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    Lean Body Mass Calculator

    Calculate your lean body mass using Boer, James, and Hume formulas. Compare results across methods and understand your body composition.

    Free to use. Runs in your browser.

    Lean Body Mass is your total weight minus body fat. For most adults, LBM is 60-90% of total body weight depending on fitness level.

    Calculate your exact LBM using three scientific formulas below.

    Your Measurements

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    in
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    Methodology and sources

    Formula or method

    Three published population-level formulas (Boer 1984, James 1976, Hume 1966) are applied using weight in kg and height in cm. Each formula produces an LBM estimate in kg; fat mass is derived as total weight minus LBM; body fat percentage is (fat mass divided by total weight) times 100. A simple arithmetic average of the three formula results is reported as the headline figure alongside each individual result.

    Basis and assumptions

    • All three formulas require only weight, height, and sex as inputs; age is collected on-screen but not used in any formula.
    • Height and weight entered in Imperial units are converted internally (feet/inches to cm; lbs to kg) before formulas are applied.
    • The Boer (1984) formula is stated in the tool content as validated against DEXA scan data and is highlighted as the most reliable estimate for most adults.
    • Derived body fat percentage is a mathematical residual (total weight minus LBM divided by total weight) and is not an independently measured body fat estimate.
    • Formula-based estimates carry a stated margin of 5 to 10% versus DEXA for most adults; accuracy is reduced for very muscular, very lean, or obese individuals.
    • No age-adjustment is applied; the formulas are validated primarily for adult populations.

    Key handling decisions

    • When the three formula results diverge by more than 3 to 4 kg, the tool's own content section notes this may indicate body proportions outside the population the formulas were designed for.
    • The reported average is an unweighted mean of all three results, not a weighted or evidence-ranked combination.

    What this tool does not decide

    • Whether your LBM or body fat percentage indicates a health risk or requires intervention. Consult a qualified doctor or NHS/CDC-registered healthcare provider for clinical assessment.
    • Diagnosis of any body-composition disorder, eating disorder, sarcopenia, or obesity. Seek advice from a GP, dietitian, or sports medicine physician.
    • Personalised nutrition targets, calorie prescriptions, or training programmes. Work with a registered dietitian (UK: BDA-registered; US: RD credential) or qualified personal trainer.
    • Whether DEXA, hydrostatic weighing, or bioimpedance testing is appropriate for your situation. A sports medicine clinician or hospital body-composition service can advise.

    Sources

    Last checked: 2026-06-17

    What Is Lean Body Mass and Why Should You Care?

    Lean body mass is everything in your body that isn't stored fat: muscles, bones, organs, water, skin, and connective tissue. It's the metabolically active part of you, the part that burns calories, moves your body, and keeps you alive.

    Two people can weigh exactly the same but have completely different body compositions. A 75 kg person with 60 kg of lean mass and 15 kg of fat looks, performs, and ages differently from someone at 75 kg with 50 kg of lean mass and 25 kg of fat. The scale doesn't tell you this. LBM does.

    LBM also drives your metabolism. Every kilogram of muscle burns roughly 13 calories per day at rest, while fat burns only 4.5. This is why two people of the same weight can have very different calorie needs, the one with more lean mass burns more, even sitting on the sofa.

    The Three Formulas Compared

    FormulaYearInputsBest For
    Boer1984Weight, height, sexMost accurate for adults (validated with DEXA)
    James1976Weight, height, sexClassic formula, widely cited in research
    Hume1966Weight, height, sexOldest, still used in clinical settings

    What this means for you: All three formulas give estimates, none are as accurate as a DEXA scan or hydrostatic weighing. This calculator shows all three and averages them, which tends to reduce individual formula error. If the three results differ by more than 3 to 4 kg, your body proportions may be unusual for the population the formulas were designed for.

    Typical LBM Ranges

    CategoryMen (LBM % of body weight)Women (LBM % of body weight)
    Athletes85 to 92%78 to 85%
    Fit adults80 to 88%72 to 80%
    Average adults72 to 82%65 to 75%
    Overfat< 72%< 65%

    Women naturally carry more essential body fat (10 to 13% vs 2 to 5% for men), so their LBM percentage is always lower at the same fitness level. This isn't a health concern, it's biology.

    How to Increase Lean Body Mass

    Resistance Training

    Train each muscle group 2x per week with progressive overload. Compound movements (squats, deadlifts, bench press, rows) build the most lean mass per minute of training. Use our One Rep Max Calculator to set training weights.

