All dimensions must be in the same unit (inches) and the volume is in the cube of that unit (cubic inches). For US gallons, divided by 231 cubic inches per gallon.
Using the calculator:
Full: 104338.6 in³ = 451.7 gallons
7" of fuel: 13203 in³ = 57.2 gallons
Note: The full figure is physical volume, but a safe fill allows around 5% headspace for thermal expansion as temperature changes. Max safe fill would be around 429 gallons.