1. Because you know the number of gallons per square foot. You would want to use cubic feet for the measurement of the tank.
2. Convert all dimensions to feet:
1 yard = 3 feet
18 inches = 1.5 feet.
Volume of tank = 3 x 1.5 x 1 = 4.5 cubic feet.
Multiply gallons per cubic foot by cubic feet of tank:
7.5 x 4.5 = 33.75 gallons.