The Lean Body Mass Calculator estimates the portion of body weight that is not fat, using three published equations: Boer, James and Hume. Enter height, weight and sex, and the tool returns each formula's result, a simple mean of the three, and the complementary fat-mass figure implied by that mean.
Lean mass includes muscle, bone, organs and water. It is the compartment that drives much of resting metabolism and that strength training aims to protect or grow. These equations estimate it from height and weight alone, so they inherit the limits of any two-input model. The page shows the spread and treats every kilogram as an estimate for general information, not a scan.
Calculate your lean body mass
Lean body mass (sometimes called fat-free mass in everyday use, with small technical distinctions in research) is total weight minus fat mass. Direct measurement needs a clinic or lab. Field equations estimate lean mass from sex, height and weight so that nutrition, training or educational tools have a starting figure when a DEXA scan is not available.
The Lean Body Mass Calculator runs Boer (1984), James (1976) and Hume (1966) on the same inputs. Showing all three avoids pretending a single regression is definitive. The mean is a convenience summary; the individual results show how sensitive the estimate is to formula choice.
Adult anthropometric equations of this type are not validated as diagnostic tools. Under-18 growth, very high muscularity and some medical conditions can move true composition away from what height and weight predict. The estimate supports education and rough planning. It does not replace clinical body-composition assessment.
Compare the LBM formulas
Each published equation returns lean mass in kilograms from the same two anthropometrics, yet the coefficients differ by sex and by author. Substituting one person into all three usually produces results within a few kilograms of each other, and sometimes a wider gap that the page intentionally leaves visible.
Boer `` Male: LBM = 0.407×kg + 0.267×cm − 19.2 Female: LBM = 0.252×kg + 0.473×cm − 48.3 ``
James `` Male: LBM = 1.1×kg − 128×(kg/cm)² Female: LBM = 1.07×kg − 148×(kg/cm)² ``
Hume `` Male: LBM = 0.32810×kg + 0.33929×cm − 29.5336 Female: LBM = 0.29569×kg + 0.41813×cm − 43.2933 ``
Source: Boer 1984, James 1976, Hume 1966, as recorded in the site health reference. Weight is in kilograms and height in centimetres. James uses the squared ratio of weight to height, which behaves differently at very high or low BMI than the more linear Boer and Hume forms.
No single equation is framed here as best for everyone. Boer is common in clinical estimation contexts; Hume and James remain widely cited alternatives. The calculator's job is to surface the comparison, not to crown a winner. When the three sit close together, confidence in a rough estimate rises a little. When they fan out, that scatter is itself useful information.
Calculate lean mass and fat mass
Worked arithmetic shows both the lean estimates and the fat-mass complement that falls out of them. Take an adult male weighing 80 kg at 180 cm. Run each published formula in turn, then form a mean and subtract from total weight.
1. Boer 0.407×80 + 0.267×180 − 19.2 = 32.56 + 48.06 − 19.2 = 61.4 kg
2. James 1.1×80 − 128×(80/180)² = 88 − 128×0.1975 ≈ 88 − 25.28 = 62.7 kg
3. Hume 0.32810×80 + 0.33929×180 − 29.5336 = 26.248 + 61.072 − 29.534 ≈ 57.8 kg
4. Mean and fat-mass complement Mean LBM ≈ (61.4 + 62.7 + 57.8) / 3 ≈ 60.6 kg Fat-mass complement ≈ 80 − 60.6 = 19.4 kg Implied fat percentage ≈ 19.4 / 80 ≈ 24%
Hume sits a few kilograms below Boer and James in this example; that gap is exactly why the page shows all three. The fat-mass complement is only as good as the lean-mass estimate it rests on. It is not a Navy tape or DEXA result. For a dedicated fat-percentage estimate with an explicit ±3 to 4 point error band, use the Body Fat Calculator.
On a different day, with weight changed by fluid or food, the same formulas will move. Short-term swings of a kilogram in total weight create matching swings in the implied fat complement even when true fat tissue has barely changed. That is another reason to prefer trends and clinical methods over a single web reading.
