How Do I Convert Cubic Inches To Gallons?

There are exactly 231 cubic inches in a US liquid gallon. To convert from cubic inches to gallons, multiply by (1 gallon/(231 in³)). The conversion factor is different for other definitions of gallon.

Example
1 ft³ = (12 in)³ = 1728 in³
  = (1728 in³)*(1 gal/(231 in³)) = (1728/231) gal
  = 7.480519 gal    (the underlined digits repeat indefinitely)
  = 7 37/77 gal

For liquid measurements there are 231 cubic inches in a gallon.
For dry measurements there are 268.8 cubic inches in a gallon.

Your formula would then be to divide your number of cubic inches by 231 for liquid gallons and 268.8 for dry gallons.