Scuba Air Consumption at Depth Calculator

It calculates ambient pressure for salt or fresh water, litres used per minute at depth, cylinder pressure lost per minute, usable gas, and an idealized duration before the stated usable pressure is consumed. Enter cylinder water volume rather than its marketed gas capacity, and enter only the pressure available after preserving the reserve required by your dive plan. The result is a planning estimate, not a substitute for training, live gauge checks, gas-sharing allowances, ascent gas, or conservative team procedures.

Start with a representative SAC rate

Surface air consumption expresses breathing-gas use as a surface-equivalent volume per minute, normally litres per minute at one atmosphere. Use a SAC rate calculated from your own logged dives with reliable starting and ending pressures, cylinder water volume, average depth, and elapsed time. A relaxed shallow dive may produce a number that is too optimistic for a cold, strenuous, deep, low-visibility, or current-exposed dive. For planning, choose a representative value for the conditions, and consider a deliberately higher stress or contingency rate where your training protocol calls for one. This calculator assumes the entered rate remains constant, although real breathing changes throughout a dive. It does not infer SAC from a pressure log and does not treat a single unusually calm result as a personal constant. If you only know a pressure-use rate in bar per minute, first convert it with the cylinder's internal water volume and the ambient pressure of the measurement. Keep units consistent: the input is litres per minute, not cubic feet per minute, and cylinder volume is the stamped internal water capacity in litres. Sound planning begins with conservative, traceable inputs rather than the most favorable number in a logbook.

Convert depth into gas use and pressure loss

Ambient pressure increases as a diver descends, so an open-circuit regulator supplies denser gas at depth. The calculator approximates salt-water pressure as one ATA at the surface plus one ATA for each 10 metres, while fresh water uses 10.3 metres per additional ATA. It multiplies SAC by that ambient pressure to estimate litres per minute at the selected depth. For example, a SAC of 18 litres per minute at 20 metres in salt water is evaluated at 3 ATA and becomes 54 litres per minute. Dividing this depth-adjusted volume rate by cylinder water volume gives the approximate gauge-pressure loss in bar per minute. A 12-litre cylinder in that example loses about 4.5 bar per minute. This relationship also explains why two cylinders at the same pressure can offer different breathing time: the larger internal volume stores more surface-equivalent gas per bar. The model uses a single fixed depth. It does not integrate a multilevel profile, ascent, descent, safety stop, temperature-related pressure change, regulator characteristics, or nonideal high-pressure gas behavior, so its duration should be interpreted as a transparent baseline calculation.

Use available pressure without spending the reserve

Enter usable pressure, not necessarily the pressure shown on the gauge at the start. Subtract the reserve, ascent allocation, team-sharing requirement, and any other amount required by your plan before entering the value. If a cylinder starts at 200 bar and the plan protects 50 bar, the simple usable amount is 150 bar only when that treatment is compatible with your training and the rest of the gas plan. The calculator multiplies usable pressure by cylinder water volume to obtain surface-equivalent usable litres, then divides by the depth consumption rate to estimate idealized minutes at one constant depth. Do not use the displayed duration as a no-decompression limit or a promise of remaining time. It does not evaluate decompression status, oxygen exposure, narcosis, gas suitability, minimum gas, thirds, rock-bottom calculations, buddy consumption, leaks, equipment failure, gauge uncertainty, or an emergency ascent. Check the live pressure gauge frequently and turn the dive according to the most limiting member of the team and the applicable procedure. This tool is best used to compare scenarios and audit arithmetic after formal training, with conservative rounding and a separate complete gas plan.