Simple....
Take tank dimensions in cm's (or inches) & multiply all 3 together. e.g. my small tank measures:
Width 61cm. Height: 36cm. Depth: 31cm (24” x 14” x 12”).
So that's 68,076 cm cubed, (or 4,032 inches cubed)
Type in Google search box:
68076 centimeters cubed in us gallons (or 4032 inches cubed in us gallons)
viola! 17.98 us gallons (or 17.45 us gallons, in other words the same answer of course.)
Naturally, you'd also want to know the volume in litres, so replace the word 'gallons' with litres in the example above:
68076 centimeters cubed in litres
Gives 68 litres. Just as well becasue I have a Jewel Rekord 60 tank.
Don't forget to loose a bit of water volume becasue you don't fill the tank to the brim (about 1" below it) and the substrate, if you're intending to use some.
Andy