An FFMI calculator divides your fat-free mass in kilograms by your height in meters squared, a bit like BMI but counting only lean mass. Formula: FFM (kg) = weight × (1 − body fat); FFMI = FFM ÷ height²; normalized FFMI = FFMI + 6.3 × (1.8 − height in m). A 180-pound, 70-inch person with 18% body fat has 67.0 kg of fat-free mass, an FFMI of 21.2 and a normalized FFMI of 21.3.
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How to use the FFMI calculator
- Enter your weight in pounds.
- Enter your height in inches.
- Enter your body fat percentage from a scan, calipers or another method.
The FFMI calculator shows your fat-free mass in kilograms, your FFMI and your normalized FFMI.
What formula does the FFMI calculator use?
Weight is converted to kilograms and height to meters. Fat-free mass is weight times (1 minus body fat). FFMI is fat-free mass divided by height squared. The normalized FFMI adds 6.3 × (1.8 − height in meters), a correction published by Kouri and colleagues in 1995 to adjust values to the height of a 1.8-meter man. Taller people get a small reduction and shorter people a small increase.
FFMI calculator worked example
Results from the FFMI calculator:
| Person | Fat-free mass | FFMI | Normalized FFMI |
|---|---|---|---|
| 180 lb, 70 in, 18% fat | 67.0 kg | 21.2 | 21.3 |
| 160 lb, 66 in, 15% fat | 61.7 kg | 22.0 | 22.7 |
| 200 lb, 74 in, 12% fat | 79.8 kg | 22.6 | 22.1 |
| 140 lb, 64 in, 25% fat | 47.6 kg | 18.0 | 19.1 |
The 74-inch person has the most lean mass but a lower normalized value than raw FFMI, because the correction scales results to 1.8 meters, about 70.9 inches.
Where the “25” figure comes from
The 1995 study measured 157 male athletes, 83 who used anabolic steroids and 74 who did not. Normalized FFMI in the non-users reached a limit of 25.0, and 20 Mr. America winners from 1939 to 1959 averaged 25.4. Many users were above 25, and some above 30. The authors called these findings preliminary. A number above or below 25 says nothing certain about any one person, and the study included men only.
Why body fat accuracy matters
Your result is only as good as the body fat number. A 5-point error in body fat on a 180-pound person changes fat-free mass by about 4 kg and FFMI by more than one point. Use the same method each time if you are tracking change, and compare trends rather than single readings. Our body fat calculator gives a tape-measure estimate you can use as the input.
Tracking FFMI over time
FFMI is most useful as a trend. If you are training, measure body fat the same way, at the same time of day, every few weeks, and log your weight and the FFMI calculator result each time. A rising FFMI with steady or falling body fat suggests you are adding lean mass. Short-term changes in water, food in the gut and the time of your last workout can move a body fat reading by a point or more, so do not read too much into one result.
Because height barely changes in adults, the normalized value moves in step with raw FFMI. For most people, it is enough to watch one of the two numbers.
FFMI compared with BMI
BMI divides total weight by height squared, so it cannot tell muscle from fat. A muscular person and a person with more body fat can share the same BMI. FFMI removes fat from the top of the ratio, so it focuses on lean tissue. The trade-off is that you need a body fat estimate, which adds error. Running the FFMI calculator next to a BMI check gives a fuller picture than either one alone.
What FFMI does not tell you
FFMI is a ratio, not a health score or a medical classification. It does not show strength, fitness, bone density or where fat is stored. If you have questions about your weight, muscle or body composition, talk to your doctor.
For information only. This is not medical advice or a diagnosis; talk to your doctor about your results.
Related converters and calculators
Related calculators:
- Body fat calculator: estimate body fat
- Lean body mass calculator: lean mass
- BMI calculator: body mass index
Sources
Questions about this calculator
What is FFMI?
Fat-free mass index: fat-free mass in kilograms divided by height in meters squared.
How do I calculate FFMI?
Multiply weight in kg by (1 − body fat), then divide by height in meters squared. The FFMI calculator does it for you.
What is normalized FFMI?
FFMI plus 6.3 × (1.8 − height in meters), from Kouri et al., 1995.
Is an FFMI of 25 the natural limit?
The 1995 study found non-user athletes reached up to 25.0, but the authors called it preliminary. It is not a rule for any one person.
Does FFMI apply to women?
The 1995 study measured men only, so its numbers do not carry over directly.
Check the assumptions and units before relying on a result. Financial, tax and health estimates do not replace advice for your individual circumstances.
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