Body mass index cannot tell a lean 200 lb man from a heavy one, because it only knows total weight. Fat-free mass index divides lean mass by height squared instead of total mass. That turns it into a measure of how much muscle, bone, and organ tissue you carry for your frame.
The calculator below returns FFMI, normalised FFMI, lean body mass, fat mass, and waist-to-height ratio from one set of inputs.
Subscribe for more
Join our newsletter to get the latest news, tips, and stories from Opt Health.
FFMI and body composition calculator
Enter height, weight, and body-fat percentage. The tool returns fat-free mass index, normalised FFMI, lean body mass, fat mass, and waist-to-height ratio if you add a waist measurement.
Fat-free mass index
…
Normalised FFMI
…
Lean body mass
…
Fat mass
…
Waist-to-height ratio
…
Body mass index
…
Every figure here is only as good as the body-fat percentage you put in. Consumer bioimpedance scales carry errors of several percentage points against reference methods.1 Use these to track your own trend. They do not diagnose anything.
What each number is
Fat-free mass index
FFMI is your fat-free mass in kilograms divided by your height in metres squared. It was introduced as a body-composition index that separates lean tissue from fat, which BMI cannot do.2 Because it is scaled to height, it lets you compare your own lean mass across time even as your weight changes.
Normalised FFMI
Raw FFMI is slightly biased by height, so taller people score marginally lower for the same build. Normalised FFMI corrects the figure to a 1.8 m reference height using the height adjustment published by Kouri and colleagues.3 If you are near 5 ft 11 in, the two numbers will be almost identical.
Lean body mass and fat mass
Lean body mass is your weight minus your fat mass. It is not the same as muscle mass, because it also contains bone, organs, connective tissue, and the water held in all of them. That distinction matters when you interpret a change: several pounds of lean mass gained in the first two weeks of creatine supplementation is water held in muscle. It is not new tissue.4
Waist-to-height ratio
Waist circumference divided by height. It captures central adiposity, which BMI misses entirely, and it has performed as well as or better than BMI at identifying cardiometabolic risk across a range of populations.5 A ratio of 0.5 is the widely used screening threshold, and the phrase attached to it is that your waist should be less than half your height.5
Practical takeaway: track FFMI and waist-to-height together. Rising FFMI with a flat or falling waist ratio is the direction that actually reflects a better body composition. Rising weight alone tells you nothing about which tissue it went to.
Getting a usable body-fat percentage
Every output here inherits the accuracy of the body-fat figure you enter, and the methods differ a great deal.
| Method | Typical error vs reference | Cost and access | Best used for |
|---|---|---|---|
| DEXA scan | Small. Commonly used as the reference in research | $50 to $150, imaging centre or gym | A trustworthy baseline and a periodic re-check |
| Air displacement (Bod Pod) | Close to DEXA in most adults | $40 to $80, university or clinic | An alternative baseline where DEXA is not available |
| Skinfold calipers | Depends heavily on the tester | Low, needs a trained practitioner | Repeat measurements by the same person |
| Bioimpedance scale | Several percentage points, and shifts with hydration1 | Low, at home | Trends only, measured under identical conditions |
| Visual estimate | Wide | Free | A rough starting input, nothing more |
If you use a home scale, take the reading first thing in the morning, before food or fluid, at the same point in the week. Hydration status alone moves bioimpedance readings by more than a real week of training will.1
What a body-fat error does to the number
Because every output here is computed from the body-fat percentage you enter, it is worth seeing how far a typical measurement error moves the result. Take the same 185 lb, 5 ft 10 in man at three different body-fat readings.
| Body fat entered | Lean body mass | FFMI |
|---|---|---|
| 15% | 157 lb | 22.6 |
| 18% | 152 lb | 21.8 |
| 21% | 146 lb | 21.0 |
A three-point error in either direction moves FFMI by about 0.8 and lean mass by roughly 5 lb. Consumer bioimpedance readings differ from reference methods by around that much, and they also shift with hydration.1 A year of good training might add one to two FFMI points, so a measurement error of this size can hide or invent most of a year's progress. The fix is not a better formula. It is one accurate baseline and a consistent method afterwards.
What FFMI does not tell you
There are two limits.
The first is that FFMI has been discussed as a way to infer anabolic steroid use, based on a 1995 analysis of bodybuilders.3 That inference does not survive contact with an individual. The distributions overlap, the original sample was small and specific, and the number says nothing about how any one person trained, ate, or what they took. This page does not publish those thresholds, because a body-composition figure is not a drug test and reading it as one is a mistake in both directions.
