A dry US gallon (non-liquid) is easy to figure out. You take the cages size in cubic inches (18x18x24=7776), so 7776, and multiply it by 0.0037202 7776x0. 037202 equals 28.928 dry US gallons.

Also, how many gallons is the Exo Terra 36x18x18? 50

Herein, how many gallons is Exo Terra?

Exo Terra Outback Terrarium 24 x 18 x 12, 20 Gallon.

What can I put in a 12x12x18 Exo Terra?

Smaller geckos would do well (bibron, flying gecko, day gecko, etc.). Or a small arboreal snake. Rough Green Snakes are beautiful and could fit in there.