Fat-free mass index: lean mass in kilograms divided by height in metres squared, normalized to 1.80 m by adding 6.1 × (1.8 − height). It is BMI with the fat taken out — muscle relative to height.

How fast does FFMI go up?

At the muscle-gain rates the projection uses — 0.8–1.2 kg a month new to lifting, 0.15–0.25 kg after three years — and slower the closer you are to the ceiling.

Why does my FFMI look too high?

Almost always because the body-fat figure is too low. Underestimating body fat by five points inflates FFMI by more than a full point.

Free calculators

What is a good FFMI, and what is the natural limit?

Short answer

FFMI is lean mass ÷ height², normalized to 1.80 m. Drug-free athletes top out at about 24–25; an untrained man is typically far below that, and gains slow sharply as you approach it. The calculator gives your index and your distance from the ceiling.

Fat-free mass index is muscle relative to height — BMI with the fat taken out. It is the most honest single number for how much muscle you carry, and for how much more is realistically available to you without drugs.

Units

Weight

Body fat

An honest guess is fine; ±3 points moves FFMI by about 1.Height

Your FFMI

20.2

Natural ceiling: about 24–25

Solidly trained — clear muscle, with room still ahead.

Lean mass

63.8 kg

About 11.9 kg of lean mass between you and an FFMI of 24 — years of training, not months.

An estimate for healthy adults, not medical advice. How every number is made, with its citations: Sources & method.

Lean mass at the ceiling, by height

The same 24–25 range, turned back into kilograms of lean mass for a height. Body weight at that lean mass depends on body fat — at 12% add about an eighth.

Lean mass at the natural FFMI ceiling by height
Height	FFMI 24	FFMI 25
165 cm	63 kg	66 kg
170 cm	68 kg	70 kg
175 cm	73 kg	76 kg
180 cm	78 kg	81 kg
185 cm	83 kg	87 kg
190 cm	89 kg	92 kg

How the number is made

Lean mass is your body weight minus its fat: weight × (1 − body fat %). FFMI is lean mass in kilograms divided by height in metres squared, then normalized to a height of 1.80 m by adding 6.1 × (1.8 − height), so that tall and short lifters can be compared.

What the natural ceiling means

In the study this index comes from, athletes who had never used anabolic steroids topped out at a normalized FFMI of about 25, while users went well beyond it. It is a population finding, not a law — a few people will sit slightly above it — but it is the best evidence-based answer to “how big can I get naturally?”

Gains slow sharply as you approach it. A beginner can add most of a kilogram of muscle a month; someone at an FFMI of 23 is fighting for a fraction of that in a year.

The weak link is body fat

FFMI is only as good as the body-fat figure you put in, and most people guess low. Underestimating body fat by five points inflates FFMI by more than a full point. If your result looks too good, that is the first thing to check.

Questions

See your six-month body. Then go earn it.

Month6 uses this same ceiling to project your lean mass month by month, at a rate set by your training experience — so the six-month body it shows you is one you can actually reach.