FFMI Calculator

FFMI = lean mass (kg) ÷ height (m)² · normalized FFMI = FFMI + 6.1 × (1.8 − height in metres)

weight

Body weight, in kilograms (kg)

body fat %

A body fat percentage you already have — 15 means 15 per cent

lean mass

Weight minus the fat: weight × (1 − body fat % ÷ 100) (kg)

height

Standing height, in metres (m)

Use it when you have a body fat reading and want to know what it amounts to for your height, or when you are comparing two people of different sizes and BMI keeps telling you the taller one is heavier. It is not a health screen: BMI is the index with published risk bands behind it, and FFMI has none. Read the 25 line as a note about measurement, never as a verdict on a person.

1. 180 cm, 80 kg, 15 per cent body fat

   1. Lean mass: 80 × (1 − 0.15) = 68 kg
   2. Height in metres: 1.80
   3. FFMI: 68 ÷ 1.80² = 21.0
   4. At exactly 1.8 m the correction term is 6.1 × (1.8 − 1.8) = 0, so the normalized column repeats 21.0
   5. The 25 line at this height: 25 × 1.80² = 81 kg of lean mass

   At the reference height the two columns always agree. The correction only moves the number for people who are not 1.8 m tall, which is why a 180 cm reading is the one case where the raw index is already comparable.

2. 195 cm, 95 kg, 20 per cent body fat

   1. Lean mass: 95 × (1 − 0.20) = 76 kg
   2. Height in metres: 1.95
   3. FFMI: 76 ÷ 1.95² = 20.0
   4. Correction: 6.1 × (1.8 − 1.95) = −0.92, so the normalized figure is 19.1
   5. The 25 line at 1.95 m: 25 × 1.95² = 95.1 kg of lean mass

   The same index means more lean mass the taller you are, so the raw column flatters tall athletes and the normalized one is the column to compare. Reaching 25 at 195 cm takes nearly as much lean tissue as most people carry in total.

FFMI is only as good as the body fat percentage you feed it, and every field method for body fat carries an error of several percentage points; a four-point error in that input moves the index by about a full point. The height correction is not a law of nature. Kouri and colleagues fitted it as the slope of a regression line through a small sample of athletes, it applies a straight line to an index that is not linear in height, and it was never validated outside that setting. The 25 figure came from 83 steroid users and 74 non-users measured in the early 1990s, most of them white American men, and the authors themselves called the finding preliminary. Treat it as a screening hint for investigators, not a diagnosis and not a ceiling on anyone's potential. This page measures nothing, cannot tell whether the percentage you entered is accurate, and is not intended for children, adolescents, or pregnancy.

What is a good FFMI?

There is no published healthy range for it, so there is no honest answer to what counts as good. The only figure with research behind it is 25, and that one is a screening threshold for steroid use rather than a goal: in the 1995 study the non-users reached a well-defined limit of about that number while many users went past it. Most men who train recreationally and are not lean land somewhere between 18 and 22, and a leaner physique at the same muscle mass raises the index without a single extra kilogram of tissue.

How is the fat-free mass index different from BMI?

BMI divides total weight by height squared, so fat and muscle count the same and a heavy, lean athlete reads as obese. The fat-free mass index divides only the lean part, which is why a 195 cm and a 170 cm athlete with the same muscularity land on the same number here and on two very different numbers on BMI. The trade is that it needs a body fat percentage, and that percentage is the least reliable input on the page.

Why is FFMI normalized to 1.8 metres?

Because the index is not quite height-independent: taller people carry proportionally more lean mass for the same muscularity, so the raw figure drifts upward with height. Kouri and colleagues drew a regression line through the non-users in their sample and used its slope, 6.1 kg per cubic metre, to shift every reading to the height of a 1.8 m man. The published abstract of that paper says 6.3 instead of 6.1, which is a discrepancy in the source itself; this page uses the 6.1 from the body of the article.

Does a high FFMI mean someone is using steroids?

No. It is a screen that was proposed for situations where a measurement is all an investigator has, and the authors described it as preliminary. A single reading above 25 has innocent explanations — an inaccurate body fat estimate, a very lean and well-trained lifter, a height at the extreme of the range, and the correction term's own error — and most of those errors push the number up rather than down. Accusing anyone on the strength of one index is not what the study supports.

Which body fat percentage should I enter?

The one you trust most, and the same method every time you come back. Calipers, a tape measure, a smart scale and a DEXA scan do not agree with each other, and they do not have to: what makes the number useful is that the method is fixed, so a change over months means something. If you have no measurement at all, use the body fat calculator or the Navy tape method first and bring the result here.

• Fat-free mass index in users and nonusers of anabolic-androgenic steroids — Kouri EM, Pope HG Jr, Katz DL, Oliva P — Clinical Journal of Sport Medicine, 1995
• Body Mass Index (BMI) — healthy weight — US Centers for Disease Control and Prevention