James,

Correction...In the above, I said 1 acre foot was about 326k gallons. To calculate, an acre is 43560 sq. ft. or a square 208.7 feet per side.

If you use this link:

http://www.dep.state.pa.us/dep/deputate/waterops/Redesign/calculators/volcalchtm.htm

and put in 208.7 feet for each side and 1 foot deep (1 acre foot) press calculate gallons and presto...326k gallons. If you know your dimensions and average water depth, you can get a pretty good estimate of your water volume this way.