How many gallons in a 24 inch by 10 inch by 13.5 tank?

There are two different answers to this question depending on whether you are refering to US Gallons or Imperial gallons.

24x18x14 = 6048 cubic inches

US gal = 231 cu.in.

Imp. gal = 277.42 cu.in.

6048 / 231 = 26.1818 US gal.

6048 / 277.42 = 21.8009 Imperial gal.