What is FFMI — and how accurate is it?

BMI cannot tell a bodybuilder from someone who is overweight: both are "heavy for their height". Fat-free mass index (FFMI) fixes that by removing fat from the equation. It divides your lean mass by your height squared, so it rises when you build muscle and ignores fat entirely.

The formula

1. Lean mass = bodyweight × (1 − body-fat %).
2. FFMI = lean mass (kg) ÷ height (m)².

At 1.80 m, 80 kg and 15% body fat, lean mass is 68 kg and FFMI is 68 ÷ 3.24 = 21.0.

Raw FFMI rises slightly with height even when two people are equally muscular. In their 1995 study of steroid users and non-users, Kouri and colleagues introduced a correction to a 1.80 m reference: normalised FFMI = FFMI + 6.1 × (1.8 − height in m). Use the normalised number when you compare yourself with anyone else, or with reference ranges. The FFMI calculator shows both.

What is a good FFMI?

For men, normalised:

• 18–20 — typical untrained adult
• 20–22 — a couple of years of consistent training
• 22–23.5 — well trained
• 23.5–25 — advanced; many years of excellent training
• 25+ — rare without drugs

For women, subtract roughly 3–4 points: around 15–16 is typical, 18–19 well trained, and 20+ exceptional. Research on women is much thinner, so treat those numbers as looser.

Where "25" comes from

Kouri's group measured 157 male athletes. The drug-free lifters topped out just under a normalised FFMI of 25; the steroid users routinely exceeded it. It is a useful ceiling for typical drug-free men — not a law. People with unusual genetics exceed it, and so does anybody whose body-fat estimate is too low.

How accurate is FFMI?

The arithmetic is exact. The input is not. FFMI inherits every error in your body-fat estimate:

• A tape-measure or smart-scale reading is typically within ±3–4 percentage points.
• At 80 kg and 1.80 m, a 4-point body-fat error changes lean mass by 3.2 kg — about 1 FFMI point.

So treat a single FFMI reading as a range of roughly ±1. For tracking yourself, the error largely cancels out if you use the same method, at the same time of day, every time: the trend is reliable even when the absolute number is not.

What FFMI does not tell you

• Where the muscle is. Two people with FFMI 22 can look very different.
• Water and glycogen. A few days of high carbohydrate intake can raise lean mass by a kilogram or more.
• Your ceiling. That depends on frame, genetics and training history — the question the Muscle Potential calculator is built for.

How to use it

Measure body fat consistently, calculate FFMI every 4–8 weeks, and watch the direction. A rising FFMI with a stable waist is muscle being built. A rising FFMI with a growing waist is worth a closer look at the body-fat estimate.