For acreage of a square or rectangle, multiply the Length (in feet) by Width (in feet) = Square Footage. Then divide Square Footage by 43,560 (amount of square feet in 1 acre) = Acreage.
Example: I have a rectangular pond that is 100′ long by 50′ wide. 100′ x 50′ = 5,000 square feet. Then 5,000 / 43,560 = 0.11 acres
For acres per foot, multiply the Acreage by the Average Depth (in feet) = Acre/feet
Example: I have a 1.25 acre pond with a 6′ average depth. 1.25 x 6′ = 7.5 acre/feet.
For gallons, multiply acre-feet by 325,829 (gallons in 1 acre – foot) = Gallons.
Example: I have 7.5 acre-feet of water. 7.5 x 325,829 = 2,443,717 gallons.