I don't know why people's answers are so large: I think maybe they calculated the diameter as radius.

Approximate:

7d x 7d x 0.785(circular portion of the area) x 15h x 7.48(gal/cu.ft) = 4315.773 gallons

Precise (7 decimal places):

3.5r x 3.5r x 3.14159265(PI) x 15h x 7.48051948(gal/cu.ft) = 4318.2618968 gallons

While PI and gal/cu.ft contain 8 decimal places, the answer is only accurate to 7 due to unrepresented decimal places in PI.

Approximate:

7d x 7d x 0.785(circular portion of the area) x 15h x 7.48(gal/cu.ft) = 4315.773 gallons

Precise (7 decimal places):

3.5r x 3.5r x 3.14159265(PI) x 15h x 7.48051948(gal/cu.ft) = 4318.2618968 gallons

While PI and gal/cu.ft contain 8 decimal places, the answer is only accurate to 7 due to unrepresented decimal places in PI.