To convert from the inside "water volume", usually in cc or CI.... to the amount of air it will hold in CF.... you need to know the fill pressure (in bar).... If the tank is listed in cc, convert to CI by dividing by 16.4 (cc/CI).... If you know the pressure in psi, convert to bar by dividing by 14.5 (psi/bar)....

First multiply the water volume by the fill pressure, to give the number of CI or air inside the tank at that pressure.... Then divide by 1,728 to convert CI to CF....

For example if the inside volume is 144 cc, that is (144 / 16.4) = 8.78 CI.... If the fill pressure is 3,000 psi, that is (3,000 /14.5) = 207 bar.... When filled to 207 bar, that tank will hold (8.78 x 207) = 1,817 CI of air at 1 bar.... Converted to CF, that is (1,817 / 1,728) = 1.05 CF....

The above is according to Boyle's Law for ideal gasses, and works up to about 3,000 psi.... It gets a bit more complicated when the tank is filled to more than 3,000 psi because of the VanDerWaals effect.... If we use the above calculation for a standard 1 hour SCBA tank (which is 550 CI water volume) we get the following at 4500 psi (310 bar)....

(550 x 310) = 170,500 CI.... (170,500 / 1728) = 98.7 CF (according to Boyle's Law).... However, that tank only holds 88 CF of air at 4,500 psi.... about 11% less than you calculated.... Up to 3,000 psi, you can use the basic Boyle's calculation as I detailed it.... **At 4,500 psi, you need to reduce that by 11%** because of the VanDerWaals effect.... At 6,000 psi the effect is much greater, about 18% compared to at 3000 psi.... At 10,000 psi a tank only holds 64% as much air as the simplified calculations tell you.... Sorry, but there is no easy formula to use once the pressure goes over 3,000 psi....

Bob