FFMI Calculator — Fat-Free Mass Index

Background.

Two men each stand 175 cm and weigh 82 kg. Both have a BMI of 26.8, which the standard bands call overweight. One is 14% body fat and lifts weights four days a week; the other is 30% fat and sits at a desk. BMI cannot tell them apart, because it divides total weight by height squared and has no idea what the weight is made of.

The fat-free mass index does the same division on the lean part only. Take away the fat, divide what is left by height in metres squared, and the first man scores 23.03 kg/m² while the second scores 18.74. Do the same with the fat alone and you get the fat mass index, 3.75 against 8.03. The two indices always add up to BMI, so between them they show how that single BMI figure is divided.

VanItallie and colleagues proposed the pair in 1990 to solve a problem with body-fat percentages: fat and lean percentages are locked together, so losing fat automatically raises the lean share even when no lean tissue was gained. Dividing each compartment by height squared instead gives two numbers that can move independently.

In gyms, FFMI is better known for a second number. In 1995 Kouri and colleagues added a height correction to put everyone on the scale of a 1.80 m man, and reported that drug-free athletes in their study topped out at a normalised FFMI of 25.0. That figure is now widely repeated as a natural limit. The evidence for it is thinner than its fame suggests, and you will find the counter-evidence below.

An FFMI is a screening index, not a diagnosis. Its reference values differ by sex, age and ethnicity, and it inherits every error in the body-fat percentage you supply.

What is ffmi calculator?

The fat-free mass index is fat-free mass in kilograms divided by height in metres squared, in kg/m². Fat-free mass is everything in the body that is not fat: skeletal muscle, but also bone, organs, skin, connective tissue and a great deal of water, which makes up roughly 70 to 75% of it. FFMI is therefore not a muscle score, though for a healthy adult a rise over months usually reflects added muscle. Its companion, the fat mass index, divides fat mass by the same height squared, and because the two compartments add up to total weight, FFMI + FMI = BMI exactly. Clinical nutrition uses FFMI to identify low lean mass in malnutrition; sport and fitness use it, usually height-normalised, to compare muscularity between people of different heights. The two uses rest on different reference data and should not be mixed.

How to use this calculator.

1. 01Choose your sex. It does not change any arithmetic; it selects the reference values quoted beside the result, Schutz 2002 medians and ESPEN 2015 cut-offs.
2. 02Enter your weight in kilograms or pounds, taken at a consistent time of day, ideally in the morning before eating.
3. 03Enter your height in centimetres or inches, measured rather than remembered. Height is squared, so a 1 cm error moves FFMI by about 1%.
4. 04Enter a body-fat percentage you have actually measured, as a percentage between 1 and 70 (14, not 0.14). DEXA is the most precise of the common methods and skinfolds taken by a trained tester come next; tape-measure formulas and home bioimpedance scales carry larger errors.
5. 05Read raw FFMI first, then the normalised figure if you want to compare yourself with people of different heights. Check that FFMI plus FMI equals the BMI shown; if it does not, an input was mistyped.

The formula.

