http://store.solar-electric.com/grsqpu.html

http://cetulare.ucdavis.edu/pubgrape/ig796.htm

One Acre-Foot = 325,851 gallons
One Acre-Inch = 27,154 gallons

These are volume measurements, the volume of water that cover an acre of land one foot or one inch deep:

One acre = an area of land that is 43560 ft2

One cubic foot (ft3) = 7.48 gallons

One acre-foot = 43560 ft2 x 1 foot water depth

= a volume that is 43560 ft3

One acre-foot = 43560 ft3 x 7.48 gallons/ft3
= 325,851 gallons

One acre-inch
= 43560 ft2 x .0833 ft (1 inch) a volume that is 3630 ft3

One acre-inch = 3630 ft3 x 7.48 gallons/ft3
= 27,154 gallons