Scuba Tank Air Consumption Calculator

Introduction: what this scuba air consumption calculator estimates

This calculator answers two different questions that recreational divers often conflate. The first is how long will my gas last at depth, which is a straightforward division of usable volume by consumption rate. The second is how much gas must I refuse to touch, so that two divers sharing a single cylinder can still reach the surface with a safety stop. The second number, known as rock bottom or minimum gas, is very often larger than the 50 bar or 500 psi habit most divers carry, and the calculator works it out explicitly.

Work in metric (litres, bar, metres) or imperial (cubic feet, psi, feet); the tool converts pressures into gas volumes using your cylinder's own pressure-to-volume factor. It is written for certified divers who already understand basic gas planning. It is a planning and teaching aid, not a substitute for training, a submersible pressure gauge, or a dive computer.

How to use the scuba air consumption calculator

Pick your unit system first, because every label below it changes. In metric mode you enter the cylinder's water capacity in litres and its pressures in bar — a 12 L cylinder filled to 200 bar holds 2,400 L of free gas. In imperial mode a cylinder is sold by the free gas it delivers, so you enter both its rated capacity in cubic feet and the rated pressure that capacity assumes: an aluminium 80 is 77.4 cu ft at 3,000 psi. The calculator divides one by the other to get cubic feet per psi and uses that throughout.

Surface air consumption is your breathing rate at one atmosphere, in litres or cubic feet per minute. If you do not know yours, 20 L/min (about 0.7 cu ft/min) is a reasonable planning figure for an average recreational diver; measured values across the population fall roughly between 12 and 25 L/min. Measure your own by recording start and end pressures over a known time at a known depth, then dividing back through the ambient pressure factor.

Stress factor only affects the rock-bottom figure. It multiplies both divers' consumption during a shared-gas ascent, because nobody breathes at their relaxed rate while donating a regulator. Two is the usual default, 2.5 for demanding conditions, 3 for deep or task-loaded dives.

The result panel reports your gas plan as a table plus a pressure-versus-time profile that draws your reserve line and your rock-bottom line on the same axes. Where the rock-bottom line sits above your reserve line, your habitual reserve is not enough for the depth you have entered.

How ambient pressure drives gas consumption

A regulator delivers gas at the surrounding pressure, so every breath at depth contains proportionally more molecules than the same breath at the surface. Seawater adds close to one bar for every 10 metres, or one atmosphere for every 33 feet, on top of the one already pressing down at the surface.

A diver breathing 20 L/min at the surface therefore consumes 80 L/min at 30 m and empties a cylinder four times as fast. The relationship is strictly linear in absolute pressure, which is why the second half of a descent costs proportionally less than the first: going from 10 m to 20 m adds 50 % to your burn rate, but 30 m to 40 m adds only 25 %.

Formula for dive time and for rock-bottom minimum gas

Write for the cylinder's gas-per-unit-pressure factor: litres per bar in metric, cubic feet per psi in imperial. In metric it is just the water capacity; in imperial it is rated capacity divided by rated pressure. Usable gas between the start pressure and the reserve is:

The ambient pressure factor at depth uses the 10 m or 33 ft gradient:

and bottom time to reserve is the quotient:

Rock bottom is a different construction. It asks how much gas two divers, both breathing at an elevated stress rate , would consume during a worst-case shared-gas ascent from the planned depth. The ascent is split into three segments: one minute at depth to sort out the problem and pass a regulator, the ascent itself at 9 m/min (30 ft/min) to the safety-stop depth, and a three-minute stop at 5 m (15 ft). Each segment is charged at its own average ambient pressure:

with the ascent duration set by the ascent rate:

Dividing that volume back through the cylinder factor converts it into a pressure you can read on a gauge:

Because the rock-bottom pressure scales inversely with cylinder size, the same dive plan demands a much higher gauge reserve on a small cylinder than on a large one — a fact that a fixed “turn at 50 bar” rule cannot express.

Worked example: a 12 L cylinder at 20 m

A diver plans a flat 20 m dive on a 12 L cylinder filled to 200 bar, with an SAC of 20 L/min and a habitual 50 bar reserve. Stress factor 2.

Bottom time to the 50 bar reserve. Usable gas is 12 × (200 − 50) = 1,800 L. The ambient factor at 20 m is 1 + 20/10 = 3.0, so the burn rate is 20 × 3 = 60 L/min. That gives 1,800 ÷ 60 = 30 minutes.

