How many gallons of water is needed to fill a 21ft by 52in pool?

First, figure the surface area. 21 ft diameter is 10.5 foot radius. Convert to inches, since that is easier for me. 10.5 ft is 126 inches. Area is Pi ( about 3.14159) times R squared. So 3.14159 x 126 x 126 = roughly 49,876 square inches. That is the surface area of the bottom of the pool. Now multiply that times the height of the water. I will use 52 inches, though you won't fill it completely up to the rim. 49,876 times 52 = about 2,593,552 cubic inches of water. One gallon of water takes up 231 cubic inches. So divide 2,593,552 by 231, and you get 11, 227.5 gallons. Roughly speaking. Depends on how high you fill it. My 9th grade science teacher, Mrs. Monel, gets the credit for teaching me this method.