Lean Body Mass Calculator
Split your weight into lean body mass and fat mass using the Boer, James and Hume equations, and see your fat-free mass index.
Lean Body Mass Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Results are a model, not a quotation — an institution's own figures govern.
- Every input is an assumption; change one and the answer changes with it.
- Rounding is applied only at the display layer, so totals may differ by a cent from a statement that rounds each line.
In short: Lean body mass is everything in your body that is not fat — muscle, bone, organs, blood and water. The Boer equation, the most widely validated, gives LBM = 0.407 × weight (kg) + 0.267 × height (cm) − 19.2 for men, and 0.252 × weight + 0.473 × height − 48.3 for women. Typical adults carry 60–90% of their weight as lean mass.
Formula
The Boer equation. W is weight in kilograms and H is height in centimetres; the sex-specific constants reflect systematic differences in average body composition.
Worked Example
- Take the weight term. 0.407 × 82 = 33.374.
- Take the height term. 0.267 × 178 = 47.526.
- Sum and subtract the constant. 33.374 + 47.526 − 19.2 = 61.7 kg lean mass.
- Derive fat mass. 82 − 61.7 = 20.3 kg, or 24.8% body fat.
- Compute FFMI. 61.7 ÷ 1.782 = 19.5 kg/m2, a normal-to-above-average level of muscularity.
Clinical note. The three equations return 61.7 kg, 63.0 kg and 57.8 kg for this subject — a 5.2 kg spread. Equation-based LBM is an estimate from height and weight only; it cannot see how muscular you actually are, which is why a tape-measure method is more informative if you have five minutes.
Strengths & Limits Of This Model
Where this engine is strong
- Runs entirely in your browser — no figure you type is transmitted or stored.
- Shows the full working, so every number can be traced and challenged.
- Free, unmetered and free of affiliate incentives.
Where it stops
- Generalised assumptions cannot capture every individual circumstance.
- Jurisdiction-specific rules and mid-year changes may not be reflected.
- A model output is not a substitute for a professional review of your position.
Practical Use Cases
Dosing drugs that distribute into lean tissue
Anaesthetic agents and several chemotherapy protocols are dosed on lean body mass rather than total weight, because adipose tissue takes up far less of the drug. This is the original clinical purpose of the Boer equation, and it pairs with the Ideal Weight Calculator in dosing protocols.
Setting protein intake during a cut
Protein requirements track lean mass far better than total weight, since muscle is the tissue you are protecting in a deficit. Take the lean figure into the Protein Calculator to set a target that will not scale absurdly for someone carrying substantial fat mass.
Judging whether a physique goal is realistic
Fat-free mass index caps out around 25 kg/m² in drug-free trained men, and progress towards it slows dramatically. Combine this with the Body Fat Calculator to see whether a target weight implies a plausible amount of muscle.
Methodology & Editorial Standards
Three published equations are evaluated in parallel: Boer (1984), James (1976) and Hume (1966), each with its own sex-specific coefficients, using weight in kilograms and height in centimetres. The Boer result is presented as the headline figure because it is the most widely validated in clinical dosing contexts. Fat-free mass index is lean mass divided by height in metres squared; the normalised variant adjusts to a 1.8 m reference stature using the conventional 6.1 coefficient. All three equations are regressions on height and weight and cannot detect individual muscularity — a direct measurement method will always be more informative. This engine implements the published formula exactly as specified in the source literature, with inputs validated for physiological plausibility before evaluation. Values outside the domain of the formula return an em-dash rather than a misleading figure.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Lean Body Mass Calculator — 20 Expert FAQs
20 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What exactly counts as lean body mass?
Everything that is not adipose tissue: skeletal muscle, bone, organs, connective tissue, blood and body water. It is sometimes confused with muscle mass alone, but skeletal muscle typically accounts for only around half of total lean mass.
Which equation should I trust?
Boer is the usual clinical reference and is the headline figure here. James tends to read slightly higher and Hume slightly lower at average builds. Where the three disagree substantially, treat the spread as the genuine uncertainty rather than picking the most flattering number.
How is this different from a body fat calculation?
This engine estimates lean mass from height and weight only, then derives fat mass as the remainder. A body fat calculator works the other way, estimating fat directly from tape measurements, which sees your actual shape and is generally more informative.
What is a good FFMI?
Roughly 18–20 kg/m² is average for men and 15–17 kg/m² for women. Around 22 indicates a well-trained physique, and values near 25 represent the practical ceiling for drug-free trained men. Sustained readings meaningfully above 25 are uncommon without pharmacological assistance.
Why normalise FFMI to 1.8 metres?
Raw FFMI retains a mild height bias, favouring shorter people. The normalised form adds 6.1 × (1.8 − height in metres), adjusting every reading to a common 1.8 m reference so that athletes of different stature can be compared fairly.
Can I increase my lean body mass?
Yes, primarily through progressive resistance training with adequate protein and sufficient total energy. Realistic rates are modest: roughly 0.25–0.5 kg of muscle per month for a beginner, and considerably less for trained lifters.
Do I lose lean mass when I diet?
Some loss is normal, but it can be minimised. Keeping the deficit moderate, eating adequate protein and continuing to lift preserves most lean tissue; aggressive crash dieting without resistance training can see a quarter or more of the weight lost come from lean mass.
Does lean body mass include water weight?
Yes. Body water is part of lean mass, which is why the figure can swing by a kilogram or more day to day with hydration, sodium intake and glycogen status. Compare readings taken under similar conditions.
Why do men and women have different equations?
Because average body composition differs systematically by sex. Women carry a higher proportion of essential fat, including breast, hip and thigh tissue required for normal physiological function, so the same height and weight imply a lower lean fraction.
Is lean body mass the same as fat-free mass?
They are used interchangeably in practice, though strictly fat-free mass excludes all lipid whereas lean body mass includes the small quantity of essential lipid in cell membranes and organs. The difference is a few percent and immaterial for practical purposes.
Why does the calculator refuse some inputs?
These are regression equations fitted to adult populations. At extreme height and weight combinations they can return a lean mass at or below zero, which is physiologically meaningless, so the engine reports an em-dash rather than a nonsense figure.
How often should I re-measure?
Monthly is sensible. Because the equations depend only on height and weight, the estimate moves solely with your weight, so more frequent checks simply track daily fluctuation rather than real change in body composition.
Is this lean body mass calculator free to use?
Yes. It is free, requires no account, and has no usage limits. ApexConverter is funded by contextual advertising, never by selling user data.
Is my data sent to a server?
No. The engine runs as Vanilla JavaScript inside your browser under our Zero-Server Client-Side Execution model. Your figures are computed locally and are never transmitted, logged, or stored.
How accurate is this calculator?
It applies the standard closed-form formula in IEEE-754 double precision, rounding only at the display layer. The engine is reconciled against an independent reference implementation before release.
Does it work on mobile?
Yes. The interface is mobile-first with numeric keypad hints and is tested down to a 320-pixel viewport with no horizontal scrolling.
Can I use it offline?
Largely, yes. Because computation is client-side, the page continues to calculate without a network connection once it has loaded.
Which currency does it use?
Amounts display in US$ accounting format, but the underlying mathematics is currency-agnostic. The result is identical in any currency, so simply read the figures in your own.
Why does a result show an em-dash?
An em-dash indicates the calculation is not defined for the inputs given — typically a division by zero or a value outside the valid domain. We show a dash rather than a misleading number.
How do I report an error?
Email apexconverter.praxiscalc@gmail.com with the tool URL, your exact inputs, the output received and the output you expected. Verified mathematical errors are patched within 72 hours.