Oxygen Tank Duration Calculator October 15, 2020 If you don’t have access to the calculator above simply use the following formula to calculate the minutes remaining in your oxygen tank: Here’s what you will need: Tank size & conversion factor for that tank PSIG (amount of pressure remaining in the tank) Flow rate (LPM) Formula: Minutes remaining = PSIG * Tank conversion factor/Flow rate Tank Size Conversion factor E 0.28 D 0.16 H 3.14 M 1.56