Dave,
There are 231 cubic inches in an American gallon so you first need to calculate the volume of the pool in cubic inches. Your pool is
18 ft = 18 12 = 216 inches, by
12 ft = 12 12 = 144 inches, by
4.5 ft = 4.5 12 = 54 inches.
Thus the volume of your pool is
216 144 54 = 1679616 cubic inches
Now divide by 231 to find the volume in gallons.
Penny
