Metric vs imperial dive math: bar vs psi, meters vs feet

The briefing is in meters and bar. The rental gauge reads psi. Your buddy’s computer shows feet because the last renter set it that way. The physics is the same in both systems, but each has its own shortcuts, and mixing them in one calculation is where the mistakes start.

How do metric and imperial units compare for diving?

Metric diving uses meters, bar, liters and kilograms; imperial diving uses feet, psi, cubic feet and pounds. The table gives the exact conversions, from the US National Institute of Standards and Technology (NIST), next to the shortcut each system uses.

Both depth shortcuts are rounded: 10 m is 32.8 ft and 1 atm is 1.013 bar, so “1 bar per 10 m” and “1 atm per 33 ft” differ by less than 1%. Divers Alert Network (DAN) pairs them the same way. The gap is harmless for planning, but it’s a good reason to stay in one system from the first line of a calculation to the last.

How does the same calculation look in each system?

The steps are identical in both systems; only the numbers change. Three calculations every diver meets, all at 20 m (66 ft):

The two answers differ, 30 minutes against 32, because the cylinder and the reserve differ. A 12 L cylinder at 200 bar holds about 2,400 L (85 cu ft); the 80 cu ft cylinder holds about 2,265 L. And 500 psi is about 34 bar, not 50: it keeps about 380 L in the cylinder, where 50 bar in a 12 L cylinder keeps 600 L. When a course converts an example from one system to the other, expect the answer to shift.

Octo asks the same question. Its imperial version uses 0.6 cu ft/min, so the times differ again:

Try a question from the app

gas supply time

A full 12 L cylinder at 200 bar, a 50 bar reserve and a surface consumption of 20 L/min: how long can you stay at 20 m before reaching the reserve, ignoring the ascent?

1. AAbout 40 min
2. BAbout 45 min
3. CAbout 30 min
4. DAbout 90 min

Show the answer

Answer C. 150 bar × 12 L = 1800 L usable; at 3 bar, 20 × 3 = 60 L/min; 1800 ÷ 60 = 30 min. 40 min also spends the reserve; 45 min uses the gauge pressure; 90 min forgets the depth. The ascent and safety stop need gas too.

More on breathing rates in air consumption and SAC rate, and every formula on one page in our dive theory formulas.

Why are metric and imperial cylinders named differently?

Metric cylinders are named by their water volume; US cylinders are named by the gas they hold at their rated pressure. That changes how you work out what’s inside.

• Metric: gas in liters = water volume × pressure in bar. A 12 L cylinder at 100 bar holds about 1,200 L.
• Imperial: gas in cubic feet = rated capacity × pressure ÷ rated pressure. A cylinder rated 80 cu ft at 3,000 psi holds 40 cu ft at 1,500 psi.

Two footnotes. The classic “aluminum 80” holds about 77 cu ft when full, not 80. And the metric shortcut is rounded too: cylinder specialist Mark Derrick notes it overstates real contents by several percent at high pressure, because a bar isn’t exactly an atmosphere and air doesn’t compress perfectly. Course math uses the simple versions, which is fine as long as you know they’re rounded.

Breathing rates in psi per minute need care: they describe one cylinder only, because the same breathing takes fewer psi from a bigger one. Convert to cubic feet per minute before you change cylinders. On a cylinder rated 80 cu ft at 3,000 psi, 20 psi/min is 20 × 80 ÷ 3,000 ≈ 0.53 cu ft/min. On one rated 100 cu ft at 3,000 psi, that same breathing shows as only 16 psi/min.

Which unit traps catch divers out?

The traps come from mixing systems, forgetting the atmosphere and trusting rounded pairs.

1. Gauge vs absolute. Your cylinder gauge and depth reading start from zero at the surface. Gas laws need absolute pressure: bar absolute, or ata. Adding the 1 is the step people skip. See gauge pressure.
2. Mixing systems mid-calculation. A breathing rate in cu ft/min with a cylinder in bar gives nonsense. Convert everything first, then calculate.
3. Treating round pairs as conversions. 50 bar (500 psi), 18 m (60 ft) and 5 m (15 ft) are each system’s round numbers, not exact equivalents: 500 psi is about 34 bar, 18 m is 59 ft, 15 ft is 4.6 m. Use the figure your briefing gives, in the unit your gauge shows.
4. Converting a rounded limit. Work out a nitrox maximum operating depth in your own system and round down. EAN32 at a ppO2 of 1.4 gives 33.75 m, so 33 m, or 111.4 ft, so 111 ft. Convert 111 ft and you get 33.8 m; round that up to 34 m and you’ve crossed 1.4. More in oxygen toxicity and nitrox.
5. Everyday degrees in gas laws. Gas laws need kelvin (°C + 273) or Rankine (°F + 460). Warming a cylinder from 10 to 20 °C raises its pressure by about 3.5%, not 100%. See Charles’s law.

Try a question from the app

MOD of EAN32

What is the maximum operating depth of EAN32 for a ppO2 of 1.4?

1. A43 m
2. B40 m
3. C34 m
4. D33 m

Show the answer

Answer D. 1.4 ÷ 0.32 = 4.375 bar; (4.375 − 1) × 10 = 33.75 m, rounded down to 33 m. 43 m forgets to subtract the atmosphere; 40 m is the limit at 1.6; 34 m rounds up and crosses 1.4.

Which units should you learn dive theory in?

Learn in the units of your course and your gauges first, then the other system well enough to follow a briefing abroad. If you’re heading for divemaster, unit slips matter even more, because the problems get longer: see how hard divemaster theory is.

Frequently asked questions

How many psi is 200 bar?

200 bar is about 2,900 psi, since 1 bar equals 14.5 psi. The other way, 3,000 psi is about 207 bar, and a 500 psi reserve is about 34 bar. Round figures paired in briefings, such as 50 bar and 500 psi, are each system’s convention rather than exact conversions, so use the reserve your briefing sets, in your gauge’s unit.

Is 10 meters of seawater exactly 33 feet?

No. Ten meters is 32.8 ft, and 33 ft is 10.06 m. Dive training uses 1 bar per 10 m in metric and 1 atm per 33 ft in imperial: two rounded rules that differ by less than 1%. Both are fine for planning, as long as you don’t mix them inside one calculation.

How do you convert liters to cubic feet for scuba?

Divide liters by 28.3 to get cubic feet, or multiply cubic feet by 28.3 to get liters. A 12 L cylinder at 200 bar holds about 2,400 L, or 85 cu ft. A cylinder rated 80 cu ft holds about 2,265 L. A breathing rate of 20 L/min is about 0.7 cu ft/min.

What’s the difference between bar, atm and ata?

Bar and atm are both units of pressure: 1 atm is 1.013 bar, close enough that dive math treats them alike. Ata means atmospheres absolute, the pressure counted with the air above the water included, which is what gas laws need. At 20 m (66 ft), the gauge pressure is 2 bar (2 atm) and the absolute pressure is 3 bar (3 ata).

Should you learn dive theory in metric or imperial?

Learn it in the system your course, your gauges and your dive computer use, so every number you calculate matches what you’ll read underwater. Then learn the key conversions, 1 bar = 14.5 psi, 1 m = 3.28 ft and 1 cu ft = 28.3 L, so you can follow a briefing in the other system.

Sources

1. NIST, Guide to the SI, Appendix B.8, Factors for Units Listed Alphabetically
2. Joseph Gessert, Shallow-Water Arterial Gas Embolisms, Alert Diver, Divers Alert Network (July 19, 2021)
3. Mark Derrick, SCUBA Cylinder Capacity and Tank Tables, N2 Diving