Lean Body Mass and FFMI: How to Calculate Muscularity Beyond BMI
October 21, 2026
Lean body mass equals weight times (1 − body fat fraction). For an 80 kg person at 15% body fat, that is 80 × 0.85 = 68 kg. The fat-free mass index (FFMI) divides lean mass by height squared: 68 ÷ 1.80² = 21.0 kg/m². Unlike BMI, FFMI separates muscle from fat, which makes it useful for people who lift.
Skip the math and get your answer instantly:
Open the Body Fat Calculator →The Formulas
Fat mass = weight × body fat %. Lean mass = weight − fat mass. FFMI = lean mass ÷ height² (in meters). Some versions apply a height adjustment, FFMI + 6.1 × (1.8 − height), to normalize for tall or short people; at 1.80 m the adjustment is zero.
The result depends on the accuracy of the body fat estimate. A 3-point error in body fat shifts lean mass by about 2.4 kg here and FFMI by about 0.7.
| Weight | Body fat | Lean mass | FFMI at 1.80 m |
|---|---|---|---|
| 80 kg | 20% | 64.0 kg | 19.8 |
| 80 kg | 15% | 68.0 kg | 21.0 |
| 80 kg | 10% | 72.0 kg | 22.2 |
| 90 kg | 15% | 76.5 kg | 23.6 |
Interpreting FFMI
Typical values for untrained men are around 18 to 20, and for untrained women around 14 to 17. Regular lifters often reach 21 to 23. A study of male athletes in 1995 suggested that values above about 25 were rare among drug-free individuals, and that has been used as a rough natural limit, though it is not a hard line and body-fat measurement error affects it.
Women have lower typical values because of differences in muscle mass and essential fat.
Using It to Track Progress
If your weight is stable but your waist shrinks and your strength rises, FFMI may be climbing even if BMI is unchanged. Measure body fat with the same method each time, and look at changes over months rather than weeks.
Remember that FFMI includes organs, bone, and water, not just muscle, so it is an estimate of muscularity, not a direct measure.
Frequently Asked Questions
How do I calculate lean body mass?
Multiply total weight by (1 − body fat percentage as a decimal).
What is a good FFMI?
Untrained men are typically 18 to 20, and trained lifters 21 to 23 or higher.
Is FFMI better than BMI?
For muscular people, yes, because it excludes fat mass.
How accurate is FFMI?
As accurate as the body fat estimate behind it, so errors of a few percent are common.