Question 1

What is lean body mass?

Accepted Answer

Everything in your body that is not fat: muscle, bone, organs, skin, blood and water. It is what you feed with protein and what burns most of your calories, which is why protein targets, drug dosing and some fitness standards are set per pound or kilogram of lean mass rather than total weight. Typical lean mass is 70–90 % of body weight for men and 65–85 % for women.

Question 2

Which formula is most accurate?

Accepted Answer

None of the height-weight formulas can see your body composition; they predict the average person of your size. Boer (1984) is the usual default and the one used in medical dosing calculators, James works well in normal-weight adults but drifts at high BMI, and Hume is the oldest. A measured body fat percentage beats all three — enter it and the calculator uses it directly.

Question 3

How do I calculate lean body mass from body fat percentage?

Accepted Answer

Lean mass = weight × (1 − body fat ÷ 100). At 180 lb and 25 % body fat: 180 × 0.75 = 135 lb lean and 45 lb fat. Calipers, a DEXA scan, a Bod Pod or a decent bioimpedance scale all give a body fat figure; the body fat calculator estimates it from tape measurements with the US Navy method.

Question 4

What is a good lean body mass?

Accepted Answer

It scales with height and frame, so the percentage is more useful than the pounds: 80 % or more lean (20 % fat or less) is fit for men, 75 % or more (25 % fat or less) for women. A 5′10″ man typically carries 130–150 lb of lean mass; a 5′5″ woman 95–110 lb. Gaining lean mass is slow — around 0.5–1 lb a month for trained lifters.

Question 5

How is lean body mass used?

Accepted Answer

Protein intake (0.8–1.2 g per lb of lean mass), keto and cutting macros, basal metabolic rate formulas such as Katch-McArdle (370 + 21.6 × lean kg), anesthetic and chemotherapy dosing, and tracking whether weight loss is coming from fat or muscle. Re-measure every few weeks and aim for the lean number to hold steady while the fat number drops.

Formula	Lean body mass	Implied body fat
Boer (1984)	135.6 lb (61.5 kg)	24.7 %
James (1976)	138.5 lb (62.8 kg)	23.1 %
Hume (1966)	126.9 lb (57.6 kg)	29.5 %

Formula	Men (kg, cm)	Women (kg, cm)
Boer	0.407 × kg + 0.267 × cm − 19.2	0.252 × kg + 0.473 × cm − 48.3
James	1.1 × kg − 128 × (kg ÷ cm)²	1.07 × kg − 148 × (kg ÷ cm)²
Hume	0.3281 × kg + 0.33929 × cm − 29.5336	0.29569 × kg + 0.41813 × cm − 43.2933
From body fat	kg × (1 − BF%)	kg × (1 − BF%)

Lean Body Mass Calculator

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