How Much Air Is in a Scuba Tank? Litres, Cubic Feet, Bar and PSI

How much air is in a scuba tank?

Multiply the tank's water volume by its pressure in bar, then divide by 1.01325, the air pressure at sea level in bar. The result is the volume that air would fill at the surface. A 12 L tank at 200 bar works out at 2,369 L, or 83.6 cubic feet. Our dive unit converter runs the same sum for any size and shows the US rating beside it.

The table takes the water volume and pressure from each maker's spec sheet. The last column is the maker's own figure for the air inside, which only US tanks rated in cubic feet carry. It sits 3-5% under the formula, for a reason explained below. Weight, size and buoyancy for the same tanks are in our scuba tank size chart.

Bar, psi, litres and cubic feet: what each number tells you

Bar and psi measure pressure, how hard the air is squeezed. Litres and cubic feet measure volume. Describing what is in a tank takes one of each: the space inside, given as water volume, and the pressure it is filled to. NIST defines 1 bar as 100,000 Pa and 1 psi (pound-force per square inch) as 6,894.757 Pa, so 1 bar is 14.504 psi. One standard atmosphere is 1.01325 bar, about 14.7 psi.

Diving gauges show gauge pressure, which leaves the atmosphere out. NOAA's diving course labels it psig and adds 14.7 to get absolute pressure (psia), so a tank at 3,000 psig holds air at 3,014.7 psia. Tank sums use the gauge figure, because the last atmosphere of air cannot flow out of the tank and is no use to you.

Europe and the US name tanks differently. A European steel tank is sold by water volume and pressure, such as 12 L at 232 bar. A US tank carries its service pressure on the shoulder and is named by the air it holds in cubic feet. Catalina's crown marking guide shows both systems on one aluminum 80: DOT-3AL 3000, the US mark in psi, and TC-3ALM 207, the Canadian mark in bar.

Metric divers multiply litres by bar because 1 bar is within 1.3% of one atmosphere, so 12 L × 200 bar = 2,400 L is quick and close enough for a dive plan. Our converter divides by 1.01325 so that its litres line up with cubic feet measured at one atmosphere. That gives 2,369 L.

200 bar to psi, 232 bar to psi and other quick conversions

Multiply bar by 14.5038 for psi, and divide psi by the same number for bar. For the air in a tank, multiply water litres by bar and divide by 1.01325, then divide by 28.317 if you want cubic feet. Each line below uses the constants in our converter.

• 200 bar to psi: 200 × 14.5038 = 2,900.8 psi, about 2,900 psi.
• 232 bar to psi: 232 × 14.5038 = 3,364.9 psi, about 3,365 psi.
• 3,000 psi to bar: 3,000 ÷ 14.5038 = 206.8 bar. That is why an aluminum 80 is stamped 207 bar.
• 300 bar to psi: 300 × 14.5038 = 4,351 psi. Luxfer lists its hoop-wrapped LALW106 at 300 bar and 4,350 psi.
• 12 L at 200 bar to litres: 12 × 200 ÷ 1.01325 = 2,369 L, or 83.6 cu ft. The quick method gives 2,400 L.
• 50 bar to psi: 50 × 14.5038 = 725 psi. Going the other way, 500 psi is 34.5 bar.

Why does an aluminum 80 hold 77.4 cubic feet, not 80?

Because air at 200 bar is harder to squeeze than the formula assumes. The ideal gas law treats air molecules as points that never push against each other. At filling pressures they do, so a real tank holds less than the sum says.

Engineers measure the gap with the compressibility factor, Z: real volume is the ideal volume divided by Z. In the table for air at 300 K (27 °C) taken from Perry's Chemical Engineers' Handbook, Z is 1.007 at 150 bar, 1.033 at 200 bar, 1.067 at 250 bar and 1.109 at 300 bar. The correction is small at moderate fills and grows quickly above them.

At 200 bar the loss is about 3%, the same gap as the aluminum 80's: Luxfer rates it at 77.4 cu ft, while 11.1 L at 3,000 psi gives 80.0 cu ft by the formula. Faber's HP100 is 5% under at 237 bar. For planning at 200-232 bar the ideal figure is close enough, and our calculators use it. At 300 bar it overstates the air by about a tenth, so treat it as a ceiling.

Working pressure, test pressure and fill pressure: what the stamps mean

Working pressure, called service pressure on US tanks, is what the tank is meant to be filled to. Test pressure is the much higher pressure it is proved at, when new and at each hydrostatic retest. Fill pressure is whatever the compressor actually put in, and once the tank has cooled it should not be above the working pressure. Here is how the stamps read:

• US aluminum, DOT 3AL: the service pressure (3000 psi on an aluminum 80) may not be exceeded at 21 °C (70 °F). The minimum test pressure is 5/3 of it, 5,000 psi (345 bar).
• US steel, DOT 3AA: the same 21 °C limit, but a plus sign after the test date allows a fill 10% over the marked service pressure. The rule names 3A, 3AA and related steel specifications, not 3AL aluminum.
• Faber HP steel, sold in the US under a DOT special permit: the shoulder reads 3442 PSI for service pressure and TP 5250, the hydrostatic retest pressure in psi.
• European tanks with a PED mark: PW is the working pressure, the one to fill to (232 bar on Luxfer's example). PT is the test pressure, 348 bar there, which is 1.5 times PW. PS 287 bar at 60 °C is the design limit, and Luxfer warns not to confuse it with the working pressure.

What should a full scuba tank read on the gauge?

Its working pressure, at room temperature: about 3,000 psi (207 bar) for an aluminum 80, 232 bar for a 232 bar steel tank and 3,442 psi (237 bar) for a Faber HP100. Your submersible pressure gauge (SPG) shows the gas left in the tank, not the water pressure around you, which is a separate number covered in our water pressure at depth chart.

Temperature moves the reading. In a closed tank, pressure rises and falls with absolute temperature (Gay-Lussac's law). NOAA's diving course puts the change at roughly 0.7 bar per °C for an aluminum tank: its rule of thumb is 6 psi per °F, and 5 psi for steel. Its worked example cools an aluminum 80 holding 207 bar (3,000 psi) at 30 °C down to 5 °C, where it reads 190 bar (2,750 psi). That is a 17 bar drop with no air used.

US transport rules build temperature in: service pressure is the limit at 21 °C, and at 55 °C (131 °F) the pressure may be no more than 5/4 of it. So a tank filled quickly reads high while warm and lower once it sits in cool water. A reading a few bar under the working pressure on a cold morning can still be a full tank.

How much of the air in the tank can you actually use?

Less than the table shows, because you surface with a reserve. Our tank duration calculator multiplies the pressure you can spend, fill minus reserve, by the water volume. A 12 L at 200 bar with a 50 bar reserve leaves (200 - 50) × 12 = 1,800 L. At 18 m (60 ft) a diver who breathes 20 L/min at the surface uses 56 L a minute, which gives 32 minutes before the reserve.

The same reserve in bar holds back different amounts of air: 50 bar is 555 L in an aluminum 80, 600 L in a 12 L and 750 L in a 15 L. DAN's gas management advice adds a turn point on top of it, for example heading back when the first diver in a pair has used a third or half of the gas, not counting the reserve.

The calculators multiply litres by bar with no 1.01325 divisor and no real-gas correction. Both effects are small at 200 bar and both make the result a little generous, so read the minutes for a full tank as a ceiling. Minutes by depth and breathing rate, and the gas an ascent needs, are in our article on how long a scuba tank lasts.