How many gal in a square 12x12x12?

Answer 1

First of all, that's not a square. It's a cube.

If every side of the cube is 12 inches long, then its volume is 1,728 cubic inches. (1 cubic foot.)

Every 231 cubic inches is 1 gallon.

1,728 / 231 = 7.48 gallons, plus another 0.12 cubic inch left over.