The link I use for that is here:
I just noticed this is kind of a weird web site. I only use the body fat calc there, none of the paranormal or psychic stuff!
I don't know how accurate this method is compared to other methods, but all you need is a measuring tape and when it comes to measurement method, I don't think it matters too much. The basic thing is to get some baseline body fat number, and see go down over time. I personally don't get too excited over my lean body mass fluctuating half a pound or so from week to week (I do measurements every 2 weeks), as long as I'm seeing a weight drop during that time and my measurements are decreasing indicating fat loss.