Lean Body Mass Calculator

This calculator uses lean body mass to mean the mass remaining after subtracting fat. It includes muscle, bone, organs, blood and other non-fat components; it is not a muscle-mass measurement.

Height and total mass cannot distinguish two people with different body composition. Boer, James and Hume are predictions from those inputs. An entered body-fat percentage instead supplies a separate arithmetic source.

At 70 kg and 175 cm with male Boer coefficients, the equation gives 56.015 kg, displayed as 56.0 kg. Entering 30% body fat gives 21.0 kg fat and 49.0 kg remaining mass; that value does not replace the independent 56.0 kg Boer comparison.

Several formulas have been developed to estimate lean body mass without requiring specialized equipment. Each formula has its own advantages and was developed with different populations in mind. The calculator retains three named historical reference equations:

1. Boer Formula (1984)

Boer (1984) studied body-fluid normalization in healthy subjects: blood volume in 66 people and extracellular fluid volume in 54. See the original study; its publication does not establish which formula best fits an individual. Source checked 30 September 2026.

2. James Formula (1976)

The QIBA FDG-PET profile, Appendix H documents these kg/cm James coefficients and warns that the equation is inaccurate at extreme BMI. This later technical source is not the original 1976 report; the original full text was not verified. Source checked 30 September 2026.

3. Hume Formula (1966)

Hume (1966) estimated lean mass from measured total body water in 29 men and 27 women. Its provenance does not validate a universal personal target. Source checked 30 September 2026; the original full text was not independently readable in this review.

Direct Calculation Method

If you know your body fat percentage (from a DEXA scan, bioelectrical impedance scale, skinfold measurements, etc.), our calculator can also directly calculate your lean body mass using:

This subtraction inherits the quality and assumptions of the entered body-fat percentage. It is arithmetic from that input, not a new independent measurement.

A formula value outside 0 to the entered total mass is physically incompatible with that mass and is displayed as unavailable. No clipping is applied. A plausible output still does not establish accuracy.

Each comparison remains tied to its named equation. The main lean/fat outputs come from the selected equation unless a body-fat percentage is supplied; in that case their source is the entered percentage.

The comparisons can disagree without identifying which one fits an individual. At 50 kg and 200 cm, Boer predicts 54.55 kg. Because that exceeds total mass, its lean mass and negative residual fat are unavailable; the independent James value may still be shown.

The allowed 0–100% body-fat input expresses an arithmetic percentage domain. It is not a recommended range or proof that an endpoint is biologically suitable.

Raw lean mass divided by height² is sometimes called a lean-mass index. No universal target bands, obesity categories or athlete grades are inferred here.

Actual body composition includes more than total weight. Muscle contributes to movement and strength; lean mass also includes tissues that are not skeletal muscle. A change in predicted lean mass cannot identify which tissue changed.

Boer’s source concerned normalization of body-fluid volumes. That study purpose does not turn this web form into medication-dosing, nutritional-prescription or disease-diagnosis software.

These equations cannot verify muscle gain or choose a training or nutrition program. Repeating a height/weight equation mainly reflects changes in its inputs.

For a useful comparison, keep the method, units and source of any entered body-fat percentage consistent. Changes in hydration or measurement conditions can affect composition estimates; an unexplained change may need assessment beyond this calculation.

Is lean body mass the same as muscle mass?

No. Non-fat mass includes bone, organs, blood and other tissues as well as muscle.

Why does entering body fat change the main answer?

It supplies a separate source: remaining mass = total mass × (1 − percentage/100). The historical equation comparisons stay visible independently.

Why is an equation unavailable?

A nonfinite prediction cannot be displayed. A finite prediction below zero or above total mass cannot describe a component of that total, so it is declined rather than clipped.

Which prediction should I use?

This comparison does not determine accuracy for an individual or certify use in children, pregnancy, athletes or unusual body proportions.