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FFMI Calculator

The Fat-Free Mass Index (FFMI) measures how much muscle you carry relative to your height. Unlike BMI, FFMI accounts for lean mass rather than total body weight, making it a far more useful metric for people who strength-train. An adjusted version normalises results to a reference height of 1.8 m so you can compare scores across different body sizes.

Quick Answer

Most natural males score an FFMI between 18 and 25. An adjusted FFMI above 25 is widely considered the upper limit achievable without performance-enhancing drugs.

These results are estimates based on general formulas and are not a substitute for professional medical advice. Consult a healthcare provider before making health decisions.

Leave blank to estimate lean mass from height, weight, and sex.

Enter your details above to calculate your FFMI.

How the Formula Works

  1. Determine your lean body mass (LBM). If you know your body fat percentage, use the direct formula. Otherwise, the Boer equation estimates LBM from height and weight.

    LBM (kg) = weight (kg) x (1 - body fat % / 100) Boer (males): LBM = 0.407 x weight(kg) + 0.267 x height(cm) - 19.2 Boer (females): LBM = 0.252 x weight(kg) + 0.473 x height(cm) - 48.3

  2. Convert your height to meters and calculate the raw FFMI.

    FFMI = LBM (kg) / height (m)^2

  3. Apply the height-normalisation correction to obtain the Adjusted FFMI.

    Adjusted FFMI = FFMI + 6.1 x (1.8 - height (m))

  4. Compare your Adjusted FFMI to the classification table to see where you fall relative to population norms.

Methodology & Sources

Reviewed and updated April 4, 2026 · Prepared by GetHealthyCalculators Editorial Team

This calculator uses the standard FFMI formula with a height-normalisation correction factor (6.1 x [1.8 - height in meters]). When body fat percentage is not provided, lean body mass is estimated via the Boer (1984) equation. Classification thresholds are based on research by Kouri et al. (1995) and subsequent validation studies.

References

  • Fat-free mass index in users and nonusers of anabolic-androgenic steroids · Clinical Journal of Sport Medicine (Kouri et al., 1995)
  • A new predictive equation for resting energy expenditure in healthy individuals (Boer, 1984) · American Journal of Clinical Nutrition
  • Prevalence of Abnormal Body Composition in Athletes · Sports Medicine

Limitations

  • Body fat percentage input significantly affects accuracy — if your body fat estimate is off, FFMI will be too.
  • When body fat is not entered, the Boer equation provides a rough LBM estimate that may not suit all populations.
  • FFMI thresholds were originally established using male subjects; female-specific norms are less well-defined.
  • The classification table does not account for age, training experience, or genetic variation.

Frequently Asked Questions

What is FFMI and why does it matter?
FFMI (Fat-Free Mass Index) is a way to measure how muscular you are relative to your height. It matters because BMI does not distinguish between fat and muscle, so a muscular person can have a high BMI without being overfat. FFMI gives a more accurate picture of body composition.
What is the difference between FFMI and Adjusted FFMI?
Raw FFMI is simply lean mass divided by height squared. Adjusted FFMI adds a correction factor — 6.1 times (1.8 minus your height in meters) — so that taller and shorter individuals can be compared on a level playing field. The adjusted version is what most references use for classification.
Is an FFMI of 25 really the natural limit?
Research by Kouri et al. found that 25 was the approximate upper limit in drug-tested natural athletes. A few genetically exceptional individuals may slightly exceed it, but scores well above 25 are extremely rare without anabolic assistance.
Can women use this calculator?
Yes. Women can calculate FFMI using the same formula. However, female norms are generally 3-5 points lower than male norms, so the classification table (designed primarily from male data) should be interpreted with that offset in mind.
Do I need to know my body fat percentage?
Not necessarily. If you leave the body fat field blank, the calculator estimates your lean body mass using the Boer equation based on your height, weight, and sex. For the most accurate result, however, entering a measured or DEXA-derived body fat percentage is recommended.
How often should I recalculate my FFMI?
Recalculating every 2-3 months is reasonable if you are actively training. Body composition changes slowly, so more frequent checks are unlikely to show meaningful differences.

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