Lean Body Mass Calculator

Formula sex:

This selects published equation constants; it is not a statement about identity.

Headline route:

Use the same route across check-ins; method changes can be larger than a real short-term trend.

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Known body fat:

Enter 1–70% from a recent, documented measurement or estimate.

Age:

Enter 18–100 completed years.

years

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Display precision:

Choose one or two decimal places without changing the calculation.

Measure	Value	Meaning	Copy
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Method	Lean mass	Difference	Use note	Copy
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Body weight combines fat, water, protein, organs, bone, and other tissues. Lean body mass is an estimate of the portion outside stored body fat; it is not the same as skeletal muscle mass. A change in lean mass can reflect hydration, glycogen, organs, bone, or measurement noise as well as muscle.

Without an imaging or laboratory measurement, adult lean mass is commonly estimated from body size or from an estimated body-fat percentage. Height-and-weight equations apply coefficients derived from particular study populations. Body-fat routes first estimate or accept body fat, then subtract the corresponding fat mass from body weight. The methods answer a similar question through different assumptions, so their results need not agree.

Lean mass: Estimated body weight remaining after the modeled fat mass is removed. It should not be read as muscle alone.
Formula spread: The difference between the highest and lowest of four height-and-weight equation results. A wide spread signals method sensitivity.
FFMI: The selected lean-mass estimate divided by height squared, abbreviated FFMI. It standardizes the estimate for height but does not diagnose health or muscularity.

Sex-specific coefficient sets are part of the published equations. Selecting a formula path chooses those constants; it is not a statement about identity. Age is used only by the BMI-and-age body-fat screen, while neck, waist, and hip measurements are used only by the circumference route. A known body-fat percentage can be entered directly, but its error transfers directly into the lean-mass estimate.

Method consistency matters more than apparent decimal precision when tracking change. Switching from Boer to Hume, changing tape landmarks, or comparing a scale estimate with a circumference estimate can create a larger difference than a real short-term change. Use the same method, measurement conditions, units, and time of day for repeat checks.

These adult estimates are informational. They are not a diagnosis, body-composition scan, nutrition prescription, treatment decision, medication-dosing instruction, military certification, or substitute for advice from a qualified health professional.

How to Use This Tool:

Choose one headline route for the question you are asking, then keep that route fixed when comparing later measurements.

Select Formula sex and Headline route. Consensus median compares four height-and-weight equations; the other routes use one equation or a body-fat estimate.
Enter height and weight with their correct units. The adult model accepts resolved heights from 120 to 230 cm, weights from 35 to 250 kg, and BMI from 12 to 65 kg/m².
Add the route-specific measurement. Enter a documented body-fat percentage for Known body fat, age for the BMI + age screen, or consistent neck, waist, and hip circumferences for the Navy route.
Review Lean mass snapshot and Method comparison. Check the formula spread and the gap between a body-fat-derived result and the four-equation median before treating a small difference as meaningful.

Interpreting Results:

The headline lean mass belongs to the selected method. The consensus result is the median of Boer, Hume, James, and Janmahasatian, not a clinically validated combined equation. It reduces the influence of the single highest and lowest estimates but does not prove that the middle pair is correct for an individual.

Read Formula spread as uncertainty between equations, not as a confidence interval. A body-fat-derived result adds a Direct gap against the consensus median. Large gaps can come from tape technique, hydration-sensitive body-fat estimates, population mismatch, or different definitions of the body component being estimated.

FFMI is calculated from the selected headline lean mass. Do not compare it with a reference range unless the source uses the same body-composition definition and method. For trends, a stable method and repeatable measurement conditions are more useful than one or two displayed decimal places.

Technical Details:

Length is converted with 1 in = 2.54 cm and weight with 1 lb = 0.45359237 kg. All four direct lean-mass equations use kilograms and centimetres. The calculation keeps full precision; display precision affects visible and document values only.

Formula Core:

Body mass index (BMI) is used by the Janmahasatian equation and the BMI-age body-fat screen.

BMI = \frac{W_{kg}}{{H_{m}}^{2}}

The four published equation routes are evaluated for every valid height-and-weight input. The consensus route takes their median.

