FFMI calculator
FFMI (fat-free mass index) is like BMI for everything except fat: your lean mass divided by your height squared. It shows how much muscle you carry for your height, which BMI cannot.
FFMI
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Normalised FFMI
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Fat-free mass
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Normalised FFMI adjusts the number to a height of 1.80 m, so tall and short people can be compared. In the 1995 study that introduced it, male athletes who did not use steroids reached a normalised FFMI of up to about 25; the authors called that finding preliminary, so read it as a rough guide, not a limit. FFMI depends on your body fat figure: an estimate from a tape measure is less precise than a body scan.
How to use the FFMI calculator
Choose kilograms and centimetres, or pounds with feet and inches. Enter your weight, your height and your body fat percentage. The calculator shows three figures straight away: your FFMI, your normalised FFMI and your fat-free mass in kg or lb.
The body fat box takes values from 3% to 60%. If you don’t have a figure, work one out with the body fat calculator from your neck, waist and, for women, hip measurements.
How FFMI is worked out
FFMI stands for fat-free mass index. The calculator works it out in three steps.
- Fat-free mass: your weight × (1 − body fat ÷ 100). That is everything except fat, including muscle, bone, organs and water.
- FFMI: fat-free mass in kg ÷ your height in metres, squared.
- Normalised FFMI: FFMI + 6.3 × (1.80 − your height in metres).
The 6.3 correction comes from the 1995 study that introduced the normalised figure. Its authors adjusted every result to the height of a 1.8 m man. At 1.80 m the correction is zero. Below that height it adds a little, and above it, it takes a little away.
Worked examples
The calculator opens at 80 kg (176 lb), 178 cm (5 ft 10 in) and 18% body fat.
- Fat-free mass: 80 × 0.82 = 65.6 kg (144.3 lb).
- FFMI: 65.6 ÷ (1.78 × 1.78) = 65.6 ÷ 3.1684 = 20.7.
- Normalised FFMI: 20.7 + 6.3 × 0.02 = 20.8.
Height makes a bigger difference further from 1.80 m. Take a person of 160 cm and 55 kg with 25% body fat. Their fat-free mass is 41.25 kg and their FFMI is 16.1. The correction adds 6.3 × 0.20 = 1.26, so the normalised FFMI is 17.4.
Now take a person of 190 cm and 90 kg with 15% body fat. Fat-free mass is 76.5 kg and FFMI is 21.2. The correction takes off 0.63, giving a normalised FFMI of 20.6.
What the 25 figure means
The 1995 study measured 157 male athletes. Among the athletes who had never used steroids, normalised FFMI went up to a clear limit of 25.0. Winners of the Mr America contest from before steroids were widely used averaged 25.4. The authors said their findings must be treated as preliminary.
So 25 is a rough reference point for men, drawn from one study. A reading above 25 is not proof that someone has used steroids, and a reading below it says nothing either way. The study included no women, so the figure does not apply to them.
Limits of an FFMI result
FFMI can only be as good as the body fat figure behind it. A tape-measure estimate can be several percentage points out, and every point changes your lean mass. At 80 kg, a body fat reading of 15% instead of 18% adds 2.4 kg of fat-free mass and raises FFMI by about 0.8.
Fat-free mass includes water, bone and organs as well as muscle. FFMI is therefore a measure of lean tissue in general, and only part of it is muscle.
Related tools and guides
The BMI calculator gives the whole-weight version of the same sum. For calories and protein, the TDEE calculator sets both from your weight, height, age and activity. If you fast and train, the 16:8 fasting schedule page shows how to fit that protein into an 8-hour eating window.
Frequently asked
What is a good FFMI?
No official body sets FFMI bands. The best-known figure comes from a 1995 study of 157 male athletes. No athlete in it who had stayed off steroids scored much above 25 on the normalised scale. The authors called the finding preliminary, so treat 25 as a rough marker for men and track your own number over time.
What is the difference between FFMI and normalised FFMI?
FFMI is your lean mass in kg divided by your height in metres squared. Normalised FFMI adds 6.3 × (1.80 − your height in metres), so every result is scaled as if the person stood 1.80 m tall. Tall people get a little taken off and shorter people get a little added, so heights can be compared.
Do I need to know my body fat percentage to use the FFMI calculator?
Yes. FFMI starts from lean mass, which is your weight minus the fat. If you don't know your body fat percentage, get an estimate from the body fat calculator, which needs only a tape measure and a scale. Then type that percentage into the body fat box.
Is FFMI useful for women?
The FFMI sum works for anyone, and a woman can track her own number as she trains. The 25 marker came from a study of men only, so it does not apply to women. No equivalent figure for women was set out in that study.
How is FFMI different from BMI?
BMI divides your whole weight by your height squared, fat included. FFMI does the same sum with lean mass only. A muscular person and a person with more fat can share a BMI and still differ by several FFMI points.