Ive been wanting to know the same thing.... many thanks moses224. Here in the UK its inches not gallons why cant it be the same for us all so theres no complications
http://www.convertunits.com/from/cubic+inches/to/gallon
3.5 gallon
10 x 10 x 8 = 800 cubic inches
then enter into calculator
cool thanks for those charts I can estimate sizes now although some taper so it might decrease the volume a little.