This one is good if you have calipers or a good measurement with a tape.

http://www.linear-software.com/online.html


Personally, I just cut to about 5.75%, and don't want to go any lower. I can almost guarentee that no one is under 5%... you would seriously be close to death at that point.

Body builders are on average between 5-8%. No athletes, from any sport are suggested to be under 5%.