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FFMI Calculator: How Much Muscle for Your Height
Fat-free mass index tells you how much lean mass you carry for your height. Enter height, weight and body fat, then try a recomposition goal.
Short answer
FFMI is lean mass in kilograms divided by height in metres squared. The median for adults aged 18–34 is about 18.9 for men and 15.4 for women. Drug-free male athletes in the original FFMI study topped out at a normalised 25.0. An 80 kg (176 lb), 178 cm (5 ft 10 in) man at 15% body fat has an FFMI of about 21.5.
BMI treats every kilogram the same, which is why a lean, muscular person and an overweight sedentary one can share a number. FFMI fixes that by counting only the fat-free part of your weight.
It is most useful for people who lift: a way to track whether you are adding muscle rather than just weight, and to set a realistic recomposition goal.
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FFMI Calculator
Fat-free mass index: how much lean mass you carry for your height, and what a body recomposition goal would weigh.
No body-fat figure? Estimate one with the body fat calculator first.
What your number means
Recomposition target
Keeping your 68 kg of lean mass, 12% body fat is a scale weight of 77.3 kg. Building muscle on the way moves that number up; it does not mean the plan failed.
An FFMI is only as accurate as the body fat behind it.
SlimAI's AI Body Assessment estimates body fat, fat mass and lean mass from two photos, and charts the trend across scans, so your FFMI can follow real numbers instead of a guess. Full results are part of SlimAI Pro.
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FFMI is only as good as the body-fat figure behind it — every 2 points of body-fat error moves FFMI by about half a point. The 25 ceiling was measured in male athletes; there is no equivalent published cut-off for women. Not medical advice.
Reading Your FFMI
There are no official categories, but two reference points make the number meaningful.
18.9 men · 15.4 women
Median FFMI for adults aged 18–34 in a study of 5,635 people. Near this number means typical lean mass for your height.
Above the median
Each kilogram of lean mass adds about 0.3 to FFMI at 1.78 m. Years of consistent training typically move men several points above the median.
25 (men)
Drug-free male athletes in Kouri’s 1995 study topped out at a normalised 25.0. A higher reading usually means the body-fat figure is too low.
Lean body mass and FFMI: why they beat BMI for people who lift
BMI divides weight by height squared. FFMI divides lean mass by height squared. The only difference is removing fat from the top of the fraction, and that one change is what makes it useful for anyone carrying muscle.
A 90 kg, 180 cm man has a BMI of 27.8 whether he is 12% or 30% body fat. His FFMI is 24.4 at 12% and 19.4 at 30%. One number calls both of them overweight; the other tells them apart.
Your FFMI is only as good as your body-fat number
SlimAI’s AI Body Assessment estimates body fat, fat mass and lean mass from two photos and charts the trend across scans. Full results are part of SlimAI Pro.
3-day free trial · Cancel anytime · No card charged during trial
The 25 ceiling, and what it does and does not mean
Kouri and colleagues measured 157 male athletes in 1995, 83 of whom used anabolic steroids. The non-users’ normalised FFMI extended up to a well-defined limit of 25.0, and a group of Mr. America winners from before steroids were available averaged 25.4.
That makes 25 a reasonable upper reference for drug-free men, not a rule. A few people exceed it naturally, and a reading above it far more often means the body-fat estimate is a few points too low. The study included no women, so it says nothing about a female ceiling.
Using FFMI for a recomposition goal
Body recomposition means losing fat and keeping or gaining muscle at the same time. Holding lean mass constant, you can work out the scale weight at any body-fat target: lean mass divided by one minus the target percentage.
If you build muscle along the way, that goal weight moves up, and your FFMI rises with it. That is the point of tracking both: a heavier scale at a lower body fat is progress, not failure.
FFMI by Lean Mass and Height
Raw FFMI for common lean masses. Normalised FFMI adds about 0.9 at 165 cm and takes off about 0.3 at 185 cm.
| Lean mass | 165 cm (5 ft 5 in) | 175 cm (5 ft 9 in) | 185 cm (6 ft 1 in) |
|---|---|---|---|
| 50 kg (110 lb) | 18.4 | 16.3 | 14.6 |
| 55 kg (121 lb) | 20.2 | 18.0 | 16.1 |
| 60 kg (132 lb) | 22.0 | 19.6 | 17.5 |
| 65 kg (143 lb) | 23.9 | 21.2 | 19.0 |
| 70 kg (154 lb) | 25.7 | 22.9 | 20.5 |
| 75 kg (165 lb) | 27.5 | 24.5 | 21.9 |
Lean mass = weight × (1 − body fat %). Normalised FFMI adds 6.3 × (1.8 − height in metres).
