XYZ square feet x 3.125 ft (37.5 inches, the average annual rainfall in Portland OR)
= XYZ cubic feet of rainwater
x 7.48051948 (to convert to gallons)
= XYZ gallons
EXAMPLE
89,000 sf x 3.125 ft (=37.5 inches, the average annual rainfall in Portland OR)
=278,125 cubic feet of rainwater
*7.48051948 (to convert to gallons)
=2,080,519.48 gallons