Body Math and Fitness Formulas

Three interrelated health measurements are total body fat, percent body fat (%BF), and lean body mass (LBM). Total body fat is the total weight of fat in your body. Percent body fat is the percentage of your weight that is fat. And lean body mass is the total weight of non-fat tissue in your body. For example, if a woman weighs 140 lbs and has 30 lbs of fat, then

total body fat = 30 lbs

% body fat = 30/140 = .214, or 21.4%

lean body mass = 140 30 = 110 lbs

There are many ways to compute the numbers. The traditional technique of determining how much fat a person has is by hydrostatic testing. Your body is submerged in a tank of water and your buoyancy is used to determine percentage body fat. The principle behind this method is the fact that fat floats in water. So, the more buoyant you are, the higher your percent body fat. This also gives the truest estimate of % body fat.

In lieu of of hydrostatic measurement (which is costly) you can also compute body fat with these methods.

Skinfold Analysis: Thickness of subcutaneous fat (the fat under your skin) is another predictor of fat. The traditional way to do this is to use calipers to record the thickness of folds on seven areas of the body, add them all up, and then plug that sum into an equation.

Girth Methods for Men: For men, body fat and lean body mass can be estimate by the circumference of the waist and neck, relative to weight and height. The simplified formula for men is

LBM = 94.42 - 4.15X + 1.082Y

where X is the waist measurement in inches and Y is the weight in pounds. This gives your lean body mass in pounds.

Girth Methods for Women: You can also estimate female body and fat and LBM with waist, wrist, hip, and forearm circumferences. The approximation formula for women is

LBM = .732Y - .157X + .318C - .249D + .434E + 8.98

where Y is weight in pounds, X is waist measurement, C is wrist measurement, D is hip measurement, and E is forearm measurement. Again, this formula returns lean body mass in pounds.

All these formulas were developed and tested by physiologists and health scientists. When the equation results are compared to the hydrostatic testing results, they are often accurate within 3%.