How much water?

Question

How much water is in a pool 18 feet round by 4 feet deep.

Answer 1

Volume of a Column = height x Pi x Radius^2

Circumference = 2 x Pi x Radius

Circumference = 18 feet
Height = 4 feet

18 = 2 x Pi x R

R = 2.86 feet

Volume = 4 x Pi x (2.86)^2

Volume = 102.79 cubic feet

1 cubic foot = 7.480 519 481 gallons

102.79 cubic feet = 768.9 gallons

Answer 2

7615 gallons

Now for the 18 feet, is that the diameter or the circumference? As you can see, my answer and Greenio's differ by almost a factor 10.