True capacity is volume x pressure x z-factor.
Best way is to use water volume in liters and pressure in bar then convert to imperial.
Why do you need to find exact capacity? Most are very close
@TBone is correct, working in metric units is easier and quicker. The issue is the need to make conversions for local requirements.
Available Gas
Cylinder volume x cylinder fill pressure = Amount of available Gas.
10 (litre) x 210 (bar) = 2100 litres of Gas
Usable Gas
Using rule of thirds
(2100 / 3 ) 2 = 1400 litres (leaving a reserve of 700 litres)
How long will it last?
Surface breathing rate [1] x Absolute pressure [2] = gas consumed at depth
25 (litres/min) x 4 (30m) = 100 litres/min at 30m
1400/100 = 14 minutes
Most modern computers can be switched between imperial and metric units.
With the increase in air integrated computers, switching to metric is much easier now than in the past.
Being British - we use a mix of Imperial and metric in normal life anyway (at least my generation does).
The issue for you is how well you equate the metric units.
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[1] 25 litres/minute is a conservative rate, normally used for example calculations.
[2] Every 10m of depth increases pressure by 1 bar (gauge)
10m = 1 bar (gauge) = 2 bar (absolute)
20m = 2 bar (gauge) = 3 bar (absolute)
20m = 3 bar (gauge) = 4 bar (absolute)
[3] 14.7 psi = 1 bar ---- for ease 15 psi = 1 bar.
[4] 33ft = 10m