Scuba Tank Size Converter: Cubic Feet to Litres

Updated November 2026 · free, works in your browser

To convert a scuba tank from cubic feet to litres, you need two numbers: the rated gas capacity in cubic feet and the working pressure. A standard aluminium 80 (80 cubic feet at 3000 psi) has an internal water volume of about 11.1 litres. The converter above does the same sum for any cylinder, and it also works in reverse, from litres and bar to cubic feet.

How to use

1. Pick a direction: "Cu ft → litres" if the tank is marked in cubic feet, or "Litres → cu ft" if it is marked in litres.
2. Enter the capacity. For cubic feet, use the rated gas volume (for example 80). For litres, use the water capacity stamped on the cylinder (for example 12).
3. Enter the working pressure and choose psi or bar. Switching the unit converts the number you typed.
4. Read the result. The large figure is the converted capacity, and the smaller panels show the free gas volume in litres and the water capacity in cubic feet.
5. Use the copy button to share a link that reopens the same values.

How it is calculated

Cylinder ratings rely on Boyle's law: at a constant temperature, the volume of gas at atmospheric pressure equals the internal water volume multiplied by the pressure ratio.

Water volume (L) = gas volume (cu ft) × 28.3168 × 14.696 ÷ working pressure (psi)

Gas volume (cu ft) = water volume (L) × working pressure (psi) ÷ (28.3168 × 14.696)

• 28.3168 is the number of litres in one cubic foot.
• 14.696 psi is one standard atmosphere.
• Working pressure is the rated fill pressure. One bar equals 14.5038 psi, which is how bar values are converted.

The method treats the gas as ideal. Real air is slightly compressible at high pressure, so manufacturers' published figures can differ from this result by a few percent. For example, an aluminium 80 is commonly listed at 77.4 cubic feet at 3000 psi.

Common cylinder sizes

Frequently asked questions

What is the difference between water capacity and gas capacity?

Water capacity is the internal volume of the cylinder, measured in litres in most of the world. Gas capacity is how much gas that cylinder holds once it is filled to its working pressure, expressed as the volume the gas would occupy at normal atmospheric pressure. US cylinders are usually labelled with gas capacity in cubic feet, while European cylinders are labelled with water capacity in litres.

How many litres of gas does a 12-litre tank at 200 bar hold?

By the simple litres × bar rule it is about 2,400 litres of free gas. The converter uses the psi-based formula, so the result differs slightly, but both are approximations of the same idea.

Why doesn't an AL80 hold exactly 80 cubic feet?

The "80" is a nominal size. The rated capacity is listed at 77.4 cubic feet at 3000 psi, and the exact figure depends on the manufacturer and on how the rating accounts for real gas behaviour.

Does a higher fill pressure change the water capacity?

No. The internal volume of a cylinder is fixed. Filling to a higher pressure only changes how much gas it holds, so the same cylinder has a different rated gas capacity at a different working pressure.

Can I use this to decide how much I can fill a tank?

No. Use it to compare sizes only. Always follow the markings on the cylinder, its test dates and the instructions of the fill station.

Planning a dive trip

Dive centres around the world quote tank sizes differently, so converting them in advance helps you compare how much gas you will have for each dive. Once you know which cylinders a centre offers, you can plan the rest of the trip, such as choosing accommodation within easy reach of the dive site or harbour. Staying close to the boat departure point makes early starts and equipment handling much easier.