# SAC Rate and RMV Calculator (Dive Kit): Free Scuba Air Consumption Rate Calculator

_Last updated: 2026-10-30_

> A free web calculator at https://divekit.app/tools/sac-rmv-calculator/ that turns one measured stretch of a dive (cylinder, average depth, time, start and end gauge pressure) into SRMV (surface respiratory minute volume, L/min or ft³/min) and SAC (bar/min or psi/min on that cylinder). It runs the same @divekit/core code as the Dive Kit app's SAC/SRMV tool, with real-gas litres. Free, no account.

## What do SAC and RMV mean?

- **SAC** (surface air consumption): the gauge pressure your cylinder loses per minute, scaled to the surface. Units: bar/min or psi/min. Tied to one cylinder: the same diver reads a lower SAC on a bigger cylinder.
- **RMV / SRMV** (respiratory minute volume at the surface): the gas volume you breathe per minute at 1 atm. Units: L/min or ft³/min. Independent of the cylinder, so it is the figure to carry into a gas plan.

## Who is the SAC and RMV calculator best for?

- **Divers building a gas plan** who want a measured breathing rate instead of a guess.
- **Divers changing cylinders** (AL80 to steel 12 L, single to twinset): SRMV carries over, SAC does not.
- **Instructors** teaching why SAC and RMV differ: the calculator shows both from the same dive.

## Formula

```latex
\mathrm{SRMV} = \frac{V_\text{start} - V_\text{end}}{t \times P_\text{amb}}, \qquad
\mathrm{SAC} = \frac{p_\text{start} - p_\text{end}}{t \times P_\text{amb}}, \qquad
P_\text{amb} = \frac{1.01325 + d / m_\text{bar}}{1.01325}
```

V is the surface gas the cylinder still delivers at each gauge reading (real gas, 15 °C), p the gauge pressure, t the minutes, d the average depth, m_bar the metres of water per bar (10.00 in EN 13319 water, the default).

## How does Dive Kit compute it?

- Gas used comes from real-gas (compressibility) tables, not pressure drop × cylinder size. For air in a 12 L cylinder the ideal-gas sum overstates what it delivers by about 2% at 200 bar and 10% at 300 bar; over a 200 to 140 bar drop it says 711 L where the cylinder delivers 648 L. Compressibility data: NIST Chemistry WebBook (https://webbook.nist.gov/chemistry/fluid/).
- The load is time × ambient pressure in true atmospheres, with depth converted for the chosen water type: EN 13319, salt, fresh or Red Sea.
- The same conventions run in the app's gas planner, so an SRMV measured here plans the same dive back to the same end pressure.
- Worked example (computed by the page at build time): one steel 12 L cylinder, 15 m average, 20 minutes, 200 to 140 bar gives about 13 L/min SRMV and 1.2 bar/min SAC.

## How does it compare?

| | Dive Kit web calculator | Dive Kit app (SAC/SRMV) | Typical online SAC calculator | Subsurface |
|---|---|---|---|---|
| Output | SRMV and SAC | SRMV and SAC per cylinder | SAC or RMV | SAC per logged dive |
| Gas model | Real gas, 15 °C | Real gas, 15 °C | Usually ideal gas | Logged pressures |
| Water type | EN 13319, salt, fresh, Red Sea | Same | Usually one fixed | From settings |
| Several legs or cylinders | One measurement | Yes, weighted by load | Rarely | Per dive |
| Price | Free, no account | Free tool in the app | Free | Free, open source |

## Facts at a glance

- **Page:** https://divekit.app/tools/sac-rmv-calculator/
- **Inputs:** cylinder (38 from the Dive Kit open cylinder dataset, single or twinset), average depth, time, start and end pressure, water type
- **Outputs:** SRMV (L/min or ft³/min), SAC (bar/min or psi/min), gas used, average ambient pressure
- **Units:** metric and imperial toggle
- **Works without an account:** yes; the default result is in the page HTML
- **Same code as the app:** yes, @divekit/core `calculateRmv`
- **In the app:** the SAC/SRMV tool adds several measurements per dive and saves the SRMV for the gas planner; free on iOS and Android
- **Guide:** https://divekit.app/docs/tools/rmv-sac-calculator/
- **Limits:** a planning aid, not a dive computer. A rate measured on a calm dive is a snapshot; cold, current and work raise it.
- **Last updated:** 2026-10-30

## Related

- [Gas consumption calculator](https://divekit.app/tools/gas-consumption-calculator/): plan the next dive from this SRMV
- [Scuba cylinder sizes](https://divekit.app/tools/scuba-cylinder-sizes/): water volume and real capacity of 38 cylinders
- [SAC, RMV and SRMV explained](https://divekit.app/blog/sac-rmv-srmv/)