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 definitions and rounded factors
Only some unit relationships are exact. The foot, the pound, the bar, and the standard atmosphere have exact definitions; most everyday factors, such as 14.5038 psi per bar, are rounded from them. Calculators use the rounded factors, which are far more precise than any gauge or cylinder marking.
Unit definitions and conversion factors
Exact definitions are from NIST SP 811, Appendix B. Rounded factors are the values DiveCalc calculators use, rounded to six significant figures. Mental shortcuts are deliberately coarse for orientation and cross-checking only; the coarsest, 15 for bar and psi, is off by about 3.4%.
| Quantity | Exact definition | Rounded factor | Mental shortcut |
|---|---|---|---|
| Pressure | 1 bar = 100,000 Pa | 1 bar ≈ 14.5038 psi | × or ÷ 15 |
| Standard atmosphere | 1 atm = 101,325 Pa = 1.01325 bar | 1 atm ≈ 14.696 psi | ≈ 1 bar |
| Length and depth | 1 ft = 0.3048 m | 1 m ≈ 3.28084 ft | × 3.3 |
| Mass | 1 lb = 0.45359237 kg | 1 kg ≈ 2.20462 lb | × 2.2 |
| Gas or water volume | 1 cu ft = 28.316846592 L | 1 cu ft ≈ 28.3168 L | × 28.3 |
These are unit conversions only. Depth-to-pressure shortcuts such as 10 m33 ft per atmosphere are separate seawater approximations, explained below.
Try common dive conversions
Enter values in your preferred units to see their equivalents.
The full dive unit converter also handles temperature and pressure-aware cylinder capacity in either direction.
Useful planning shortcuts
At sea level, 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: the US Navy treats 33 feet of seawater as one atmosphere, while metric practice defines 10 msw as 1 bar. That is why separately calculated metric and imperial plans can differ slightly. The pressure and ATA guide shows the formula behind them.
Fixed reserve pressures deserve the same care. The commonly taught end-of-dive reserve is written as 50 bar / 500 psi, but that pairing is a convention, not a conversion: 50 bar is about 725 psi, and 500 psi is only about 34.5 bar. When a plan written in one unit is read on a gauge in the other, convert the number and choose the rounding direction deliberately. Rounding 725 psi up to 750 psi keeps the full 50 bar; rounding down to 700 psi leaves about 48 bar. A fixed reserve also ignores depth, cylinder size, and team size, which the rock-bottom gas guide accounts for.
The cylinder-volume trap
A metric “12 L” cylinder usually states internal water volume. An imperial “80 cu ft” cylinder is named for its nominal free-gas capacity at working pressure. The common aluminum 80 is rated by Luxfer at 77.4 cu ft at 3,000 psi—about 2,192 L of free gas—from an internal water volume of about 11.1 L. Those 2,192 L describe surface-pressure gas, not the size of the shell. The rated capacity is also lower than the linear estimate of 11.1 L × 207 bar ≈ 2,296 L, because air compresses less than an ideal gas at high pressure; the real-gas calculator models that effect.
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: “1,000 psi,” not “1,000.”
- 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.
Mental shortcuts are for orientation and cross-checking. Use exact values from the configured calculator, cylinder markings, tables, computer, and team plan where rounding direction matters.
Sources
- NIST Guide to the SI (SP 811), Appendix B.8: Factors for Units Listed Alphabetically · National Institute of Standards and Technology — Lists the bar (100 kPa), standard atmosphere (101,325 Pa), and foot (0.3048 m) as exact; footnote 22 gives the exact pound (0.45359237 kg). The psi and cubic-foot factors are rounded.
- U.S. Navy Diving Manual, Revision 7 · Naval Sea Systems Command (GovInfo archive) — Chapter 2 treats 33 feet of seawater as one atmosphere (14.7 psi), tabulates 33/66/99 fsw as 2/3/4 ata, distinguishes gauge from absolute pressure, and notes that metric practice defines 10 msw as 1 bar.
- Luxfer Aluminum Cylinder Specifications · Luxfer / XS Scuba — Rates the Luxfer S80 aluminum cylinder at 77.4 cu ft at 3,000 psi from about 11.1 L of internal water volume.
- Scuba Cylinder Rundown · Divers Alert Network
- 14 Tips to Improve Your Air Consumption · PADI — An example of agency material pairing a typical end-of-dive pressure as “500 psi/50 bar”; the two values are a convention, not equivalent pressures.
Always use current training materials and operational procedures for dive planning.