Diving guide
How Long Will My Scuba Tank Last?
Estimate cylinder duration from usable gas, depth, and RMV, and understand why a constant-depth estimate is not a complete dive plan.
“About an hour” is not a property of a scuba cylinder. The same tank may support a long shallow dive or only a fraction of that time at depth. A useful estimate combines the cylinder's usable gas, your surface breathing rate, and ambient pressure.
The duration formula and its inputs
Duration = usable gas ÷ (RMV × ambient pressure)
For metric cylinders, usable gas in liters = cylinder water volume × (start bar − reserve bar).
- Usable gas: cylinder gas between starting pressure and the reserve you must retain.
- RMV: your surface-equivalent gas consumption per minute.
- Ambient pressure: the depth multiplier that makes gas disappear faster underwater.
For example, a 12 L cylinder from 200 to 50 bar contains 12 × 150 = 1,800 usable liters. At 18 m, ambient pressure is 2.8 ATA. With an RMV of 18 L/min, duration is 1,800 ÷ (18 × 2.8) = 35.7 minutes. In imperial units, an AL80 from 3,000 to 700 psi has 61.3 usable cu ft; at 60 ft and 0.65 cu ft/min, the same calculation gives about 33.5 minutes.
So a direct answer to the title question is: in this representative case,about 34–36 minutes at depth, before any allowance for descent, ascent, changing workload, or a larger emergency reserve.
Try it: Tank Duration Estimate
Estimate usable time at a constant depth before reaching your selected reserve.
Allowed range: 0 to 50 m.
Allowed range: 8 to 40 L/min.
Allowed range: 100 to 300 bar.
Allowed range: 30 to 100 bar.
Allowed range: 7 to 24 L.
Not a complete dive plan: descent, ascent, stops, exertion, emergencies, and changing depth all affect consumption.
Why depth changes everything
At the surface, ambient pressure is approximately 1 ATA. At 10 meters or 33 feet it is approximately 2 ATA, so a diver with the same breathing pattern consumes twice the surface gas volume per minute. At 20 meters or 66 feet, consumption is roughly three times the surface rate.
Roughly how long a full cylinder lasts
Assumes an 18 L/min RMV, a full fill, and holding back a 50 bar reserve.Assumes a 0.64 cu ft/min RMV, a full fill, and holding back a 700 psi reserve. Constant depth only.
| Depth | AL80 | AL100 | Steel 12L |
|---|---|---|---|
| 10 m33 ft | 48 min48 min | 65 min62 min | 61 min61 min |
| 20 m66 ft | 32 min32 min | 44 min41 min | 40 min40 min |
| 30 m99 ft | 24 min24 min | 33 min31 min | 30 min30 min |
| 40 m132 ft | 19 min19 min | 26 min25 min | 24 min24 min |
These are gas-supply estimates, not bottom times. A real dive spends gas on descent, ascent, and stops, and no-decompression limits often end the dive well before the cylinder does.
SAC, RMV, and cylinder size
RMV expresses actual surface gas volume and works across cylinders. A pressure-based SAC value only describes the cylinder on which it was measured unless it is converted. Use the SAC and RMV calculator with recent dive data rather than relying on a generic rate.
Why this is not your planned bottom time
The simple estimate holds depth and breathing rate constant. A real dive consumes gas during descent, ascent, stops, swimming, current, and unexpected delays. Cold, effort, stress, and equipment configuration can raise consumption. The reserve gas must remain protected rather than being treated as extra bottom time.
A flat estimate versus a real dive profile
Turn the estimate into a plan
Use a conservative RMV, model each part of the profile, retain the appropriate reserve, and plan around the team member whose available gas reaches the limit first. The full gas consumption planner is designed for that next step.
Sources
References and further reading
These official and specialist sources support the concepts in this guide. Always use current training materials and operational procedures for dive planning.