Understand why lean body mass matters
Resting energy use scales with metabolically active tissue, which is why lean mass shows up in everyday nutrition maths and training plans alike. Equations such as Katch-McArdle estimate basal metabolic rate from lean mass once a composition figure is available:
BMR ≈ 370 + 21.6 × lean_body_mass_kgThat relationship is why composition-aware calorie tools ask for a body-fat or lean-mass figure when they can get one. A higher lean mass usually supports a higher maintenance calorie need than the same total weight with less lean tissue, though individual metabolism still varies and no equation captures every person.
Strength and physical function track lean mass as well, especially muscle. People recovering from illness, ageing adults watching sarcopenia, and athletes managing training load all have reasons to care about the lean compartment. Protecting muscle during a moderate calorie change matters more than chasing a scale number alone. Drug dosing in some clinical settings also uses lean or ideal mass rather than total weight; those decisions belong with a clinician, not with a web estimate.
None of these uses turns a Boer or Hume result into a verdict on health. Lean mass is one useful number among many. The Lean Body Mass Calculator supplies an educational estimate so the concept is concrete, with the spread across formulas kept in view. Pair it with strength progress, how clothes fit, lab work when relevant, and professional guidance when the stakes are clinical.
Athletes and people new to lifting sometimes expect lean mass to jump week to week. Tissue change is slower than scale noise. A few weeks of consistent training may move strength before it moves the estimated lean kilograms in a height-weight equation, because those equations barely see training at all. That gap between lived progress and a two-input model is another reason to keep the number in its place.
Frequently asked questions
What is lean body mass?
Lean body mass is the part of body weight that is not fat, including muscle, bone, organs and water. Everyday language often treats it as interchangeable with fat-free mass; research sometimes draws finer distinctions. This tool estimates the lean compartment from height, weight and sex.
Which formula should be used?
Boer, James and Hume are all published adult estimates. None is universally best. Comparing all three and noting the spread is more honest than locking onto one number. The mean is a convenience summary only.
How is fat mass derived on this page?
Fat mass is shown as total weight minus the mean lean-mass estimate. If the lean estimate is off, the fat complement is off by the same amount. For a circumference-based fat percentage with a stated ±3 to 4 point error band, use the Body Fat Calculator.
Does lean mass differ by sex in the formulas?
Yes. Each equation uses different coefficients for male and female adults. Enter the sex that matches the formula set. The structure (height and weight in, kilograms out) is the same.
Can this replace a DEXA scan?
No. Height-weight equations are convenient estimates. DEXA and related clinical methods measure composition more directly. Use the calculator for education and rough planning; use clinical testing when precision matters.
Why does lean mass matter for calories?
Lean tissue contributes much of resting metabolic rate. The Katch-McArdle BMR equation uses lean mass explicitly. Composition-aware calorie planning can be more suitable for muscular or very lean people than weight-only equations, still as an estimate.
Is this suitable for teenagers?
These adult equations are a poor fit for growing bodies. Paediatric composition assessment needs age-appropriate methods and clinical guidance. Do not treat a Boer or Hume result as a target for anyone under 18.
What if the three formulas disagree a lot?
Large spreads appear more often at unusual BMI values or body proportions the regressions never saw in force. Treat a wide spread as a warning that the estimate is shaky, and prefer a direct composition method if the number will drive a decision.
How does lean mass relate to body-fat percentage?
If total weight and lean mass are known, fat mass is the difference and fat percentage is fat mass divided by weight. That path inherits every error in the lean estimate. Circumference or clinical fat methods approach the percentage more directly.
Does hydration change the reading?
Equations based on scale weight move when weight moves, including from fluid. True tissue change is slower. Hydration shifts are one more reason a single estimate is a snapshot, not a verdict.
Summary
The Lean Body Mass Calculator estimates lean mass with Boer, James and Hume from height, weight and sex, reports each result and their mean, and shows the fat-mass complement implied by that mean. The equations are adult field estimates, not scans, and they can disagree by several kilograms.
Lean mass matters for metabolism, training and some clinical contexts, yet every figure here remains general information. Pair it with better composition tools when precision matters, and with professional guidance when health decisions hang on the number.