Lean Body Mass Calculator (Boer Formula)

Lean body mass is everything you weigh that is not fat — muscle, bone, organs, blood and water. The calculator below estimates it, and the fat mass left over, from weight, height and sex. Enter your numbers and read both halves together.

The equation, and where it comes from

This is the Boer formula, published by P. Boer in the American Journal of Physiology in 1984 for a purpose that has nothing to do with fitness: normalizing body fluid volumes.

• Men: LBM = 0.407 × weight in kg + 0.267 × height in cm − 19.2
• Women: LBM = 0.252 × weight in kg + 0.473 × height in cm − 48.3

Notice how differently the two equations weight their inputs. For men, weight carries most of the prediction; for women, height does. That asymmetry is not a rounding artifact — it encodes the different average relationship between body size and fat-free tissue in each sex.

Why radiologists care about this number

Lean mass is not a gym metric that escaped into medicine. It is a dosing variable. Iodinated contrast media distribute through the extracellular fluid and plasma, which live in lean tissue, not in adipose tissue. Dose by total body weight in a heavier patient and you overshoot, because a large share of that weight cannot take up the agent.

A 2018 study in BioMed Research International compared the Boer and James formulas for exactly this task and concluded that the Boer formula should be the method of choice for lean body weight estimation in obese patients, producing an accurate contrast amount and optimal liver enhancement. The equation on this page is the one a CT suite trusts to scale a dose — which is a better credential than most body composition formulas can claim.

Reading your two numbers

A 180 lb, 5 ft 9 in man (82 kg, 175 cm) comes out around 61 kg of lean mass and 21 kg of fat. Convert the fat share and you are near 25 percent, which lands just outside the healthy band that the National Athletic Trainers’ Association places at 10 to 22 percent for men and 20 to 32 percent for women.

That cross-check is the useful move here. Boer and a body fat estimate approach the same body from opposite directions, and when they disagree by a wide margin one of them is being fooled by your build. Run the body fat percentage calculator on the same inputs and compare.

Why the fat-free half matters

• Metabolism. Resting energy expenditure tracks fat-free mass far better than it tracks total weight, which is why two people of the same weight can burn noticeably different amounts at rest.
• Holding on to it while dieting. Weight loss always removes some lean tissue. Keeping the deficit moderate, protein adequate and resistance training in place is what shifts the ratio toward fat. The athletic trainers’ position statement puts a safe pace at about 1 to 2 lb (0.5 to 0.9 kg) per week and not more than 1.5 percent of body weight in a week.
• Aging. Lean mass declines with the decades whether or not the scale moves, and that quiet substitution is what pushes fat percentage up without any change in weight.

For the shape side of the same picture, which no weight-based formula can see, the conicity index uses your waist to score how centrally that mass is arranged.

This content is informational and does not replace evaluation by a health professional.