Just had to do this for my permit renewal. The thing missing from this thread is not to waste time trying to measure a complex pond shape, there is an easier way.
If your pond is a simple shape use Bill's formulas. If it is irregular like mine, the easiest and most accurate way is to obtain an aerial photograph from the county tax assessor's office. The photo quality will vary based on the technical capabilities of the county. My county is flown every five years and I was lucky to get a 6-month old 24" x 36" photograph with a scale of 1" = 100' for $25.
First look at the link that Cecil provided. Now take a ruler and draw a series of rectangles that best match the outline of your pond. The rectangles do not have to be uniform. Use all different sizes. Cover as much of the open water with one big square or rectangle then fill in the rest. They do not have to fit the shoreline perfectly, just try to balance the amount of land within the rectangles to the amount of water not in the rectangles (you'll see what I mean when you get started). I drew mine with permanent markers on a transparency overlay in order to preserve the photo. My pond has a very irregular shoreline so I had to draw approximately 10-12 rectangles. Number the rectangles and calculate the area of each by multiplying the length times width (convert to feet). Add up all the rectangles to get the total square footage of the pond. Divide by 43,560 to convert to acres. It's also nice to have the aerial photograph on hand.
