Works out the free gas in a cylinder, quad or bank from its water capacity and pressure, and how much of it is usable down to a minimum pressure.
The formula: Free gas = water capacity x gauge pressure. Change any box and the full working updates. For study only, never for dive planning.
The IMCA ADS, BDS and LST papers ask this as a four-option question, with a calculator on screen. A free account gives you 15 practice questions for each exam, each with the worked answer and the document it comes from.