Lean Body Mass Calculator

What lean body mass actually is

Lean body mass (LBM), also called fat-free mass, is your total body weight minus all of your fat. It bundles together skeletal muscle, bone, organs, skin, blood and the large volume of water your body carries. Because almost everything metabolically active lives inside that fraction, LBM is what drives your resting energy expenditure, how much protein you can usefully absorb, and how quickly your body clears many medications. Two people can weigh exactly the same 80 kg and differ by 15 kg of lean mass, which is why the scale on its own tells you very little.

There are two honest ways to get the number. If you have a measured body fat percentage from a DEXA scan, a bioimpedance device or skinfold calipers, LBM is simple arithmetic: weight multiplied by (1 minus body fat divided by 100). If you do not, you fall back on a regression formula that predicts LBM from height, weight and sex. This calculator does both, and shows all three of the classic formulas side by side so you can see how much the estimate depends on which one you pick.

Worked example: a man at 175 cm and 80 kg

Boer (1984): LBM = 0.407 × 80 + 0.267 × 175 − 19.2 = 32.56 + 46.73 − 19.2 = 60.1 kg (132.5 lb).

James (1976): LBM = 1.1 × 80 − 128 × (80/175)² = 88 − 128 × 0.2090 = 88 − 26.75 = 61.3 kg (135.0 lb).

Hume (1966): LBM = 0.32810 × 80 + 0.33929 × 175 − 29.5336 = 26.25 + 59.38 − 29.53 = 56.1 kg (123.7 lb).

The spread is 5.2 kg, or roughly nine percent. That is normal, and it is the single most important thing to understand about these formulas: they are population averages, not measurements of you.

The same man with a measured body fat percentage

Now suppose a DEXA scan puts him at 18 percent body fat. The direct calculation gives LBM = 80 × (1 − 0.18) = 65.6 kg, with 14.4 kg of fat mass and an LBM share of 82 percent. Every formula underestimated him, because at 175 cm and 80 kg the average person in those source populations carried more fat than he does. Run it again at 25 percent body fat and you get 80 × 0.75 = 60.0 kg, almost exactly the Boer prediction. The formulas are, in effect, guessing your body fat percentage for you: Boer implicitly assumes about 25 percent here. If you know better than that guess, the direct method always wins, which is why this tool promotes it to the headline whenever you supply a number.

A worked example for women

For a woman at 165 cm and 65 kg, Boer gives 0.252 × 65 + 0.473 × 165 − 48.3 = 16.38 + 78.05 − 48.3 = 46.1 kg of lean mass, implying 18.9 kg of fat and a body fat percentage near 29 percent. That sits inside the typical healthy range for adult women, which runs roughly 21 to 33 percent, versus about 8 to 20 percent for men. The sex coefficients exist precisely because essential fat is higher in women; using the male equation on a woman overstates lean mass by several kilograms.

Which formula should you use?

Boer is the usual default in clinical and nutrition settings and is the one this calculator features when you have no body fat reading. James applies a squared penalty term to the weight-to-height ratio, so it drifts low as BMI climbs and tends to underestimate lean mass in people with obesity. Hume, derived from hospital patients in the 1960s, generally sits below the other two for tall subjects. None of them handle very muscular bodies well: a 100 kg strength athlete at 12 percent body fat has 88 kg of lean mass, and no height-and-weight formula will ever produce that figure.

Putting the number to work

The most common use is protein. Scaling intake by total weight overshoots for anyone carrying substantial fat, because adipose tissue has almost no protein turnover. Research on lean athletes in a calorie deficit points to roughly 2.3 to 3.1 g of protein per kg of LBM per day to preserve muscle while cutting; at 60.1 kg of lean mass that is about 140 to 185 g. Resting metabolic rate formulas that use LBM, such as Katch-McArdle, are also more accurate than weight-only equations for very lean or very heavy people, and several drug classes are dosed on lean or adjusted body weight for the same reason.

The second use is tracking. A diet that drops 6 kg of scale weight but only 1 kg of lean mass went well; one that drops 6 kg with 4 kg from lean mass did not. To see that you need consistency, not precision: use the same method, the same scale, the same time of day, and compare trends over four to eight weeks rather than day to day. Hydration alone can swing a bioimpedance reading by a kilogram or more.

Finally, the limits. These are estimates for healthy adults, not for children, pregnancy, oedema, amputation or serious illness, all of which break the assumptions the regressions were fitted on. DEXA remains the practical reference standard, with hydrostatic weighing and air displacement plethysmography behind it. Treat the numbers here as a well-calibrated starting point for planning, and let a measured body composition test settle anything that actually matters.