I purchased a GPS unit that has all this in it. All you have to do is walk/ride around your food plot. When you get back to where you started click and you get a map and acreage for your plot that you can save for next time/year. Works great.
I use: http://www.findlotsize.com/
which works great to find the area of a field from a google earth photo. I've checked it with seveal surveys and it is always very, very close.
I see that acme.com link also works almost the same way!
Everyone save yourselves some time and do it the easy way. Type your lenghth and width into this caculator and it will tell you how much of an acre you have. That way you dont over or underseed your foodplots.