Lean Body Mass Calculator

What Lean Body Mass Actually Measures

Lean body mass (LBM) is your total body weight minus fat mass. It includes muscle, bone, organs, water, and everything that isn't stored fat.

LBM matters because muscle tissue burns more calories at rest than fat tissue does. Two people at the same total weight can have very different metabolic rates depending on how much of that weight is lean mass.

The Formula

Lean body mass = total weight x (1 - body fat % / 100)

Worked Example

An 80 kg person with 18% body fat:

LBM = 80 x (1 - 0.18) = 80 x 0.82 = 65.6 kg
Fat mass = 80 - 65.6 = 14.4 kg

This person carries about 65.6 kg of lean mass and 14.4 kg of fat mass.

Why This Feeds Directly Into Katch-McArdle

The Katch-McArdle formula calculates BMR from lean body mass instead of total weight:

BMR = 370 + (21.6 x lean body mass in kg)

For the example above: 370 + (21.6 x 65.6) = 1,787 kcal/day. This tends to be more accurate than Mifflin-St Jeor for people who are notably leaner or more muscular than average, since it accounts for body composition directly rather than using total weight as a proxy.

The Input That Matters Most: Body Fat Accuracy

This calculator is only as accurate as the body fat percentage you enter. A rough guess or an unreliable bioelectrical impedance reading can throw off the lean mass number by several kilograms. If you don't have a trusted body fat reading, our Body Fat Calculator using the US Navy tape-measure method is a reasonable free starting point.

Frequently Asked Questions

How is lean body mass different from muscle mass?

Lean body mass includes muscle, but also bone, organs, and body water. Muscle mass specifically is a subset of LBM, not the whole thing.

Can I calculate LBM without knowing my body fat percentage?

Not accurately. Formulas that estimate LBM from weight and height alone (like the Boer formula) exist, but they're really just re-deriving what Mifflin-St Jeor already estimates, so they don't add precision without a real body fat measurement.

Does more lean body mass always mean a higher BMR?

Generally yes. Since muscle and organ tissue burn more calories at rest than fat tissue, higher lean mass at the same total weight correlates with a higher resting metabolic rate.