Diving guide
Scuba Diving Units: Bar, PSI, Meters, and Feet
Convert the units divers use, understand which conversions are exact or approximate, and avoid mixing pressure, depth, and cylinder capacity.
Dive arithmetic crosses metric and imperial systems constantly. The safe skill is not memorizing every decimal; it is knowing which conversions are exact, which shortcuts are intentionally approximate, and when a label is not the same kind of quantity.
Exact conversions worth knowing
| Quantity | Exact or standard conversion | Mental shortcut |
|---|---|---|
| Pressure | 1 bar = 14.5038 psi | × or ÷ 15 |
| Depth | 1 m = 3.28084 ft | × 3.3 |
| Mass | 1 kg = 2.20462 lb | × 2.2 |
| Free gas | 1 cu ft = 28.3168 L | × 28.3 |
Try common dive conversions
Enter an imperial value to see its metric equivalent.
The full dive unit converter also handles temperature and pressure-aware cylinder capacity in either direction.
Useful planning shortcuts
Seawater adds approximately 1 ATA every 10 m or 33 ft. Those two depths are not exact conversions—10 m is 32.8 ft—but each is the conventional rounded constant in its own system. That is why separately calculated metric and imperial plans can differ slightly.
Gauge-friendly reserve pairs deserve the same care. Fifty bar is about 725 psi, so 50 bar / 700 psi is a sensible paired convention. Five hundred psi is only about 34.5 bar and should not silently stand in for 50 bar.
The cylinder-volume trap
A metric “12 L” cylinder usually states internal water volume. An imperial “80 cu ft” cylinder states nominal free gas at working pressure. Converting 80 cu ft to 2,265 L of free gas does not mean the shell has a 2,265 L internal volume; a typical AL80's water volume is about 11.1 L.
Use the exact cylinder's working pressure when converting pressure SAC to RMV or comparing capacity. The cylinder guide gives worked AL80, 12 L, and 15 L examples.
One team, one unit language
- State every number with its unit: “70 bar,” not “70.”
- Agree on one unit system for depth, reserve, turn pressure, and ascent rate.
- Set computers deliberately; do not assume a rental or backup device matches.
- Convert on the surface and write the result. Avoid mental arithmetic during a problem.
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.