The second is that FFMI cannot distinguish muscle from the rest of your lean tissue. Someone who is retaining fluid, whether from sodium, from a hard week of training, or from a medical cause, will read higher without having gained anything useful.
Reading a change over 12 weeks
The number only earns its keep when you have two of them. A realistic 12-week block of consistent training and adequate protein moves lean mass by a few pounds at most in a trained man, and rather more in someone new to lifting. Against that, a scale reading taken at a different time of day or a different hydration state can move the printed figure by a similar amount, which is why the interpretation matters as much as the measurement.
| What you see after 12 weeks | What it usually means |
|---|---|
| Weight up, FFMI up, waist ratio flat | Lean mass added. The outcome you were training for |
| Weight up, FFMI flat, waist ratio up | The surplus was too large and most of the gain is fat |
| Weight down, FFMI flat, waist ratio down | Fat lost with lean mass held. The outcome a good cut produces |
| Weight down, FFMI down | Lean mass is going with the fat. Raise protein or shrink the deficit |
| Everything flat | Either the stimulus or the nutrition is not there, or the measurement is too noisy to tell |
Body composition and hormones
Lean mass and fat mass are not independent of your endocrine system. Body composition that moves the wrong way on a correct programme is worth investigating for that reason. Testosterone is a direct determinant of how much lean mass you carry, and testosterone replacement in men with confirmed hypogonadism increases lean mass and reduces fat mass.6 The relationship also runs backwards: adipose tissue expresses aromatase, which converts testosterone to oestradiol, so a higher fat mass tends to lower testosterone and lower testosterone tends to raise fat mass.7
Growth hormone and thyroid status affect the same picture through different mechanisms. None of this means a body-composition plateau is hormonal. It means that a plateau which persists through correct calorie and macro targets, adequate protein, and consistent resistance training is a reason to look at a panel rather than to cut calories further.
Frequently asked questions
Use your own baseline rather than a category. A rise of one to two points over a year of consistent training is a real change in lean tissue, and that comparison is only meaningful if you measure body fat the same way each time.
For anyone who trains, yes, because BMI counts muscle and fat identically and misclassifies muscular men as overweight. For population-level screening BMI remains useful, since it needs no body-fat measurement.
Multiply your weight by one minus your body-fat percentage. At 185 lb and 18 percent body fat, lean body mass is about 152 lb.
Below 0.5 is the common screening threshold, which is a waist less than half your height.5 It is a screening cut-off rather than a target, and it is more informative than BMI about where fat is stored.
Every 8 to 12 weeks. Body composition changes slowly enough that more frequent measurement mostly captures measurement error and fluid shifts.
The bottom line
Use FFMI to track lean mass against your own baseline, and waist-to-height alongside it to see where any weight change is going. Get one accurate body-fat reading to anchor the series, then keep the method consistent.
When the training is right and the composition is not
Lean mass falling on a programme that used to build it is the pattern worth investigating, and no body-composition figure can tell you why. Opt Health measures the hormones that determine how much lean mass you hold and gives you a physician to interpret them. Become a member.
References
- Achamrah N, Colange G, Delay J, et al. Comparison of body composition assessment by DXA and BIA according to the body mass index: a retrospective study on 3655 measures. PLoS One. 2018;13(7):e0200465.
- 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;52(6):953-959.
- 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;5(4):223-228.
- Kreider RB, Kalman DS, Antonio J, et al. International Society of Sports Nutrition position stand: safety and efficacy of creatine supplementation in exercise, sport, and medicine. J Int Soc Sports Nutr. 2017;14:18.
- Ashwell M, Gunn P, Gibson S. Waist-to-height ratio is a better screening tool than waist circumference and BMI for adult cardiometabolic risk factors: systematic review and meta-analysis. Obes Rev. 2012;13(3):275-286.
- Corona G, Giagulli VA, Maseroli E, et al. Testosterone supplementation and body composition: results from a meta-analysis of observational studies. J Endocrinol Invest. 2016;39(9):967-981.
- Kelly DM, Jones TH. Testosterone and obesity. Obes Rev. 2015;16(7):581-606.
You might also like
Start Today
Your health, your terms. Discover how personalized care can transform not just the way you feel, but how you live.