FFMI explained: tracking lean mass over time

FFMI (Fat-Free Mass Index) compares how much lean mass you carry to your height, normalising the ratio so you can compare yourself to others of different statures. It is analogous to BMI, but where BMI uses total weight, FFMI uses lean mass — which is why it is useful for tracking muscle gain and loss.

How FFMI is calculated

Two steps:

1. Lean mass: body weight × (1 − body fat percentage / 100). If you weigh 80 kg at 20% body fat, your lean mass is 64 kg.
2. FFMI: lean mass (kg) / height (m)². At 180 cm (1.8 m), 64 kg lean mass gives FFMI = 64 / 3.24 = 19.75.

The FFMI calculator takes height, weight, and body fat percentage as inputs. It gets the body-fat figure from you — typically from the body fat calculator.

Why height adjustment exists

Raw FFMI has a built-in height bias. Taller people have higher FFMI at the same composition, simply because lean mass scales with height² faster than the FFMI denominator does. The height-adjusted version corrects for this:

Adjusted FFMI = FFMI + 6.1 × (1.8 − height in m)

At 1.8 m (180 cm), the adjustment is zero — the reference height. At 1.7 m (170 cm), it adds 0.61 to the raw FFMI. At 1.9 m (190 cm), it subtracts 0.61. The coefficient and reference height come from the original paper (Schmidt et al., 1995).

What the numbers mean

What the 25 cutoff is not

The oft-cited 25 FFMI "natural limit" was popularised from a single observational study of male bodybuilders. It is not:

• A medical threshold — no clinical guideline uses it.
• A universal cap — it is an average observed in one sample.
• A drug-status test — FFMI is an estimate, not a measurement.

This site reports the number because it is a useful tracking metric, not because it is a ceiling.

Tracking progress with FFMI

FFMI is most useful over time, not at a single point. If your weight is rising but FFMI is rising faster, you are gaining lean mass. If your weight is falling but FFMI is stable or rising, you are losing fat while keeping muscle. If both weight and FFMI are falling, you may be losing lean mass.

For the lean mass figure behind FFMI, the lean body mass calculator shows the raw number that FFMI is built from.

Formulas are estimates, not medical advice.

Sources