Inches to Gallons?

[Gizmodo ^_^]

I get really confused working out how much a fish tank would be in gallons :S
FISH TANK, WITH ORNAMENTS/ DECORATIONS SCULL, BARREL, BRIDGE, PLANT + FREE STAND | eBay
How much would this be :S

its 31 inches wide x 16 inches height x 13 inches deep?

[mangoandmimi]

I think about 20 gallons :3

[ham888]

23.24 imperial (UK) gallons total volume - wolfram alpha is a handy site for conversions and maths bits.

The calculation is 31x16x13 inches = 6448 cubic inches, multiplied by 0.003605 = 23.24