FFMI calculator (fat-free mass index)
Enter your height, weight and body fat percentage. You get your lean mass and fat mass in kilos, your fat-free mass index (FFMI), the height-normalised FFMI used in lifting circles, and an honest read on what those numbers do and don't tell you.
Don't know your body fat? Estimate it with a tape in two minutes using the body fat calculator, then come back.
FFMI is only as good as the body fat number you put in, and it describes size, not health. Reference values come from Swiss adults measured by bioimpedance (Schutz et al., 2002) and from 157 male athletes (Kouri et al., 1995); they are comparisons, not targets.
Fat-free mass index (FFMI) is your lean mass in kilograms divided by your height in metres squared. It works like BMI but strips out fat, so it tells you how much muscle, bone, organs and water you carry for your height. A 175 cm, 75 kg man at 18% body fat has about 61.5 kg of lean mass and an FFMI of about 20.1.
How FFMI is calculated
The calculator runs four steps:
- Fat mass = weight × body fat % ÷ 100
- Lean (fat-free) mass = weight − fat mass
- FFMI = lean mass ÷ height² (kg/m²)
- Normalised FFMI = FFMI + 6.3 × (1.8 − height in m)
The first three are plain arithmetic, and FFMI is defined this way in both Kouri et al. (1995) and Schutz et al. (2002). The fourth step exists because taller people tend to have a slightly higher FFMI for the same build. Kouri and colleagues added a correction that scales everyone to a 1.80 m man, so a 165 cm lifter and a 185 cm lifter can be compared fairly.
One note on the formula: the Kouri paper's published abstract gives the height correction as 6.3, which is what this calculator uses. Many FFMI calculators and articles use 6.1 instead. For anyone between about 150 and 200 cm the two give results within 0.06 of each other, so it makes no practical difference.
Worked example
You're a man, 175 cm tall, 75 kg, with a body fat estimate of 18%.
- Fat mass = 75 × 0.18 = 13.5 kg
- Lean mass = 75 − 13.5 = 61.5 kg
- FFMI = 61.5 ÷ (1.75 × 1.75) = 61.5 ÷ 3.0625 = 20.1
- Normalised FFMI = 20.1 + 6.3 × (1.80 − 1.75) = 20.1 + 0.3 = 20.4
The calculator also shows fat mass index (fat mass ÷ height²), the other half of the picture.
How to read your number
There are only two sets of published reference points the calculator uses, and both have limits.
| Reference | Who was measured | Value |
|---|---|---|
| Schutz et al., 2002 | 5,635 healthy Swiss adults, bioimpedance | Median FFMI at 18 to 34 years: 18.9 men, 15.4 women |
| Schutz et al., 2002 | Same group | Median fat mass index: 4.0 men, 5.5 women |
| Kouri et al., 1995 | 157 male athletes: 74 drug-free, 83 steroid users | Normalised FFMI of drug-free athletes went up to 25.0 |
The median is the middle value of a general, non-athletic population: half were above, half below. Being above it means you carry more lean mass for your height than the typical young Swiss adult. Indian adults weren't in that study and body composition differs between populations, so treat it as a rough reference point, not a norm for you.
The 25 figure is the one gym talk turns into a "natural limit". What the paper actually says: in its sample of drug-free male athletes, normalised FFMI extended up to 25.0, while many steroid users went well past it and some exceeded 30. It also estimated that 20 Mr. America winners from 1939 to 1959, before steroids, averaged 25.4. The authors called their findings preliminary and suggested FFMI as an initial screening measure, not proof of anything. So a normalised FFMI above 25 means you're beyond what that one group reached. It doesn't mean you're using drugs, and staying under 25 isn't a ceiling you're barred from. Kouri studied men only, which is why this calculator gives women no equivalent line.
Why the body fat number matters so much
FFMI is calculated from your body fat percentage, so any error there flows straight through. Each percentage point of body fat you get wrong shifts FFMI by your weight ÷ height² ÷ 100, which is simply your BMI ÷ 100. At a BMI of 25, a 4-point error in body fat moves FFMI by a full point.
Tape-measure formulas, calipers and bathroom smart scales each make different errors, and the same person can read several points apart on different methods. That's why the result panel shows a range for a 3-point error. If you estimate body fat with the body fat calculator, use the same method every time and watch the trend. A sudden jump in FFMI over a month usually means the body fat reading changed, not that you added 3 kg of muscle.
What FFMI is (and isn't) good for
FFMI is useful for lifters tracking whether a bulk added lean mass or mostly fat, and as a sanity check on claims you see online. Clinically, Schutz and colleagues saw value in it for spotting a deficit of fat-free mass, such as muscle loss in older adults.
It is not a health score. A higher FFMI doesn't mean better blood pressure, blood sugar or heart health, and chasing a number like 25 is a poor goal on its own. For a health check, waist size and the BMI calculator with Asian-Indian cut-offs say more. To build lean mass, the levers are dull and reliable: progressive training (the one-rep max calculator helps set loads), enough protein from the protein calculator, and a small calorie surplus.
If you're under 18, pregnant, or have a medical condition affecting weight or fluid balance, these numbers aren't meant for you; speak to a doctor.
Useful links
- Kouri EM, Pope HG, Katz DL, Oliva P. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clin J Sport Med, 1995
- Schutz Y, Kyle UU, Pichard C. Fat-free mass index and fat mass index percentiles in Caucasians aged 18–98 y. Int J Obes, 2002
This calculator is for general information and isn't medical advice.
Frequently asked questions
What is a good FFMI?
There's no official healthy range. For comparison, the median FFMI among Swiss adults aged 18 to 34 was 18.9 for men and 15.4 for women (Schutz et al., 2002). Lifters who train for years usually sit well above that.
Is an FFMI of 25 the natural limit?
Not exactly. In Kouri et al. (1995), the normalised FFMI of 74 drug-free male athletes went up to 25.0, while many steroid users exceeded it. It was a small, preliminary study, and pre-steroid-era Mr. America winners averaged 25.4, so it's a reference point, not a hard limit.
What is normalised FFMI?
It's FFMI adjusted to a height of 1.80 m: FFMI + 6.3 × (1.8 − height in metres). Taller people naturally score a little higher, so the correction lets people of different heights be compared.
Does FFMI work for women?
The formula works the same way, but the reference data are thinner. The 25 cut-off came from men only. For women, the published median from Schutz et al. (2002) is 15.4 at ages 18 to 34.
Why did my FFMI change so much in a month?
Almost always because the body fat estimate changed. Each percentage point of body fat error moves FFMI by about BMI ÷ 100, so a few points on a smart scale can look like kilos of new muscle.
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