Roughly, or exactly? Roughly, go to a spa store and ask how many gallons one has about your size. Exactly, you need to go to the manufacturer, and know the model year. You can't use math like you can for a pool, because the shape of the shell is so variable. Roughly should be good enough for most chemical additions. Over time, if your guess always comes out 10% too low after you add something; guess 10% higher!