How to Use the Scuba Dive Time Calculator
How long you can stay underwater depends on more than just your remaining air — it's also capped by the no-decompression limit (NDL) at your depth. This calculator takes your tank size, fill pressure, typical air consumption rate (SAC rate), and planned depth, then works out both the air-limited time and the NDL, reporting whichever is shorter as your real maximum dive time.
The deeper you go, the higher the ambient pressure, and the faster you burn through air at the same breathing rate. At 66ft (20m), for example, pressure is 3 times what it is at the surface (3 ATA), so you consume air roughly 3 times faster than your surface SAC rate would suggest. The calculation also sets aside a 50 bar reserve from your fill pressure for emergencies and your safety stop.
At the same time, nitrogen builds up in your body in proportion to depth and time, and once you exceed the NDL you can no longer surface directly without a decompression stop. At shallow depths, air usually runs out first, but at deeper depths the NDL more often becomes the binding constraint — so it's worth checking both every time.
Frequently Asked Questions
Keeping a minimum air reserve in the tank is a standard safety practice for emergencies and your safety stop (typically 3 minutes at 5m/15ft). Once you dip into that reserve, you should already be heading up.
Whichever is shorter is your real maximum dive time. At shallow depths you usually run low on air first, but at deeper depths you can still have air left while exceeding the NDL, which would require a decompression stop — so always check both.