The body is split first: fat mass = weight × body-fat % ÷ 100, and fat-free mass = weight − fat mass. Pounds are converted at exactly 0.45359237 kg and inches at exactly 2.54 cm before anything else. Each mass is then divided by height in metres squared: FFMI = FFM ÷ h² and FMI = FM ÷ h². Because FFM + FM is total weight, FFMI + FMI = weight ÷ h², which is BMI, so the three numbers always reconcile. The normalised value adds Kouri's correction: normalised FFMI = FFMI + 6.3 × (1.80 − h). The adjustment is positive for anyone shorter than 1.80 m, negative for anyone taller and zero at exactly 1.80 m. It exists because lean mass does not grow quite as fast as height squared, so raw FFMI tends to run higher in tall people of the same build; the slope of 6.3 was fitted in men. Some other calculators use 6.1 instead, which changes the result by only 0.02 for someone 10 cm from the reference height. A 2024 study of collegiate American football players re-fitted the correction as −3.7 × (1.81 − h), the opposite direction, which is a good reason to treat the normalised value as a comparison convention rather than a measurement. The reference line is judged before display rounding. The ESPEN and Schutz steps use raw FFMI, the index they were built on; Kouri's step uses the normalised value, because that is the index he stated 25.0 for. For men it reads below the ESPEN cut-off under a raw 17 kg/m², below the Schutz median from 17 up to 18.9, at or above the median from 18.9 until the normalised FFMI reaches 25, and at or above Kouri's contested 25.0 from there up. For women the steps are 15, 15.4 and 20; the last is not a published threshold, only the point above which the line notes that no female ceiling has been published. Each cut-off belongs to the band above it, matching ESPEN's wording of FFMI below 17, so exactly 17.0 is not below the cut-off. Because the 25.0 step is tested on the normalised index, a short man can reach it with a raw FFMI a little under 25 and a tall man can pass 25 raw and still read below it.

A worked example.

Frequently asked questions.

References& sources.

1. [1]VanItallie TB, Yang MU, Heymsfield SB, Funk RC, Boileau RA. "Height-normalized indices of the body's fat-free mass and fat mass: potentially useful indicators of nutritional status." Am J Clin Nutr. 1990 Dec;52(6):953–959. PMID 2239792. Original definition of FFMI = FFM (kg)/height (m)² and BFMI = fat mass (kg)/height (m)². Abstract open access; full text paywalled.
2. [2]Kouri EM, Pope HG Jr, Katz DL, Oliva P. "Fat-free mass index in users and nonusers of anabolic-androgenic steroids." Clin J Sport Med. 1995 Oct;5(4):223–228. PMID 7496846. Source of the height normalisation 6.3 × (1.80 m − height), the 25.0 figure in 74 non-users, and the pre-steroid-era Mr America mean of 25.4. Abstract open access; publisher full text paywalled.
3. [3]Schutz Y, Kyle UUG, Pichard C. "Fat-free mass index and fat mass index percentiles in Caucasians aged 18–98 y." Int J Obes. 2002;26(7):953–960. PMID 12080449. Reference medians used by this page: FFMI 18.9 kg/m² (men) and 15.4 kg/m² (women) aged 18–34, n = 5,635. Abstract open access; full text paywalled.
4. [4]Cederholm T, Bosaeus I, Barazzoni R, et al. "Diagnostic criteria for malnutrition — An ESPEN Consensus Statement." Clin Nutr. 2015 Jun;34(3):335–340. PMID 25799486. Source of the low-fat-free-mass cut-offs FFMI <17 kg/m² (men) and <15 kg/m² (women), and of the >10 % / >5 %-over-3-months weight-loss criteria quoted on this page. Abstract open access; full text paywalled.
5. [5]Fields JB, Jones MT, Kuhlman NM, et al. "Fat-Free Mass Index in a Large Sample of Collegiate American Football Athletes." Int J Exerc Sci. 2024;17(4):129–139. PMC11042886. Independent authority consulted as a second check: reports mean FFMI 23.6 ± 2.0 and a maximum of 27.7 kg·m⁻², re-fits the height adjustment as −3.7 × (1.81 − height), and states that the 25 kg·m⁻² limit underestimates the values it observed. Open access.
6. [6]Loenneke JP, Wilson JM, Wray ME, et al. "The Estimation of the Fat Free Mass Index in Athletes." Asian J Sports Med. 2012;3(3):200–203. PMC3445648. Uses the unnormalised index FFMI = FFM (kg)/m² and reports DEXA-derived athlete values of 21.1 (men) and 17.5 (women) kg/m². Open access.
7. [7]National Institute of Standards and Technology. "Guide for the Use of the International System of Units (SI)," NIST Special Publication 811, 2008 edition, Appendix B.9. Source of the exact conversions used for pound-to-kilogram (0.45359237) and inch-to-centimetre (2.54).