How many gallons of water are needed to fill a 15'round pool 4 feet deep?

5289 gallons.

pi r2 d = 3.14159 * 7.52 * 4 = 707 cubic feet

cf * 123 * 2.543 = 707 *123 * 2.543 = 2.002 x 107 ml

ml / 3785 = 2.002 x 107 / 3785 = 5289 gallons