How to Calculate Your FFMI
You need three numbers: height, weight and a body-fat estimate.
- 1
Estimate your body fat
A DEXA scan is best, a tape-measure method is fine. Use the same method every time you recalculate.
- 2
Work out lean mass
Weight × (1 − body fat). 80 kg at 15% body fat is 68 kg of lean mass.
- 3
Divide by height squared
68 ÷ (1.78 × 1.78) = 21.5. That is your FFMI.
- 4
Normalise for height
Add 6.3 × (1.8 − 1.78) = 0.13, for a normalised FFMI of about 21.6.
- 5
Compare to yourself, then to the references
Your FFMI three months from now matters more than any chart.
The Formulas Behind This Calculator
Computed in your browser. Nothing is uploaded and nothing is stored between visits.
Lean mass
lean (kg) = weight × (1 − bodyFat% ÷ 100)Everything that is not fat: muscle, bone, organs and water.
FFMI
FFMI = lean (kg) ÷ height (m)²Same shape as BMI, with fat removed.
Normalised FFMI
normalised = FFMI + 6.3 × (1.8 − height m)Kouri et al.’s correction to a 1.8 m man. Many calculators use 6.1; the paper uses 6.3.
Recomposition goal weight
goalWeight = lean ÷ (1 − targetBodyFat% ÷ 100)Holds lean mass constant. Muscle gained along the way raises it.
Sources and method
- Kouri EM et al. (1995) — Clinical Journal of Sport Medicine — FFMI in 157 male athletes, 83 anabolic-steroid users and 74 non-users. Non-users’ normalised FFMI extended up to a well-defined limit of 25.0; pre-steroid-era Mr. America winners averaged 25.4. Normalisation: + 6.3 × (1.80 − height).
- Schutz Y et al. (2002) — International Journal of Obesity — Reference values from 5,635 healthy adults: median FFMI at 18–34 years of 18.9 for men and 15.4 for women, and median fat mass index of 4.0 and 5.5.
FAQ
Still Wondering About SlimAI
What is FFMI?
What is FFMI?Fat-free mass index: your lean body mass in kilograms divided by your height in metres squared. It works like BMI but counts only the fat-free part of your weight, so it reflects muscle rather than total size.
How do I calculate lean body mass?
How do I calculate lean body mass?Multiply your weight by one minus your body-fat percentage. At 80 kg (176 lb) and 15% body fat, lean body mass is 80 × 0.85 = 68 kg (150 lb). Lean mass includes muscle, bone, organs and water, so it is not the same as muscle mass, but it is the figure FFMI is built on.
What is a good FFMI for a man?
What is a good FFMI for a man?The median for men aged 18–34 is about 18.9. Several points above that indicates a lot of muscle for your height. Drug-free male athletes in the original FFMI study topped out at a normalised 25.0.
What is a good FFMI for a woman?
What is a good FFMI for a woman?The median for women aged 18–34 is about 15.4. There is no published natural ceiling for women: the well-known 25 figure comes from a study of men only, so compare against your own past readings instead.
Is an FFMI over 25 possible naturally?
Is an FFMI over 25 possible naturally?Rarely. In Kouri’s study of male athletes, non-users topped out at 25.0. A reading above 25 far more often means the body-fat estimate is too low — a 4-point underestimate adds about one point of FFMI.
What is the difference between FFMI and normalised FFMI?
What is the difference between FFMI and normalised FFMI?Normalised FFMI adjusts the number to what it would be at 1.8 m tall, so short and tall people can be compared. The correction is small: 6.3 × (1.8 − your height in metres).
How do I calculate body recomposition?
How do I calculate body recomposition?Hold your lean mass constant and divide it by one minus your target body fat. With 68 kg of lean mass, 12% body fat is a scale weight of about 77.3 kg. If you gain muscle along the way, the target weight rises.
Can SlimAI estimate my lean mass?
Can SlimAI estimate my lean mass?Yes. SlimAI’s AI Body Assessment estimates your body-fat percentage, fat mass and lean mass from two photos, and charts the trend across scans. Full results are part of SlimAI Pro.
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