Lean Body Mass (LBM) Calculator
Estimate lean body mass (LBM) using several common formulas (Boer, James, Hume, Janmahasatian). Enter weight, height, sex and age to get LBM, approximate body fat mass (if body fat % provided), and compare formula outputs.
Estimating Lean Body Mass — formulas, interpretation and practical guidance
Lean body mass (LBM) — sometimes called lean mass — is the weight of everything in the body except fat tissue. That includes skeletal muscle, organs, bone, connective tissue and the water within them. LBM is an important concept used in clinical dosing (some drugs are dosed to lean mass), athletic performance analysis, and tracking changes during weight loss or bodybuilding.
Why estimate LBM?
Measuring body composition directly (DEXA, hydrostatic weighing, air displacement plethysmography) is accurate but costly and not always accessible. For many applications an estimate from simple anthropometrics — weight, height, sex, age — is adequate. Several empirically derived formulas have been published that provide robust estimates for general populations. This calculator implements the most commonly used formulas so you can compare them side-by-side.
Common formulas (what this calculator uses)
Each formula below estimates LBM from routine measurements. They were derived from different datasets and may perform differently depending on the population (age, obesity level, ethnicity, muscularity).
- Boer (1984):
- Male: LBM = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2
- Female: LBM = 0.252 × weight(kg) + 0.473 × height(cm) − 48.3
- James (1976):
- Male: LBM = 1.1 × weight(kg) − 128 × (weight(kg)/height(cm))²
- Female: LBM = 1.07 × weight(kg) − 148 × (weight(kg)/height(cm))²
- Hume (1966):
- Male: LBM = 0.32810 × weight(kg) + 0.33929 × height(cm) − 29.5336
- Female: LBM = 0.29569 × weight(kg) + 0.41813 × height(cm) − 43.2933
- Janmahasatian (2005):
Designed for pharmacokinetic applications and often used to calculate LBM for drug dosing. Uses BMI and different coefficients:
- Male: LBM = 9270 × weight(kg) / (6680 + 216 × BMI)
- Female: LBM = 9270 × weight(kg) / (8780 + 244 × BMI)
How to use the calculator
Enter weight and height in the units you have (kg or lb, cm or in). Choose a single formula or "Show all" to compare results. If you also know body fat percentage you can compute fat mass and cross-check against LBM estimates. Use step-by-step mode if you want the exact arithmetic shown for verification or documentation.
Worked examples
Example 1 — 70 kg male, 175 cm: Boer formula gives LBM ≈ 0.407×70 + 0.267×175 − 19.2 ≈ 28.49 + 46.725 − 19.2 ≈ 55.99 kg. James/Hume/Janma estimates will be similar but may vary by a few kilograms. Compare results to pick a sensible value for your context.
Example 2 — 85 kg female, 165 cm: Using Hume or Boer may produce different LBM values; athletes with higher muscle mass may have true LBM higher than these formulas predict because the equations were derived from mixed populations.
Interpreting differences between formulas
Differences of 1–3 kg between formulas are common. Larger discrepancies can appear in very obese or very muscular people where anthropometric relationships deviate from the original study populations. When high accuracy matters (e.g., research or critical drug dosing), prefer direct body-composition methods or consult clinical guidance.
Limitations and caveats
These formulas estimate lean mass, not muscle mass specifically. They also assume stable hydration — dehydration or fluid retention will affect estimates. Ethnic and age-related differences can alter accuracy. For elderly people with sarcopenia or for bodybuilders, results should be taken as approximations.
Practical uses
- Clinical dosing (some medications use LBM rather than total body weight).
- Tracking body composition changes during weight loss or muscle gain.
- Estimating resting metabolic rate when combined with fat-free mass-based equations.
Tips for better estimates
- Use measured weight and height — avoid self-reported values if possible.
- Compare multiple formulas and consider the context (athlete vs general population).
- When possible, validate with DEXA or other direct measures for critical decisions.
This calculator is intended for educational and general-purpose use. It does not replace professional medical advice. For clinical or pharmacological decisions consult a qualified healthcare professional.
Frequently Asked Questions
They are often used interchangeably. Fat-free mass (FFM) is technically the mass excluding all lipids; LBM is a practical term for the non-fat part of the body. Differences are minor for most uses.
No single formula is best for every person. Janmahasatian is commonly used in pharmacokinetics; Boer and Hume perform well in general adult populations. Compare multiple formulas.
These formulas were derived for adults. Use pediatric-specific methods for children.
If body fat % is accurately measured, converting to LBM (LBM = weight × (1 − BF%/100)) is straightforward. The key is the accuracy of the body fat measurement method.
No — the calculator accepts kg/lb and cm/in and converts internally. Just enter the correct numeric values and selected units.
Many nutritionists set protein targets per kg of lean mass (e.g., 1.6–2.4 g/kg LBM for athletes). Use professional guidance to set individualized targets.
Recalculate when weight or height (rarely) changes, or when tracking body composition across a training or diet phase — typically weekly to monthly depending on goals.
Yes — fluid shifts change body weight and apparent lean mass. For consistent tracking, measure under similar hydration conditions.
Some drugs use LBM for dosing; Janmahasatian formula is commonly referenced in pharmacokinetic literature. Always follow clinical protocols.
The formulas may under- or over-estimate true LBM. Consider direct measurement (DEXA) for best accuracy.