Rock bottom for the same dive. The ascent from 20 m to the 5 m stop covers 15 m at 9 m/min, so 1.67 minutes, and its average depth is 12.5 m for an ambient factor of 2.25. Summing the three segments per diver per unit of SAC gives 1 × 3.0 + 1.67 × 2.25 + 3 × 1.5 = 11.25 bar·min. Multiply by SAC 20, stress 2 and two divers: 11.25 × 20 × 2 × 2 = 900 L. On a 12 L cylinder that is 900 ÷ 12 = 75 bar.

What that changes. The habitual 50 bar reserve is 25 bar short of what a shared-gas ascent from 20 m actually needs. Turning the dive at 75 bar instead cuts usable gas to 12 × 125 = 1,500 L and bottom time to 1,500 ÷ 60 = 25 minutes. Five minutes of bottom time is the honest price of being able to get a buddy up.

The same dive in imperial. An aluminium 80 delivers 77.4 cu ft at 3,000 psi, so  cu ft/psi. At 66 ft with SAC 0.7 cu ft/min, the segment sum is 1 × 3.0 + 1.7 × 2.23 + 3 × 1.45 = 11.15 ata·min, giving 11.15 × 0.7 × 2 × 2 = 31.2 cu ft, or 31.2 ÷ 0.0258 ≈ 1,210 psi. That is well above the 500 psi many divers treat as their floor, and it is why 500 psi should be read as an absolute minimum rather than a plan.

Depth comparison table for the worked example cylinder

Holding the 12 L cylinder, 200 bar fill, 20 L/min SAC and stress factor 2 constant, only the depth changes.

The right-hand column is the one that matters. Bottom time computed against a fixed 50 bar reserve falls gently with depth; bottom time computed against a reserve that can actually get two divers up falls off a cliff. At 40 m a 12 L cylinder gives barely six minutes of honest bottom time, which is the arithmetic reason deep recreational dives are done on twinsets or larger cylinders rather than on a standard single.

Interpreting the results before you get wet

Treat the bottom-time figure as an upper bound rather than a target. It is the moment you reach your reserve, not the moment you should begin ascending, and it assumes a perfectly flat profile that no real dive follows. Descent and ascent both consume gas that this model charges at the bottom depth or not at all.

If your SAC input is optimistic, everything downstream is optimistic with it. Novice divers frequently run 25–30 L/min on early dives and only settle toward 15–18 L/min after a hundred or so. Planning on a number you have not measured is the single most common way these estimates go wrong.

Gas duration is also only half of a dive plan. Even when the cylinder would last, no-decompression limits, thermal exposure and your computer's ascent obligations may end the dive sooner. Whichever limit arrives first is the limit.

Limitations, assumptions, and safe use

Constant depth, constant workload. The bottom-time formula charges the entire duration at the deepest depth entered and assumes your breathing rate never changes. Currents, cold, buoyancy struggles, photography and stress all raise consumption, sometimes by half or more.

The 10 m per bar rule is an approximation. Seawater at 1,025 kg/m³ actually adds one bar every 9.95 m and fresh water every 10.2 m, so the rule of thumb is conservative in salt and slightly optimistic in fresh, by about 2 % either way. That is far smaller than the uncertainty in your SAC.

Rock bottom is a recreational construction. It assumes two divers, a direct ascent to the surface with no overhead, no decompression obligation, and one three-minute safety stop. It is not a decompression gas plan and it does not cover stage bottles, multiple gases, or team ascents of more than two.

Gas behaves ideally here. Real compressed air deviates from the ideal gas law at high pressure, so a cylinder at 232 bar holds a few percent less free gas than the linear factor suggests. The error is small at recreational fill pressures and always in the conservative direction for planning purposes.

Not for technical or decompression diving. Overhead environments, staged decompression, multiple gas mixes and long runtimes all require training and planning tools this page does not attempt to replace.

Never a substitute for instruments. Monitor your submersible pressure gauge and computer continuously, plan inside your certification and experience, and end the dive early whenever the gauge disagrees with the plan.

Sources. Depth-pressure relationship, ascent rate and safety-stop practice follow the NOAA Diving Manual, which is the standard US government reference for diving physics and procedures. Typical respiratory minute volumes of 12–25 L/min for recreational divers, and the method for measuring your own, follow DAN Europe, “Gas Planning 101: How to Calculate Your RMV”. The rock-bottom construction used here — one minute at depth, ascent at 9 m/min (30 ft/min), a three-minute stop at 5 m (15 ft), two divers at an elevated stress rate, with 500 psi treated as an absolute floor — is the widely taught recreational minimum-gas method described in Rock Bottom Gas Planning for Recreational Diving. Aluminium 80 rating of 77.4 cu ft at 3,000 psi is the manufacturer's standard specification.

Common questions about scuba gas planning