Gallons (US liquid) to cubic inches conversion calculator shown above calculates how many cubic inches are in 'X' gallons (US liquid) (where 'X' is the number of gallons (US liquid) to convert to cubic inches). In order to convert a value from gallons (US liquid) to cubic inches (from gal to cu in) type the number of gal to be converted to cu in and then click on the 'convert' button.