Scuba Tank Air Duration Calculator

Updated November 2026 · free, works in your browser

A scuba tank lasts for roughly its usable gas divided by how fast you breathe it at depth. Enter your tank size, fill pressure, reserve and surface air consumption (SAC), pick a depth, and the calculator shows the estimated minutes you can stay at that depth.

How to use

1. Choose metric (bar and litres) or imperial (psi and cubic feet).
2. Enter the tank size: water volume in litres, or the rated capacity in cubic feet for imperial cylinders. For imperial, also enter the rated working pressure.
3. Enter the fill pressure and the reserve pressure you want to keep in the tank when you finish.
4. Enter your SAC rate, which is how much gas you breathe per minute at the surface.
5. Enter the depth and read the estimated time, usable gas, gas use at depth and pressure drop per minute.
6. Use the copy button to share or save a link with your numbers.

How it is calculated

Gas is compressed by the surrounding water, so every breath at depth uses more gas than at the surface. Ambient pressure is about 1 bar at the surface and rises by roughly 1 bar every 10 metres (1 atmosphere every 33 feet). This follows Boyle's law, the standard basis for dive gas planning.

• Pressure at depth (ata) = depth ÷ 10 + 1 in metres, or depth ÷ 33 + 1 in feet.
• Gas use at depth = SAC × pressure at depth.
• Usable gas = tank volume × (fill pressure − reserve pressure). For imperial cylinders: rated cubic feet × (fill − reserve) ÷ rated working pressure.
• Time = usable gas ÷ gas use at depth.

The result is the time at one constant depth. It does not include the gas used on descent, the ascent, safety stops or a buddy emergency. It is a planning aid and does not replace your training or your agency's gas management rules.

How depth changes gas use

FAQ

What is a SAC rate?

SAC stands for surface air consumption: the volume of gas you breathe per minute, expressed at surface pressure. It differs from person to person and changes with fitness, stress, water temperature and workload.

How do I find my own SAC rate?

Note the pressure drop over a dive at a steady depth, convert it to a volume using your tank size, divide by the minutes and by the ambient pressure at that depth. Averaging several relaxed dives gives a more useful number than a single one.

Why does my tank last so much less when I go deeper?

Each breath contains more gas molecules at higher ambient pressure. At 30 m you breathe four times as much gas per minute as at the surface, so the same tank gives roughly a quarter of the time.

How much reserve should I keep?

That depends on your training, the dive and the conditions. Many divers plan to surface with a reserve of 50 bar (about 700 psi) or more. Follow the rules from your instructor or agency for deeper or overhead dives.

Does the time include descent and ascent?

No. The calculator assumes you stay at the entered depth the whole time. For a real dive, plan the descent, ascent and stops separately, and use a depth below the maximum you expect.

Gas planning is easier once you know where you will be diving. If you are planning a dive trip, check the typical depths of the sites in your area, then choose a place to stay close to the dive operator, so early boat departures and tank refills do not add stress to your day.