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FFMI

General

Fat-Free Mass Index

A measure of lean muscle mass relative to height, useful for tracking muscle-building progress in a way BMI cannot, since BMI can't distinguish added muscle from added fat.

Definition

Fat-Free Mass Index (FFMI) measures lean muscle mass relative to height, isolating the fat-free component of body weight rather than treating all weight the same way BMI does. This makes it the more relevant metric for athletes, bodybuilders, and anyone tracking muscle-building progress, since an increasing FFMI over time indicates real muscle gain rather than simply gaining weight.

The FFMI Calculator calculates this from your body fat percentage, height, and weight.

Formula

Fat-Free Mass = Weight × (1 − Body Fat Percentage ÷ 100)

FFMI = Fat-Free Mass (kg) ÷ [Height (m)]²

Adjusted FFMI = FFMI + 6.1 × (1.8 − Height in metres)

Worked Example

For a weight of 80 kg, 15% body fat, and height of 1.75 m:

  • Fat-Free Mass = 80 × (1 − 0.15) = 68 kg
  • FFMI = 68 ÷ (1.75)² = 68 ÷ 3.0625 ≈ 22.2
  • Adjusted FFMI = 22.2 + 6.1 × (1.8 − 1.75) ≈ 22.5

This falls within the typical natural range.

Key Things to Know

  • Isolates muscle from total weight: unlike BMI, FFMI specifically tracks lean mass, not total body weight.
  • Requires a body fat percentage estimate: results are only as accurate as the body fat measurement method used (skinfold, bioimpedance, or DEXA).
  • Adjusted version accounts for height: use the height-adjusted formula for more consistent comparisons across different heights.
  • Useful for tracking progress over time: a rising FFMI across months of training indicates genuine muscle gain, which weight alone can't confirm.

Related Calculators

FFMI Calculator →Calculate your Fat-Free Mass Index (FFMI) and normalized FFMI from weight, height, and body fat percentage to assess your muscle mass level accurately.
BMI Calculator →Calculate your BMI instantly using kg/lbs and cm/ft. Includes BMI category and healthy weight range using WHO standards.
One Rep Max Calculator →Estimate your one rep max (1RM) from any sub-maximal lift. Uses both Epley and Brzycki formulas to show your 1RM and training percentages at 90%, 80%, and 70%. Free calculator.

Related Terms

BMIBody Mass IndexBMRBasal Metabolic Rate

Frequently Asked Questions

Most natural, non-enhanced individuals fall between roughly 18 and 25, with values above 25 considered exceptional and often associated with either genetic outliers or the use of performance-enhancing substances.
Total body weight doesn't distinguish muscle from fat, so two people of the same weight and height can have very different amounts of actual lean tissue — FFMI isolates the fat-free component specifically to track muscle mass.
[BMI](/glossary/bmi/) uses total body weight and can't distinguish muscle from fat, while FFMI uses fat-free mass specifically, making it the more relevant metric for tracking muscle-building progress.
The adjusted FFMI formula normalises for height, since taller individuals naturally tend to have somewhat lower raw FFMI values at the same relative muscularity — the adjustment makes comparisons across different heights more consistent.
Body fat percentage is typically estimated using skinfold calipers, bioelectrical impedance scales, or a DEXA scan — the [FFMI Calculator](/ffmi-calculator/) requires this value as an input alongside height and weight.