Imperial vs Metric Units in Diving: Practical Guide

Why this matters when travelling or buying gear

Divers split roughly along a US/non-US line: the United States uses imperial (ft³, PSI, °F, ft), nearly everywhere else uses SI metric (L, bar, °C, m). A diver trained in one system who rents gear in the other faces unfamiliar gauge faces, computer displays and cylinder labels — at the exact moment when fast, reliable readings matter most. The risk is rarely catastrophic conversion errors; it's the slow drain of attention while you mentally translate every number underwater. Use the conversions tool for any field calculation, set your dive computer to your trained system, and learn the few key equivalences below by heart.

Cylinders: 80 ft³ vs 12 L

An aluminium AL80 (the classic US rec cylinder) holds 80 ft³ at its rated 3000 PSI working pressure. The metric equivalent is a 11.1 L cylinder at 207 bar — close enough that AL80 ≈ 11 L is the field shorthand. A European 12 L steel at 232 bar actually holds about 98 ft³ — meaningfully more gas than an AL80, despite the smaller internal volume label. Steel HP100 (US, 100 ft³ at 3442 PSI) lines up with a 12.2 L at 232 bar. When you book a dive boat overseas, ask for the cylinder's gas capacity ("how many cubic feet at full pressure?" or "liters × bar?") rather than just the size label, because internal volume × rated pressure is what your SAC plan actually depends on.

Pressure: PSI vs bar

1 bar ≈ 14.5 PSI. A full European cylinder at 232 bar reads about 3365 PSI; a full US AL80 at 3000 PSI reads about 207 bar. The mental shortcut: bar × 14.5 = PSI, or just bar × 15 if you're rounding underwater. Submersible pressure gauges (SPGs) are usually marked in only one unit, but dive computers often let you choose. Be aware that European "turn pressures" use neat round numbers (200 → 100 → 50 bar for rule-of-thirds on a 200-bar fill) that translate to messy 2900 → 1450 → 725 PSI. If you switch systems, pre-compute your turn and reserve pressures in the destination unit before splashing — don't try to convert during the dive.

Depth: feet vs metres

1 m ≈ 3.28 ft. Most computers used outside the US display metres; US computers typically default to feet. The classic recreational "60 ft" max depth is 18 m; 100 ft is 30 m; 130 ft (US recreational limit) is 40 m. Decompression-relevant numbers change feel: a 21 m gas-switch depth in metric is 70 ft in imperial. NDL tables differ by agency but the depth in metres is rounded to integer values (10, 12, 15, 18, 21, 24, 27, 30, 33, 36, 40, 45 m); in feet rounded to 5-ft increments (35, 40, 50, 60, 70, 80, 90, 100, 110, 130 ft). Set your computer to your trained system to avoid mental translation underwater — the depth display is the single most-read number on the dive.

Practical recommendation

Dive within the unit system you were trained in. If you trained metric, set every computer to metric on day one of any US trip; if you trained imperial, set every European computer to imperial. Carry one printed conversion card (or use the conversions tool offline) for cylinder negotiations on dive boats. The dangerous moments are: a rented computer set to the unfamiliar system, a borrowed SPG on a buddy's regulator, or a digital pressure transmitter reading a unit the owner forgot to switch. Confirm units at the briefing — "depth in metres? pressure in bar?" — and the rest of the dive runs on autopilot. Convert once, on the surface, never underwater.