Lean Body Mass Calculator

Estimate lean body mass and body-fat percentage from height and weight using the Boer and James formulas, or directly from a measured body-fat percentage.

Lean Body Mass CalculatorEn direct

Comment utiliser cette calculatrice

1Choose metric or imperial units and enter your height and weight.
2Select your sex, which the estimation formulas depend on.
3If you know your body-fat percentage, enter it for a precise result.
4Read your lean body mass, fat mass, and body-fat percentage.

Comment ça marche

Lean body mass

if body fat % known: LBM = weight × (1 − fat% ÷ 100)
Boer (men): LBM = 0.407·kg + 0.267·cm − 19.2
Boer (women): LBM = 0.252·kg + 0.473·cm − 48.3
fat mass = total weight − lean body mass

Lean body mass, sometimes called fat-free mass, is the total weight of everything in the body except fat — muscle, bone, organs, connective tissue, and body water. It is a more meaningful figure than total weight for many purposes, because it reflects the metabolically active tissue that burns most of the body's energy and is preserved (or lost) during weight change. When a body-fat percentage is known from a direct measurement, lean mass is simply the total weight minus the fat portion. Without one, formulas like Boer and James estimate lean mass from height, weight, and sex, exploiting the statistical relationship between those measures and typical body composition. These estimates are handy but blind to individual build, so a measured body-fat percentage always beats a formula.

Exemple détaillé

An 80 kg man who is 180 cm tall has a Boer lean body mass of 0.407 × 80 + 0.267 × 180 − 19.2 ≈ 61.4 kg, implying about 18.6 kg of fat, or roughly 23% body fat. If a DEXA scan instead measured his body fat at 18%, his true lean mass would be 80 × 0.82 = 65.6 kg — the difference the formula can't see.

Lean Body Mass Calculator : le guide complet

Why lean body mass matters

Total body weight lumps together two very different things: fat and everything else. Lean body mass separates out the everything-else — the muscle, bone, organs, and water that make up the working body. This distinction matters because those tissues, especially muscle, drive metabolism, strength, and function. Lean mass is the largest determinant of resting energy expenditure, which is why two people of the same weight can have quite different calorie needs depending on how much of that weight is muscle versus fat.

The concept is central to sensible weight management. The goal of a good fat-loss plan is to lose fat while preserving lean mass, and the goal of strength training is to build it. Tracking lean body mass rather than just the scale reveals whether a diet is costing you muscle — a common problem with aggressive dieting — or whether a training program is adding it even as total weight holds steady. In clinical settings, lean mass is used to scale drug doses and to assess conditions like muscle wasting that a weight reading alone would miss.

Formulas versus direct measurement

The formulas used here — Boer, James, and their relatives — estimate lean mass from height, weight, and sex. They work by encoding the average relationship between those easily measured quantities and body composition across large populations. For a person of typical build they are reasonably accurate and require nothing more than a scale and a tape measure, which is their appeal. But by design they assume an average amount of muscle and fat for your size, so they systematically misjudge people at the extremes.

A very muscular athlete will have their lean mass underestimated by a height-and-weight formula, because the formula attributes some of their above-average weight to fat that isn't there. Conversely, a lightly built but overweight person may have it overestimated. This is the fundamental limit of any formula that cannot actually see inside the body. Direct methods — DEXA scans, hydrostatic weighing, air-displacement plethysmography, or even reasonably good body-fat calipers and bioelectrical scales — measure body composition rather than infer it, and entering a body-fat percentage from one of these turns the calculation from an estimate into a straightforward subtraction.

Using lean mass in training and nutrition

Once you know your lean body mass, several practical calculations become more accurate. Protein needs, for instance, are often better set per unit of lean mass than per unit of total weight, since it is the lean tissue that protein maintains and builds — basing intake on lean mass avoids overfeeding protein to account for fat that doesn't need it. Some approaches to estimating calorie needs also key off lean mass, on the grounds that it, not fat, does most of the metabolic work.

For anyone recomposing their body — losing fat while gaining or holding muscle — lean body mass is the metric that tells the real story. The scale can be flat or even rising during a successful recomposition, because muscle gained offsets fat lost, and only by tracking lean and fat mass separately can you see that progress. Measuring periodically, ideally with the same method each time for consistency, turns a frustrating plateau on the scale into visible evidence that the underlying composition is improving. That is the deeper value of separating lean mass from the single number a bathroom scale reports.

Questions fréquentes

What is lean body mass?

Lean body mass, or fat-free mass, is the total weight of everything in your body that isn't fat — muscle, bone, organs, connective tissue, and water. It's a better guide than total weight to metabolism and function, since muscle drives most of the body's energy use.

How is lean body mass calculated?

If you know your body-fat percentage, lean mass is simply total weight times (1 − fat%). Without it, formulas like Boer estimate lean mass from height, weight, and sex — for men, 0.407 × kg + 0.267 × cm − 19.2. A measured body-fat percentage is more accurate.

Why does my lean mass estimate seem off?

Height-and-weight formulas assume average body composition, so they underestimate lean mass in very muscular people and overestimate it in those carrying more fat. They can't see your actual muscle. Entering a measured body-fat percentage from a scan or calipers fixes this.

Should I track lean mass instead of weight?

For body recomposition, yes — the scale can stay flat while you lose fat and gain muscle, and only tracking lean and fat mass separately reveals that. Lean mass also helps set protein and calorie targets. Measure with the same method each time for consistent comparisons.