How long does a scuba tank last?

There is no single duration. Tank capacity, starting pressure, reserve, depth, breathing rate, exertion, and the dive profile all change the answer.

Does a scuba tank last half as long one atmosphere deeper?

Only for the first atmosphere. Constant-depth duration is proportional to 1 ÷ ambient pressure. Using the seawater, sea-level approximation of 10 m per atmosphere, going from the surface (1 ATA) to 10 m (2 ATA) halves it, 10 m to 20 m (3 ATA) leaves about two-thirds, and 20 m to 30 m (4 ATA) leaves three-quarters. Real dives also include changing depths and workload. (Only for the first atmosphere. Constant-depth duration is proportional to 1 ÷ ambient pressure. Using the seawater, sea-level approximation of 33 ft per atmosphere, going from the surface (1 ATA) to 33 ft (2 ATA) halves it, 33 ft to 66 ft (3 ATA) leaves about two-thirds, and 66 ft to 99 ft (4 ATA) leaves three-quarters. Real dives also include changing depths and workload.)

Why should reserve gas be excluded?

Reserve gas is not planned bottom-time gas. It is held for ascent and contingencies according to the dive plan and training procedures.

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

t \approx \frac{G}{\text{RMV} \times P_{\text{ambient}}}

G \approx V_{\text{water}} \times (P_{\text{start}} - P_{\text{reserve}})

G \approx V_{\text{rated}} \times \frac{P_{\text{start}} - P_{\text{reserve}}}{P_{\text{working}}}

$t$ is the constant-depth duration in minutes, and $G$ is the usable gas in literscu ft. RMV is your surface-equivalent breathing rate in L/mincu ft/min, in the same volume unit as the usable gas.

$V_{\text{water}}$ is the cylinder's internal water volume in liters, and the pressures are gauge readings in bar. $V_{\text{rated}}$ is the manufacturer's rated capacity in cu ft at the working pressure $P_{\text{working}}$ ; all three pressures are gauge readings in psi.

$P_{\text{ambient}}$ is absolute pressure in ATA. With the seawater, sea-level approximation of 10 m33 ft per atmosphere, $P_{\text{ambient}} \approx \frac{\text{depth (m)}}{10} + 1$ $P_{\text{ambient}} \approx \frac{\text{depth (ft)}}{33} + 1$ .

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.

Worked example

A 12 L cylinder filled to 200 bar, with a 50 bar reserve, breathed at a constant 18 m with an RMV of 18 L/min.An AL80 (rated 77.4 cu ft at 3,000 psi) filled to 3,000 psi, with a 700 psi reserve, breathed at a constant 60 ft with an RMV of 0.64 cu ft/min.

G \approx 12 \times (200 - 50) = 1{,}800\ \text{L}

P_{\text{ambient}} \approx \frac{18}{10} + 1 = 2.8\ \text{ATA}

t \approx \frac{1{,}800}{18 \times 2.8} = 35.7\ \text{min}

G \approx 77.4 \times \frac{3{,}000 - 700}{3{,}000} = 59.3\ \text{cu ft}

P_{\text{ambient}} \approx \frac{60}{33} + 1 = 2.82\ \text{ATA}

t \approx \frac{59.3}{0.64 \times 2.82} = 32.9\ \text{min}

Round the duration down, not to the nearest minute: 35 minutes32 minutes is the operational ceiling for this constant-depth case.

So a direct answer to the title question is: in this representative case, roughly 3532 minutes at depth, before any allowance for descent, ascent, changing workload, or a larger emergency reserve.

The ideal-gas shortcut

The usable-gas formula treats pressure and gas volume as proportional, as an ideal gas would be. Real air compresses less than that at diving fill pressures, so between a 200–232 bar2,900–3,400 psi fill and a typical reserve a cylinder releases roughly 5–8% less gas than the shortcut suggests. In the example, that is about 1,700 L rather than 1,800 L, or about 34 minutes instead of 35.7.A manufacturer's rated capacity already includes this at the full working pressure: 11.1 L × 207 bar would suggest about 81 cu ft for an AL80, but it is rated 77.4 cu ft. Scaling that rating linearly to a partial fill is still an approximation. The real-gas cylinder calculator estimates the difference for a specific fill, gas, and temperature.

Try it: Tank Duration Estimate

Estimate usable time at a constant depth before reaching your selected reserve.

Depth

RMV

L/min

Starting pressure

bar

Reserve pressure

bar

Cylinder water volume

35 minutes

constant-depth estimate • 1800 L usable gas

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. In seawater at 10 meters33 feet it is approximately 2 ATA, so a diver with the same breathing pattern consumes twice the surface gas volume per minute. At 20 meters66 feet, consumption is roughly three times the surface rate. Duration therefore falls with 1 ÷ ambient pressure: the first atmosphere halves it, but each further atmosphere removes a smaller share.

Roughly how long a full cylinder lasts

Assumes an 18 L/min RMV, a full fill, and an example reserve of 50 bar.Assumes a 0.64 cu ft/min RMV, a full fill, and an example reserve of 700 psi. Constant depth in seawater (10 m or 33 ft per atmosphere), rounded to the nearest minute.

Depth	AL80	AL100	Steel 12L
10 m33 ft	48 min46 min	65 min61 min	61 min61 min
20 m66 ft	32 min31 min	44 min41 min	40 min40 min
30 m99 ft	24 min23 min	33 min30 min	30 min30 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.

A fixed reserve is a shallow-dive shortcut. Deeper dives need a calculated rock bottom gas reserve for a shared ascent, which at 30 m99 ft is usually well above 50 bar700 psi and shortens that row considerably.

Cells multiply water volume by pressure, an ideal-gas shortcut that overstates real gas by roughly 5–8% at these fills.The aluminum cells start from Luxfer's rated capacities (AL80 77.4 cu ft, AL100 98.8 cu ft), which already include real-gas behavior at the working pressure. The 12 L steel has no published cubic-foot rating, so its cells use the ideal-gas water-volume shortcut.

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

The calculator holds depth and breathing rate constant. A usable plan accounts for every segment of the dive and protects reserve gas throughout.

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.

The displayed time is an educational constant-depth estimate, not permission to remain at depth for that duration. Follow your training, no-decompression limits, gas plan, and dive computer.

Sources

Estimating Your Air Consumption · Divers Alert Network
Diving Medical Technician Reference and Formulas · NOAA Diving Program — Air requirement formulas: SAC in psi/min normalized by depth in ATA, cylinder constant as rated volume ÷ rated pressure, RMV = SAC × constant, available volume above a reserve, and cylinder duration.
Luxfer Aluminum Cylinder Specifications · Luxfer / XS Scuba — Source of the AL80 (77.4 cu ft at 3,000 psi; 11.1 L at 207 bar) and AL100 (98.8 cu ft at 3,300 psi; 13.2 L at 228 bar) ratings used in the examples and tables. Comparing 11.1 L × 207 bar (about 81 cu ft) with the 77.4 cu ft rating shows the real-gas shortfall of the water-volume shortcut.
Scuba Cylinder Rundown · Divers Alert Network — General overview of cylinder sizes and choosing a larger cylinder for higher consumption. It names the AL80 by its nominal 80 cu ft; the calculations here use Luxfer's 77.4 cu ft rating.
Lessons in Gas Management · Divers Alert Network

Always use current training materials and operational procedures for dive planning.