A diver’s gas supply lasts 60 minutes at 33 (10.1 m) feet. Assuming the same gas consumption rate, how long will the gas supply last the diver at 99 fsw (30.2 msw)? Scuba Diving Calculators makes doing the calculation easy. Having this handy calculator you merely enter in your SAC, Time and Feet and click calculate and our calculator will display your Cubic Feet of Gas.