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FFMI Calculator (Fat-Free Mass Index)

Calculate raw and height-adjusted FFMI, lean mass and fat mass—and understand the estimate without turning it into a verdict.

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How this calculator works

FFMI relates estimated fat-free mass to height in the same way BMI relates total weight to height. It can add useful context when body weight changes because it separates the calculation into estimated fat and fat-free components.

The result inherits every error in the body-fat estimate. A tape measure, smart scale and skinfold assessment can give different values on the same day, so decimals should not be treated as real biological precision.

Fat-free mass index

Lean mass = weight × (1 − body-fat %). FFMI = lean mass (kg) ÷ height² (m). Height-adjusted FFMI adds 6.3 × (1.80 − height in metres).

How to use the result

Repeat the same body-fat method under similar conditions and compare changes across months. Use gym performance, circumferences and photos alongside the number.

The often-quoted adjusted FFMI value of 25 came from a small historical sample of male athletes. It is not a universal natural limit, a drug-use test or a health category.

Get a more reliable result

Measure height and weight accurately and use the best body-fat estimate available. Because body-fat error flows directly into fat-free mass, use the same method over time. Normalized FFMI adjusts the result toward a reference height of 1.80 m, helping comparisons but not removing measurement error.

Worked example: at 80 kg, 15% body fat and 1.80 m, estimated fat-free mass is 68 kg and FFMI is about 21.0. At exactly 1.80 m the normalized value is also about 21.0. A change smaller than the body-fat method’s error may not be real.

Common errors that change the answer

  • Interpreting all fat-free mass as contractile muscle; it also includes water, organs and bone.
  • Treating the often-cited FFMI 25 observation as a universal natural-limit test.
  • Switching between smart scales, calipers and visual estimates when tracking progress.

What to do next

  • Use a body-fat estimate collected under repeatable conditions.
  • Compare raw FFMI with raw FFMI and adjusted with adjusted.
  • Focus on the trend and training performance, not a single cutoff.

Guides that support this calculator

How to interpret FFMI→Body recomposition guide→

Frequently asked questions

Is FFMI more useful than BMI?

It answers a different question. BMI uses total weight; FFMI uses estimated fat-free mass and therefore depends on a body-fat estimate.

Does FFMI measure muscle mass?

Not exactly. Fat-free mass also includes water, bone, organs and connective tissue.

Can FFMI prove steroid use?

No. No FFMI value can establish drug use in an individual. Genetics, measurement error and training history all matter.

Sources and methodology

Reviewed September 2026

Kouri et al. — Fat-free mass index in athletes ↗
Health note: This calculator provides a general educational estimate for adults. It does not diagnose a condition or replace advice from a doctor or registered dietitian. Seek professional guidance for pregnancy, an eating disorder, a medical condition, medication-related needs, or concerning symptoms.