Length X Width divided by 43,560 to get acreage. (43,560 square feet in an acre) However it depends somewhat on the shape of your pond and this is a rough estimate.

If you want to be more precise here is an interesting link:

http://www.sfr.cas.psu.edu/water/pond%20area.htm

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.