Lean Body Mass Calculator

What is lean body mass?

Lean body mass (LBM) is your total weight minus fat mass — muscle, bones, organs, skin, and water. It’s the metabolically active tissue that drives your calorie needs, so tracking it matters more than raw scale weight.

How this calculator works

• Enter your sex, height, weight, and body fat percentage if you know it.
• LBM is weight minus fat mass: weight × (1 − body fat / 100).
• If body fat is blank, it estimates fat mass with the Boer equation from height and weight.
• It also reports fat mass and FFMI, which normalises lean mass to height.

How to use the result

1. Use LBM to set a protein target — a common starting point is 0.8–1g per pound of lean mass.
2. Watch LBM during a cut: if it drops while you lose weight, you’re losing muscle too — slow the deficit and add protein.
3. Compare FFMI between people fairly — it removes the height bias of raw lean mass.
4. Re-measure monthly; LBM changes slowly, so a big weekly swing is water, not muscle.

What is FFMI and what’s a good value?

Fat-free mass index (FFMI) is lean body mass in kg per square metre of height. Around 18–19 is typical for a trained man, 20–22 is advanced, and sustained values above 25 are exceptional — for men, rarely achieved without performance drugs. For women the scale sits several points lower.

Common mistakes

• Confusing LBM with muscle. LBM includes bone, organs, and water — it’s not all muscle.
• Using an inaccurate body fat number. The LBM result is only as good as the fat% you enter; a poor measurement skews everything downstream.
• Judging health on LBM alone. Pair it with the body fat calculator and the BMI calculator for the full picture.

Lean mass drives the calorie side of your numbers — the BMR calculator turns it into a resting calorie baseline, and the macro calculator splits that target into protein for the muscle you’re keeping.