Hi Patti,
One inch is ^{1}/_{12} feet so one acre of water 1 inch deep is
43560 ^{1}/_{12} = 3630 cubic feet of water.
In my dictionary I found that there are 231 cubic inches in an american gallon. A cubic foot is 12 inches by 12 inches by 12 inches and thus one cubic foot is
12 12 12 = 1728 cubic inches
so 3630 cubic feet is 3630 728 = 6272640 cubic inches. Thus the amount of water needed to cover one acre to a depth of 1 inch is
^{6272640}/_{231} = 27154.3 gallons.
Penny
