The formula I found is:
%Fat = 163.205*LOG(abdomen + hip - neck) - 97.684*LOG(height) - 78.387
Where abdomen = waist measurement, and then you use your hip and neck measurements plus your height. All need to be in the same units, but it doesn't matter if you use centimeters or inches... I used inches and got a "reasonable" answer, if that helps.
I will also add that different methods of measurement might yield extremely different results. The other people on the forum that had tried out that formula both got higher values than they were getting from other methods.
Today I went in and met with a personal trainer. She did a 3-site body fat % measurement on me using calipers: 27%. She had me use a bioelectrical impedance hand-held measuring device: 36%. And based on today's measurements, the formula above says about 42% body fat... So I can certainly not say anything about the accuracy of the formula, but if you use one method consistently, your changes should also theoretically be consistent within that method. Even if the actual value you determine isn't quite right.