Measurements in inches (Length x Width x Height) x 0.004329004329004
1 inch cube = 0.004329004329004 gallon.
Or after you have your measurements you can plug it here:
http://www.unit-conversion.info/volume.html
Just make sure you have are using the correct unit of measure.
Sorta relevant :
http://youtu.be/BVtQNK_ZUJg
Logic behind the math:
http://youtu.be/KfNRArPXCjw