Dive Unit Converter
Two-way converters for pressure, depth, lead weight, temperature and gas volume, plus metric and US cylinder sizes side by side.
Unit conversions
Results update as you type.
Cylinder size
Metric cylinders are named by water volume and working pressure. US cylinders are named by the gas they hold in cubic feet. Enter one and read the other.
AL80 and AL63 are Luxfer aluminium, HP100 and HP120 are Faber steel. The metric sizes are steel cylinders filled to 232 bar.
- Gas at the surface (ideal)
- 80 cu ft
- Same gas in litres
- 2266 L
- Manufacturer rating
- 77.4 cu ft
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About the unit converter
Dive gear and dive sites mix unit systems. A US boat fills to psi, a rented computer may show feet and weight belts are often sold in pounds. Each converter works both ways: type in either box and the other one updates.
Metric cylinders are named by water volume and working pressure, US cylinders by the gas they hold in cubic feet. The cylinder converter links the two with the ideal gas law and shows the manufacturer's rating next to it.
Good to know
- Cylinder presets use manufacturer data: Luxfer for the AL80 and AL63, Faber for the steel HP100 and HP120 (spec sheets published by XS Scuba). The numbers stamped on your own cylinder take priority over a preset.
- Depth conversion is length only. For pressure at depth, each 10 m (33 ft) of seawater adds about 1 bar (14.5 psi).
- The other calculators on this site work in metric units. Convert your psi, feet or cubic feet here first, then enter them in the SAC or gas planning calculator.
Formula
Pressure
psi = bar × 14.5037738
Depth
ft = (m) ÷ (0.3048)
Lead weight
lb = (kg) ÷ (0.45359237)
Water temperature
°F = °C × (9) ÷ (5) + 32
Gas volume
cu ft = (L) ÷ (28.316846592)
Cylinder gas at the surface
Vgas = (Vwater × P) ÷ (1.01325)
Where
- Vwater: cylinder water volume (L)
- P: working pressure on the gauge (bar)
- Vgas: gas volume at the surface (L); divide by 28.316846592 for cu ft
- 1.01325: one standard atmosphere in bar, the reference for US cubic-foot ratings
How it's calculated
- Pressure: psi = bar × 14.5037738 (1 bar = 100 000 Pa, 1 psi = 6894.757 Pa).
- Depth: 1 ft = 0.3048 m exactly, so ft = m ÷ 0.3048.
- Weight: 1 lb = 0.45359237 kg exactly, so lb = kg ÷ 0.45359237.
- Temperature: °F = °C × 9/5 + 32. Values below absolute zero (-273.15 °C) are rejected.
- Gas volume: 1 cu ft = 28.316846592 L exactly.
- Cylinder gas (L) = water volume (L) × working pressure (bar) ÷ 1.01325. The divisor is 1 atm in bar, the reference pressure for US cubic-foot ratings. Divide by 28.316846592 for cubic feet.
- Working pressure is the gauge reading, so the gas that stays in an empty cylinder is not counted.
- US to metric: water volume (L) = gas capacity (cu ft) × 28.316846592 × 1.01325 ÷ working pressure (bar).
- Many metric divers multiply water volume by bar (12 L × 232 bar = 2784 L). That treats 1 bar as 1 atm and gives 1.3% more than this tool.
- Converted values are rounded to 0.1 and the gas readout to whole litres. The number you type stays exactly as typed and only the other box is rounded, so values do not creep when you move between boxes.
These tools are educational planning aids; they are not a substitute for dive training, current printed tables or a dive computer. Verify results against your training and always plan within the limits of your certification.
Quick reference
Common dive values in both unit systems, worked out with the same formulas as the converter.
| Metric | Imperial |
|---|---|
| 1 bar | 14.5 psi |
| 50 bar | 725.2 psi |
| 200 bar | 2900.8 psi |
| 206.8 bar | 3000 psi |
| 232 bar | 3364.9 psi |
| 237.3 bar | 3442 psi |
| 300 bar | 4351.1 psi |
| Metric | Imperial |
|---|---|
| 5 m | 16.4 ft |
| 10 m | 32.8 ft |
| 18 m | 59.1 ft |
| 18.3 m | 60 ft |
| 30 m | 98.4 ft |
| 30.5 m | 100 ft |
| 39.6 m | 130 ft |
| 40 m | 131.2 ft |
| Metric | Imperial |
|---|---|
| 1 kg | 2.2 lb |
| 2 kg | 4.4 lb |
| 4.5 kg | 10 lb |
| 5 kg | 11 lb |
| 9.1 kg | 20 lb |
| 10 kg | 22 lb |
| Metric | Imperial |
|---|---|
| 4 °C | 39.2 °F |
| 10 °C | 50 °F |
| 20 °C | 68 °F |
| 25 °C | 77 °F |
| 26.7 °C | 80 °F |
| 28 °C | 82.4 °F |
| Cylinder | Water volume | Working pressure | Ideal gas | Manufacturer rating |
|---|---|---|---|---|
| Luxfer AL80 | 11.1 L | 207 bar (3000 psi) | 80 cu ft | 77.4 cu ft |
| Luxfer AL63 | 9 L | 207 bar (3000 psi) | 64.9 cu ft | 63 cu ft |
| Faber HP100 | 12.9 L | 237 bar (3442 psi) | 106.7 cu ft | 101.3 cu ft |
| Faber HP120 | 15.3 L | 237 bar (3442 psi) | 126.5 cu ft | 120.1 cu ft |
| 10 L steel | 10 L | 232 bar (3365 psi) | 80.9 cu ft | Not rated in cu ft |
| 12 L steel | 12 L | 232 bar (3365 psi) | 97 cu ft | Not rated in cu ft |
| 15 L steel | 15 L | 232 bar (3365 psi) | 121.3 cu ft | Not rated in cu ft |
How to use the dive unit converter
- 1
Type a value in either box of a pair. The other box updates as you type, so it works metric to imperial and back.
- 2
For a cylinder, tap a common size such as AL80 or 12 L, or enter the water volume and working pressure stamped on the cylinder.
- 3
Read the gas at the surface in litres and cubic feet. For US cylinders, compare it with the manufacturer rating shown below it.
Metric and US cylinder sizes
A metric cylinder is named by its water volume and working pressure. A 12 L at 232 bar holds 12 litres of water when empty and is filled to 232 bar. A US cylinder is named by the gas it holds at its rated pressure: an AL80 holds about 80 cubic feet of air at 3000 psi.
To compare the two, multiply water volume by pressure and divide by atmospheric pressure. The result is the volume the gas would take up at the surface. A 12 L at 232 bar comes out at about 97 cu ft, which is more than an AL80 and less than a steel HP100.
The formula treats air as an ideal gas. At filling pressures real air is harder to compress, so a real cylinder holds a little less than the formula says. That is why the Luxfer AL80 is rated 77.4 cu ft while the formula gives 80.0 cu ft. For comparing cylinder sizes the ideal figure is close enough.