Gallons = (Length x Width x Height) / 231
For example, if you have a square tank that's 12 inches long, 12 inches wide, and 10 inches high, the calculation would be:
Gallons = (12 x 12 x 10) / 231
Gallons = 1440 / 231
Gallons ≈ 6.23
So, this square tank can hold approximately 6.23 gallons of water.