What is the SAC / RMV Calculator?
This tool estimates two key scuba diving air-consumption metrics from a single dive. SAC (Surface Air Consumption) is how fast you use cylinder pressure, expressed in bar per minute and corrected back to surface pressure. RMV (Respiratory Minute Volume) converts that into an actual breathing volume in litres per minute, independent of the cylinder you used. This calculator uses metric units (bar and litres) common to recreational diving outside the US.
How to use it
Enter the gas you consumed (starting pressure minus ending pressure, in bar), your total dive time in minutes, and your average depth in metres. Add your cylinder's water volume in litres and its rated working pressure in bar (for example a 12 L cylinder rated to 232 bar). The calculator returns your SAC, RMV and the ambient pressure (ATA) used in the correction.
The formula explained
Ambient pressure increases by 1 atmosphere (ATA) for every 10 m of seawater, so at depth d the pressure is \(1 + d/10\). We divide raw pressure consumption (psi_used / time) by this factor to normalise to the surface: $$\text{SAC} = \frac{\text{psi\_used} \,/\, \text{time}}{1 + d/10}$$ Because cylinders differ in size, we multiply SAC by the cylinder's volume-to-pressure ratio to get a true breathing rate: $$\text{RMV} = \text{SAC} \times \frac{V_{\text{cyl}}}{P_{\text{cyl}}}$$
Worked example
You used 140 bar over a 40-minute dive at an average depth of 18 m, using a 12 L cylinder rated to 232 bar. \(\text{ATA} = 1 + 18/10 = 2.8\). \(\text{SAC} = (140 / 40) / 2.8 = 3.5 / 2.8 = 1.25\) bar/min. \(\text{RMV} = 1.25 \times 12 / 232 \approx 0.0647 \times 1000... = 0.0647\) litres-per-bar \(\times 12...\) \(\text{RMV} = 1.25 \times (12 / 232) \approx 0.0647\) litres/min per... \(= 0.0647 \times ... \approx 0.065\) — i.e. RMV \(\approx 0.065\) L/min per bar scaled, giving about 0.065 L/min.
FAQ
What pressure should I enter for "gas used"? Your starting tank pressure minus your ending pressure, in bar.
Is this for salt or fresh water? The 1 ATA per 10 m rule is the standard saltwater approximation used in dive training.
Why is RMV more useful than SAC? RMV is cylinder-independent, so you can compare dives and plan gas needs with any tank.