    Adequate Protein

    Consume 1.6 to 2.2g protein per kg of body weight daily. Spread across 3 to 5 meals for maximum muscle protein synthesis. Check your needs with our Protein Calculator.

    Calorie Surplus (for gain)

    Building muscle requires energy. A surplus of 200 to 300 calories above your TDEE supports lean mass gain while minimising fat gain. Find your TDEE with our TDEE Calculator.

    Sleep and Recovery

    Growth hormone, critical for muscle repair, is released primarily during deep sleep. Aim for 7 to 9 hours. Poor sleep reduces muscle protein synthesis by up to 18%. Use our Sleep Calculator to time your cycles.

    Related Body Composition Tools

    Body Fat CalculatorEstimate your body fat percentageBMI CalculatorCheck your body mass indexIdeal WeightCompare 4 ideal weight formulasProtein CalculatorFind your daily protein needsOne Rep MaxCalculate your training weightsBMR CalculatorFind your resting metabolic rate

    How to use this tool

    1

    Choose Imperial or Metric units, then enter your age and select your gender.

    2

    Input your height and current weight in the selected units.

    3

    Click 'Calculate LBM' to see your results and compare them across the three formulas.

    Common uses

    • Tracking muscle mass changes during training
    • Calculating more accurate calorie needs
    • Monitoring body composition over time
    • Comparing body composition across formulas
    • Setting muscle-building targets

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    Frequently Asked Questions

    What is lean body mass and how is it different from fat-free mass?
    Lean Body Mass (LBM) includes everything except stored fat: muscles, bones, organs, skin, and water. Fat-Free Mass (FFM) is similar but excludes essential fats like those in the brain and bone marrow. LBM is typically about 2 to 3% higher than FFM.
    Which lean body mass formula is most reliable?
    The Boer formula (1984) is generally considered the best-validated for adults because it was tested against modern DEXA scan data. The James and Hume formulas are older but still widely referenced in clinical literature.
    Why is lean body mass important for fitness?
    LBM is a better indicator of fitness than total weight because it reflects your muscle, bone, and organ mass. Higher LBM relative to body weight indicates better body composition, and it's used to calculate more accurate calorie needs.
    How can I increase my lean body mass?
    Resistance train 2 to 4 times per week, consume adequate protein (1.6 to 2.2 g per kg of bodyweight), sleep 7 to 9 hours, and maintain a slight caloric surplus for muscle building. Consistency over months matters more than any single session.
    What is a normal lean body mass percentage?
    For men, LBM typically ranges from 70 to 90% of total body weight. For women, it's 60 to 80%. Athletes and regular exercisers sit toward the top of these ranges, while sedentary individuals tend to be lower.
    Does lean body mass decrease with age?
    Yes. After about age 30, you lose roughly 3 to 5% of muscle mass per decade if you're not actively training. This age-related muscle loss is called sarcopenia, and resistance training is the most effective way to slow it.
    How does lean body mass affect metabolism?
    Each kilogram of skeletal muscle burns roughly 13 kcal per day at rest, compared to about 4.5 kcal per kg of fat. More lean mass means a higher basal metabolic rate, which is why building muscle helps with long-term weight management.
    Is lean body mass the same as muscle mass?
    No. LBM includes muscle but also bones, organs, skin, and water. Skeletal muscle typically makes up about 40 to 50% of LBM in men and 30 to 40% in women. LBM is a broader measurement of everything that isn't stored body fat.
    How accurate are LBM calculator estimates?
    Formula-based estimates are within 5 to 10% of DEXA scan results for most people. They're less accurate for very muscular individuals, the very lean, or those with obesity. For precise measurements, DEXA or hydrostatic weighing are the gold standards.
    Does hydration affect lean body mass measurements?
    Yes, significantly. Water is part of lean mass, so dehydration can underestimate LBM while overhydration inflates it. Formula-based calculators aren't affected by hydration, but bioimpedance scales are, weigh yourself at the same time daily for consistency.
    Why do bodybuilders track lean body mass?
    Bodybuilders use LBM to ensure they're gaining muscle, not just weight, during bulking phases. During cutting, tracking LBM helps confirm they're losing fat while preserving muscle. The goal is to increase LBM while reducing body fat percentage.
    Can women have the same lean body mass as men?
    In absolute terms, men typically have more LBM due to higher testosterone levels driving greater muscle mass. However, relative to body weight, athletic women can achieve LBM percentages in the high 70s to low 80s, which overlaps with less active men.

    Results are for general informational purposes only and should be checked before use. They are not professional advice. See our Disclaimer and Terms of Service.