How To Calculate Lean Body Mass

Lean body mass is everything in the body that is not fat: muscle, bone, organs, connective tissue and the water that goes with them. A bathroom scale reports a single weight, and that number says nothing about how it divides between fat and lean tissue. Lean body mass is the estimate of that split.

The Boer formula is one of several published equations for estimating lean body mass from weight and height. For men it is 0.407 times weight plus 0.267 times height minus 19.2, with weight in kilograms and height in centimetres. For women the coefficients are 0.252 and 0.473, with a constant of 48.3 instead.

The two sets of coefficients reflect the average difference in body composition between men and women, which is largely driven by sex hormones. The formulas are population averages, not individual measurements, so any single person can sit noticeably above or below the estimate.

Because the formula takes weight and height, the difference between the estimate and total weight is taken as fat mass. Dividing that fat mass by total weight gives a body fat percentage. That is the same figure a dedicated body fat formula produces, reached by a slightly different route.

The fat-free mass index, usually shortened to FFMI, divides lean body mass by height in metres squared. The idea is borrowed from body mass index: just as BMI normalises weight for height, FFMI normalises lean mass for height so that people of different statures can be compared.

Normalising matters because taller people carry more lean mass without being more muscular in any meaningful sense. A lean mass of 65 kilograms means something different on a 165 centimetre frame than on a 195 centimetre one, and FFMI removes most of that difference.

Lean body mass is not the same as muscle mass. Skeletal muscle is typically a little over half of lean body mass, with bone, organs and water making up the rest. An increase in lean body mass can come from any of those components, and a change of a kilogram is not a kilogram of new muscle.

The formula is least reliable at the extremes. At very high body fat the equation tends to overestimate lean mass, and at very low body fat it can underestimate it, because the relationship between weight, height and composition was fitted on a general population rather than on athletes or on people with obesity.

Trends are more informative than single readings. Tracking the estimate over months while weight and training change shows direction even when the absolute value is uncertain. A falling fat mass with a stable lean mass over several months is a meaningful signal; a one-day change is not.

Anything that changes hydration shifts the estimate, because water is part of lean mass. A hard training session, a salty meal or a long flight can move the number by a kilogram or more without any change in tissue. Measuring at the same time of day under similar conditions keeps the series comparable.