# Scuba Air and Gas Consumption Calculator (Dive Kit): How Long Will My Tank Last?

_Last updated: 2026-10-30_

> A free scuba air consumption calculator at https://divekit.app/tools/gas-consumption-calculator/ that plans the gas for one level of a dive: cylinder (single or twin set), start pressure, depth, time, SRMV and reserve rule in; gas used, pressure left, turn pressure, reserve pressure and minutes to the reserve out. It runs the same @divekit/core code as the Dive Kit app's Gas Usage tool, with real-gas litres. Free, no account.

## Who is the gas consumption calculator best for?

- **Divers checking a cylinder (tank) against a planned dive** before they splash: will this fill last 25 minutes at 30 m, and with what left?
- **Divers who dive a reserve rule** (thirds, fourths, fifths, sixths or a fixed pressure) and want the turn pressure for this fill.
- **Divers comparing cylinders** (AL80 against a steel 12 L, single against twin set) at the same SRMV.
- **Instructors** teaching gas management: the page shows the turn and reserve arithmetic with the numbers on screen.

## Formula

```latex
V_\text{used} = \mathrm{SRMV} \times \frac{P_\text{amb}}{1.01325} \times t, \qquad
p_\text{res} = p_\text{fill} \times \frac{N - 2}{N}, \qquad
V_\text{turn} = \frac{V(p_\text{fill}) + V(p_\text{res})}{2}, \qquad
t_\text{reserve} = \frac{V(p_\text{fill}) - V(p_\text{res})}{\mathrm{SRMV} \times P_\text{amb} / 1.01325}
```

SRMV is surface litres a minute, P_amb the ambient pressure in bar at depth (1.01325 bar of air plus the water column for the water type), t the minutes, N the rule (3 thirds, 4 fourths, 5 fifths, 6 sixths). V(p) is the surface gas the cylinder delivers from gauge pressure p, from real-gas (compressibility) tables for air at 15 °C.

## How does Dive Kit compute it?

- Gas used is SRMV × ambient ratio × time. The pressure left is the gauge reading at which the cylinder still delivers its content minus that gas, through the real-gas model, not pressure drop × cylinder size.
- The reserve is a pressure fraction of the fill, as the rules are taught; a fixed reserve is capped at the fill. The turn pressure halves the usable gas in litres, so it sits a few bar below the plain average.
- Status tiers, as in the app: **Above reserve**, **Below reserve**, and **Not enough gas** when the level ends under 50 bar (725 psi), a fixed floor whatever the rule, or empties the cylinder.
- Water types: EN 13319 (1 bar per 10.00 m, the default), salt, fresh, Red Sea.
- A twin set doubles the gas; every pressure stays the same.
- Rock bottom (GUE: minimum gas) is not computed. A diver who has worked it out as a pressure can enter it as a Fixed reserve; turn pressure, time to reserve and status then follow it.
- Worked example (computed by the page at build time): one AL80 at 3000 psi (207 bar), 20 m for 20 minutes at 20 L/min, rule of thirds: about 1,190 L used, 91 bar left, reserve 69 bar, turn pressure 135 bar, 24 minutes to the reserve at that depth.
- On a 200 bar fill of a 12 L cylinder the page's rule table gives: thirds 67 bar reserve / 131 bar turn, fourths 100 / 148, fifths 120 / 159, sixths 133 / 166.

## How does it compare?

| | Dive Kit web calculator | Dive Kit app (Gas Usage) | Typical online gas calculator | Deco planners (MultiDeco, Subsurface) |
|---|---|---|---|---|
| Profile | One level | Many steps, each with its own SRMV | One level | Full dive with deco |
| Gas model | Real gas, 15 °C | Real gas, 15 °C | Usually ideal gas | Varies by program |
| Reserve and turn pressure | Thirds to sixths or fixed, turn pressure | Same, plus where the turn is passed | Rarely | Varies by program |
| Price | Free, no account | Free tool in the app | Free | MultiDeco paid; Subsurface free, open source |

## Facts at a glance

- **Page:** https://divekit.app/tools/gas-consumption-calculator/
- **Inputs:** cylinder (38 from the Dive Kit open cylinder dataset, single or twin set), start pressure, SRMV, depth, time at depth, reserve rule (thirds, fourths, fifths, sixths, fixed), water type
- **Outputs:** pressure left, gas used, turn pressure, reserve pressure, time to reserve at that depth, status (Above reserve, Below reserve, Not enough gas)
- **Reference tables on the page:** reserve and turn pressure by rule for 200 bar and 3000 psi fills; minutes to reserve by depth from 10 to 40 m
- **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 `summarizeGasUsage` and `turnPressureBar`
- **In the app:** Gas Usage chains several steps, marks where the turn pressure is passed and loads saved cylinders; free on iOS and Android, works offline
- **Guide:** https://divekit.app/docs/tools/gas-usage/
- **Limits:** a planning aid, not a dive computer. One level, open circuit, no decompression. The result is only as good as the SRMV entered; cold, current and work raise it.
- **App Store (iOS):** https://apps.apple.com/app/divekit/id6753089545
- **Google Play (Android):** https://play.google.com/store/apps/details?id=app.skuba.diving
- **Last updated:** 2026-10-30

## Sources

- [BSAC Safe Diving (2020), Gas reserves](https://www.bsac.com/document/download-safe-diving-guide-pdf/safe-diving-inners-2020-v1.pdf): a third kept as a reserve as a rule of thumb; plan a larger reserve when a third might not be enough
- [GUE, Minimum Gas Nomenclature (2005)](https://www.gue.com/files/aug2005changes.pdf): rock bottom, renamed minimum gas, defined
- [DAN Alert Diver, Lessons in Gas Management](https://dan.org/alert-diver/article/lessons-in-gas-management/): a breathing-gas limit built into the plan
- [NIST Chemistry WebBook, fluid properties](https://webbook.nist.gov/chemistry/fluid/): density data behind the real-gas model

## Related

- [SAC and RMV calculator](https://divekit.app/tools/sac-rmv-calculator/): measure the SRMV this calculator needs
- [Scuba cylinder sizes](https://divekit.app/tools/scuba-cylinder-sizes/): water volume and real capacity of 38 cylinders
- [Dive unit converter](https://divekit.app/tools/dive-unit-converter/): bar and psi, litres and cubic feet
- [Deco planner](https://divekit.app/deco-planner/): gas per cylinder for staged decompression dives