Esshup also posted a page that I have used in the past also but have not found it to be as accurate. One thing I should have mentioned is that the overlay map can make a big difference. ArcGIS uses maps from Bing (Microsoft) where Planimeter uses maps from Google. For my property, the Bing maps are only about 5 months old with the Google maps are about 3 years old and with a much lower resolution.
This difference may skew my measurements but I tend to find that Planimeter says everything is about 10-15% larger then my other measurements.
I recommend that you use every kind of measurement available and then average out the results!
Joe
