FFMI Calculator
FFMI Calculator
What is FFMI?
FFMI, the fat-free mass index, is your lean body mass divided by the square of your height. Lean body mass is everything that is not fat: muscle, bone, water and organs. Where BMI divides total weight by height squared, FFMI divides only the lean part by height squared, so it measures how much lean tissue you carry for your frame, independently of how much fat you carry. Two people can share a BMI and have very different FFMIs, and that difference is exactly what the index exists to show.
How to use this FFMI calculator
Enter your weight in kilograms, your body fat percentage from a measurement you trust, and your height in centimetres, then press Calculate FFMI. The tool gives your normalised FFMI first, because that is the figure published benchmarks use, followed by the raw FFMI, your lean mass and fat mass in kilograms, and your fat mass index. If you work in pounds and inches, convert first: divide pounds by 2.20462 to get kilograms, and multiply inches by 2.54 to get centimetres. Do not guess your body fat percentage if you can avoid it: the whole result inherits that input’s error, as the accuracy section below shows.
A worked example from the live tool
A person weighing 80 kg with 15 percent body fat at 180 cm tall has a normalised FFMI of 20.99 on the live tool. The steps are: fat mass is 80 times 15 divided by 100, which is 12.00 kg; lean mass is 80 minus 12, which is 68.00 kg; height is 1.8 m, and 1.8 squared is 3.24; raw FFMI is 68 divided by 3.24, which is 20.99; the normalisation adds 6.3 times (1.8 minus 1.8), which is zero, so the normalised figure is also 20.99. The fat mass index is 12 divided by 3.24, which is 3.70. A second example, 90 kg at 20 percent body fat and 175 cm, gives lean mass 72.00 kg, raw FFMI 23.51, normalised FFMI 23.83, and fat mass index 5.88, because the height correction adds 6.3 times 0.05, which is 0.315. Both examples are live tool results, and both can be checked by hand from the formula below.
How the maths works
The calculation has three steps. First, split the weight: fat mass = weight times body fat percent divided by 100, and lean mass = weight minus fat mass. Second, divide by height squared: raw FFMI = lean mass in kg divided by height in metres squared, and FMI = fat mass divided by height in metres squared, so the two indices add up to BMI. Check it on the first example: 20.99 plus 3.70 is 24.69, and BMI for 80 kg at 1.8 m is 80 divided by 3.24, which is also 24.69. Third, normalise for height: normalised FFMI = raw FFMI + 6.3 times (1.8 minus height in metres). The correction exists because lean mass scales faster than height squared across a population, so the raw index quietly favours taller people. Normalising to a 1.8 m reference frame puts different heights on one scale, and every published FFMI benchmark is quoted on that normalised scale, which is why comparing a raw figure against a normalised benchmark is an error.
Reading the result: reference bands for normalised FFMI
The bands below are population references for men, drawn from the published literature, and they describe groups, not individuals. There is no equivalent widely quoted band set for women, because the Kouri study and most follow-up work studied male athletes; a woman’s normalised FFMI should not be judged against a men’s band.
| Normalised FFMI (men) | Reading |
|---|---|
| Below 18 | Below average lean mass for the frame |
| 18 to 20 | Average |
| 20 to 22 | Above average |
| 22 to 24 | Well above average |
| 24 to 25 | Superior, near the commonly cited reference |
| Above 25 | Above the Kouri reference for drug-free male athletes |
The Kouri reference deserves care. Kouri and colleagues reported in 1995 that the drug-free male athletes in their study all scored at or below 25 on the normalised scale. That observation became a widely cited ceiling for natural muscular development. It is not a test: a score above 25 does not prove anything about any individual, and a score below 25 does not prove anything either. Training history, measurement error and body composition method all move the figure. Treat 25 as a reference point for a population, not a verdict on a person.
