SAC Rate and RMV Calculator: Scuba Air Consumption

SAC (surface air consumption) is how fast your gauge drops per minute, scaled to the surface: bar or psi per minute, on one cylinder. RMV (respiratory minute volume) is the gas volume you breathe per minute; quoted at the surface it is SRMV, in litres or cubic feet, and it stays the same whatever cylinder you carry. To calculate your SAC rate, divide the pressure your gauge dropped by the minutes and by the average ambient pressure in atmospheres.

The formula

SRMV = \frac{V_{start} - V_{end}}{t \times P_{amb}}

V is the gas the cylinder still delivers at each gauge reading (real gas, surface litres), t the time in minutes, P_amb the average ambient pressure in atmospheres.

SAC = \frac{p_{start} - p_{end}}{t \times P_{amb}} P_{amb} = \frac{1.01325 + d / m_{bar}}{1.01325}

p is the gauge pressure, d the average depth, m_bar the metres of water per bar for the water type (10.00 in EN 13319 water).

Worked example

A diver breathes one Steel 12L 232 cylinder (12 L water volume) at an average 15 m for 20 minutes, and the gauge drops from 200 to 140 bar.

1Ambient pressure at 15 m averages 2.48 atm, so the load is 20 × 2.48 = 49.6 atm·min.
2Between 200 and 140 bar the cylinder delivered 648 L of gas at the surface (real gas, 15 °C). The ideal-gas sum would say 711 L: it overstates what this cylinder holds of air by 2% at 200 bar and 10% at 300 bar.
3SRMV = 648 ÷ 49.6 = 13.1 L/min.
4SAC = (200 − 140) ÷ 49.6 = 1.2 bar/min on this cylinder.

In imperial: an AL80 at an average 50 ft for 20 minutes, 3000 to 2000 psi, gives an SRMV of 0.48 ft³/min and a SAC of 20.0 psi/min on that cylinder.

How Dive Kit computes this

The gas used is not the pressure drop times the cylinder size. Dive Kit reads how many surface litres the cylinder still delivers at the start and end pressures from real-gas (compressibility) tables at 15 °C, and takes the difference. The ideal-gas sum overstates the gas, a few percent at 200 bar and more as the pressure rises; the worked example puts numbers on it.

The load is time multiplied by ambient pressure in true atmospheres (exactly 1 at the surface), with depth converted by your water type: EN 13319 (1 bar per 10.00 m, the app's default), salt, fresh or Red Sea water. SAC stays gauge arithmetic: the pressure drop divided by the load.

These are the same conventions the app's gas planner uses, so an SRMV measured here plans the same dive back to the same end pressure. This page takes one measurement; the app adds several (a twinset leg and a stage leg, say) and weights them by load.

The app screen for this tool is documented in the user guide: SAC/SRMV in the Dive Kit guide.

What is the difference between SAC and RMV?

SAC is a pressure rate tied to one cylinder: the same lungs read a lower SAC on a bigger cylinder. RMV (or SRMV) is a volume rate and belongs to you, so it is the number to carry into a gas plan. Our article SAC, RMV and SRMV covers why divers, agencies and software use the names differently.

What is a normal SAC rate for scuba diving?

There is no single normal figure: it changes with body size, fitness, workload, water temperature, stress and experience, so an average from someone else plans someone else's dive. Measure your own SRMV on a few calm dives, plan from the highest sensible one, and allow for harder breathing when you work or get cold. The gas consumption calculator starts at 20 L/min, the Dive Kit app's default, until you replace it with your own.

Which depth should I enter?

The average depth of the stretch you measured, ideally from your dive computer's log. For a multi-level dive, measure each level separately; the app combines them weighted by load.

Why does my SAC change when I switch cylinders?

Because SAC is a pressure drop per minute, and a larger cylinder loses less pressure for the same volume of gas. Your SRMV does not change. Convert between the two with the cylinder's water volume, or use the dive unit converter.

Sources

NIST Chemistry WebBook: Thermophysical Properties of Fluid Systems : the compressibility data for oxygen, nitrogen and helium behind the real-gas tables
Span, Lemmon et al. (2000), A Reference Equation of State for Nitrogen : the nitrogen equation of state NIST uses for those values
Schmidt and Wagner (1985), A New Form of the Equation of State for Oxygen : the oxygen equation of state NIST uses for those values

Last updated 30 October 2026 · English page

SAC Rate and RMV Calculator