The gas needed to pressurise a chamber to depth, and the gas used each time a medical or equipment lock is run.
The formula: Gas = volume x depth/10. 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.