Why FFMI and BMI can disagree
BMI cannot tell muscle from fat, so it misleads in both directions. A muscular person at 90 kg and 175 cm with 20 percent body fat has a BMI of 29.39, which sits in the overweight range, yet a normalised FFMI of 23.83, which is well above average lean mass: the BMI label reflects total weight, while the FFMI reflects what that weight is made of. In the other direction, a person at 65 kg and 165 cm with 25 percent body fat has a BMI of 23.88, inside the usual range, but a normalised FFMI of 18.85, which is average lean mass carried with a higher fat share. Neither index alone describes health, and neither replaces a clinical assessment. Together, FFMI and FMI split BMI into its two parts, which is why the tool reports all three: raw FFMI 17.91 plus FMI 5.97 equals BMI 23.88 in that second example, exactly.
Why your body fat figure dominates the accuracy
FFMI is a derived figure, and a derived figure cannot be more accurate than its inputs. On an 80 kg body, reading 15 percent body fat when the truth is 18 percent overstates lean mass by 2.4 kg and FFMI by about 0.74 points, since 2.4 divided by 3.24 is 0.74. That single error moves the result by nearly a full band in the table above. Measurement methods differ in accuracy: a DEXA scan is usually quoted within 1 to 2 percentage points, professionally measured skinfold calipers are close when done well, and consumer bioelectrical impedance scales commonly carry 3 to 5 points of error, which translates into roughly 0.5 to 1.5 FFMI points. The practical advice follows directly: use the most accurate body fat measurement available, measure the same way each time if you are tracking change over time, and treat small movements in FFMI as noise unless the body fat method is consistent.
Common mistakes
The most common mistake is comparing a raw FFMI against a normalised benchmark, which flatters taller people and shortchanges shorter ones; always use the normalised figure for comparisons. The second is entering a guessed body fat percentage and treating the output as precise. The third is entering height in centimetres where metres are expected, or the reverse, which squares the error: always enter centimetres in this tool. The fourth is treating the Kouri 25 reference as a personal test. The fifth is forgetting that FFMI says nothing about fat on its own: always read it with the fat mass index, since the same FFMI can sit next to very different fat levels.
Limits you should know
This tool calculates FFMI from the figures you enter; it does not estimate body fat, measure lean mass, or assess health, fitness or drug use. It uses the 6.3 normalisation constant; some calculators use 6.1, and for heights between 5 feet and 6 feet 6 inches the two versions differ by less than 0.1 points, so the choice does not change the reading. Reference bands are published for men only, and no band in this article should be applied to women, adolescents, or anyone with a condition affecting body composition. If a health decision depends on the number, check it with a qualified clinician. Results are estimates, not medical advice.
Privacy note
The calculation runs in your browser. The weight, body fat and height you type are not uploaded or stored by this tool. If a result looks wrong, use the Contact page and include the figures you entered, so the error can be repeated.
Related tools
Try the BMI calculator for total weight relative to height, the basal metabolic rate calculator for resting energy use, the percentage calculator for shares and changes, and the calculators hub for the full set.
How do I calculate FFMI?
First find lean mass: weight times (1 minus body fat percent divided by 100). Then divide lean mass in kilograms by height in metres squared for raw FFMI, and add 6.3 times (1.8 minus height in metres) for normalised FFMI.
What is the FFMI of an 80 kg person with 15 percent body fat at 180 cm?
20.99, raw and normalised alike, because the height is exactly the 1.8 m reference. Lean mass is 68.00 kg, fat mass is 12.00 kg, and the fat mass index is 3.70.
What is normalised FFMI, and why does it matter?
It is raw FFMI corrected to a 1.8 m reference height, using FFMI + 6.3 x (1.8 – height in metres). It matters because raw FFMI favours taller people, and every published benchmark is quoted on the normalised scale.
What does an FFMI of 25 mean?
It is the reference reported by Kouri and colleagues in 1995 for drug-free male athletes, all of whom scored at or below 25 on the normalised scale. It is a population reference, not a test that can classify any individual.
Why does my FFMI change when my body fat estimate changes?
Because FFMI is computed from lean mass, which is weight minus fat mass. On an 80 kg body, a 3-point body fat error moves lean mass by 2.4 kg and FFMI by about 0.74 points, so the body fat method sets the accuracy.
Is FFMI better than BMI?
It answers a different question. BMI describes total weight for height, while FFMI describes lean mass for height. A muscular person can have a high BMI and a high FFMI at once, so the two together, with the fat mass index, describe body composition better than either alone.