Multiply the inner dimensions of your tank, i.e., length*width*height, in inches. Take the product and divide by 231, which is the number of cubic inches in a gallon. There you have it. Ex. Let's say your tank measures 36"l*18"w*21"d. This equals 13608 cubic inches. That divided by 231 equals 58.90909909090..., or about 59 gallons. Best Regards- Ed O'Briant