A 30*30*45 is about 10.7 gallon.
Yes, it's a bit of a hassle since the stubborn americans can't use a system that acually make sense (and the rest of the world uses)
For future refernence, let me give you a easy way to calculate volume:

10 cm = 1 dm (as I'm sure you know)
1 cubic dm = 1 liter

Therefore, the easiest way is to just multiply the dimensions in dm instead of cm. This way, you'll get the answer in liters.
So to calculate a 30*30*45cm exoterra, you just:
3*3*4.5dm = 40.5 cubic dm (liters)
Then, just google "40.5 liter in gallon"!
If you want to do it the other way around, just google "10.7 gallon in liter". If you want to do it the old way, just multiply the liters with 0.2646 and you'll get the answer answer in gallon:
40.5*0.2646 = 10.7

And lastly, if you want to convert gallon into liter the old way, multiply the gallons with 4.54609 and you'll get the answer in liter:
10.7*4.54609 = 48.643163

Hope this helps!

EDIT: Added related picture