Lean body mass equations in kilograms using weight in kilograms and height in centimetres
Method	Male coefficient set	Female coefficient set
Boer	0.407W + 0.267H − 19.2	0.252W + 0.473H − 48.3
Hume	0.32810W + 0.33929H − 29.5336	0.29569W + 0.41813H − 43.2933
James	1.10W − 128W²/H²	1.07W − 148W²/H²
Janmahasatian	9270W ÷ (6680 + 216BMI)	9270W ÷ (8780 + 244BMI)

The same equations are represented formally below. W is weight in kilograms and H is height in centimetres.

\begin{matrix} {LBM}_{Boer,m} & = & 0.407 W + 0.267 H - 19.2 \\ {LBM}_{Boer,f} & = & 0.252 W + 0.473 H - 48.3 \\ {LBM}_{Hume,m} & = & 0.32810 W + 0.33929 H - 29.5336 \\ {LBM}_{Hume,f} & = & 0.29569 W + 0.41813 H - 43.2933 \\ {LBM}_{James,m} & = & 1.10 W - \frac{128 W^{2}}{H^{2}} \\ {LBM}_{James,f} & = & 1.07 W - \frac{148 W^{2}}{H^{2}} \\ {LBM}_{Janma,m} & = & \frac{9270 W}{6680 + 216 BMI} \\ {LBM}_{Janma,f} & = & \frac{9270 W}{8780 + 244 BMI} \end{matrix}

Body-fat routes calculate lean mass by subtracting the estimated fat fraction. FFMI then standardizes the selected headline result by height.

\begin{matrix} LBM & = & W \times (1 - BF / 100) \\ FFMI & = & \frac{LBM}{{H_{m}}^{2}} \end{matrix}

Mechanism Core:

The BMI-age route uses the adult Deurenberg screen. The circumference route uses continuous base-10 logarithmic equations after converting measurements to inches.

Body fat estimation route formulas
Route	Male coefficient set	Female coefficient set
BMI + age	1.2BMI + 0.23age − 10.8 − 5.4	1.2BMI + 0.23age − 5.4
Circumference	86.01 log₁₀(waist − neck) − 70.041 log₁₀(height) + 36.76	163.205 log₁₀(waist + hip − neck) − 97.684 log₁₀(height) − 78.387

All four height-and-weight estimates must be finite, above zero, and no greater than body weight even when a body-fat route is selected. Body-fat estimates must resolve to 1% through 70%. The Navy route additionally restricts neck to 20–60 cm, waist to 45–180 cm, and female hip measurement to 60–200 cm.

At 180 cm and 82 kg on the male coefficient path, Boer gives about 62.23 kg, Hume 58.44 kg, James 63.64 kg, and Janmahasatian 62.58 kg. The median of the middle two is about 62.41 kg, FFMI is about 19.26 kg/m², and the formula spread is about 5.19 kg.

Limitations and Accuracy:

Prediction equations estimate group relationships and can be biased for an individual. Age, ethnicity, training status, illness, pregnancy, edema, limb difference, and body size outside the source populations can reduce agreement with a reference measurement.

Lean mass is not skeletal muscle mass, and FFMI is not a diagnosis.
Hydration and measurement technique can move body-fat and circumference estimates without a true tissue change.
The circumference route is not an official Navy Body Composition Assessment. Official procedures specify landmarks, repeated measurements, rounding, and table use.
Do not use these estimates to set medication doses, diagnose malnutrition or sarcopenia, or replace dual-energy X-ray absorptiometry or another clinically selected method.

References:

Prediction of lean body mass from height and weight, Journal of Clinical Pathology, 1966.
Estimated lean body mass as an index for normalization of body fluid volumes in humans, American Journal of Physiology, 1984.
Quantification of lean bodyweight, Clinical Pharmacokinetics, 2005.
Correction CP-1612 — James lean body mass formula coefficient, DICOM Standards Committee, November 6, 2016.
Body mass index as a measure of body fatness: age- and sex-specific prediction formulas, British Journal of Nutrition, 1991.
Guide 4 — Body Composition Assessment, U.S. Navy, January 2024.
Methodological standards for body composition, American Journal of Clinical Nutrition, 2025.