Enter the tank's shape and dimensions, and the calculator gives its capacity in US gallons and litres.
The formula
A rectangular tank holds length × width × height. A cylindrical tank, lying down or standing up, holds π × radius² × length. With the dimensions in inches that gives cubic inches, and a US gallon is 231 cubic inches. In centimetres, 1,000 cubic centimetres make a litre. The calculator uses NIST's 3.785 412 litres to the US gallon for the conversion.
Gross and usable
The answer is the volume of the shape, from outside dimensions if that's what you measured. Tanks hold less than that. Wall thickness, internal baffles, the pickup tube standing off the bottom and space left for expansion all take some away. Use the calculation to check the figure on the tank label or in the builder's specification is plausible, not to promise a number of gallons.
On the survey
A tank's stated capacity drives the range and the fuel plan. If the measured volume falls well short of the label, the tank may have been replaced with a smaller one, or the label may belong to another tank. Note the label and the tank's material and condition, and look for corrosion where it sits on its supports.
Citing it in a report
"Fuel tank, aluminium, under the cockpit sole. Label capacity 75 US gal. Measured outside dimensions 36 × 24 × 20 in, gross volume about 75 US gal, consistent with the label." Give the dimensions you measured and say whether they were inside or outside.