather than watching pressure alone.
NIST Chemistry WebBook, Thermophysical Properties of Fluid Systems : The nitrogen and oxygen density data behind the real-gas (compressibility) model that turns litres into gauge pressure.

Last updated 30 October 2026

Scuba Air and Gas Consumption Calculator