You're probably thinking of the Waist to Hip (http://www.annecollins.com/weight-loss/waist-hip-ratio.htm) ratio. You can work your ratio out by dividing the circumference of your waist (measured just below your navel) by the circumference of your hips (measured where you buttocks are largest). Women should have a ratio of 0.8 or less, men should have a ratio of 0.95 or less.

A lot of these measurement type things don't seem to be very accurate though when it comes to saying if you need to loose weight or not. The only reasonable measurement I can find is body fat percentage and its a pain to get that measured accurately; even then there is a lot of disagreement in the scientific community about what is a healthy body fat percentage of a given person.