It uses one of three methods to do so - those used by the US Navy, the US Army and the YMCA. I decided not to put the choice of method into a preference pane because I like to enter my measurements and then flick through the three methods comparing the results - and believe the lowest!
The application is so simple to use that I won't insult you with instructions. It is worth pointing out that the application does not convert units, so if you enter your body weight as 170 pounds and then switch to metric measurements, you will weigh 170 kilograms. No, even I don't put on that much weight in an instant - I did this because the the fat percentage calculated is the same if you enter your weight as 170 lb or as 77.27272727 kg - so there's no real reason for you to convert your figures to one system when you've entered them in another. But I did think that you might start to enter your weight in kilos, and then realise that the application is set to accept pounds. Because the application doesn't convert your entries, you can switch units and not have to type your measurements all over again.
I've released this application as freeware. I'm considering enhancing it with the ability to store your figures each time you measure yourself and draw pretty graphs telling you the changes in your measurements and fat percentage over time. If I do that, I'll probably charge a small amount for the application. Let me know if you'd like this feature - and you'd pay for it.
Download Fat Calculator here.
A fellow Mac owner, Pete Ford, suggested that I call the application "Lard'o'meter". It was the seed for a whole new version of the software, which you can read more about here.