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- FFMI (fat-free mass index) is like BMI but uses only your lean mass: lean mass in kg ÷ height in meters squared.
- Lean mass = weight × (1 − body fat %), so you need a body fat estimate.
- Normalized FFMI adjusts the result to a height of 1.8 m: FFMI + 6.1 × (1.8 − height).
- In a 1995 study, men who did not use steroids rarely had a normalized FFMI above about 25.
How to use the FFMI calculator
Choose sex and units
US units (feet, inches, pounds) or metric (centimeters, kilograms).
Enter height and weight
Use a recent weight.
Enter body fat
Use a body fat percentage from a scan, calipers or the body fat calculator.
Read your FFMI
You get FFMI, normalized FFMI, lean mass and fat mass, with every step shown.
FFMI formula
Fat-free mass index
Lean mass (kg) = weight (kg) × (1 − body fat %)
FFMI = lean mass (kg) ÷ height (m)²
Normalized FFMI = FFMI + 6.1 × (1.8 − height in m)
The normalization comes from Kouri and colleagues (1995). It makes tall and short people easier to compare.
How to calculate FFMI
Find your lean mass
A 180 lb (81.6 kg) man at 15% body fat has 81.6 × 0.85 = 69.4 kg of lean mass.
Square your height
5 ft 10 in is 1.778 m, and 1.778² = 3.161.
Divide
69.4 ÷ 3.161 = 22.0. That is his FFMI.
Normalize
22.0 + 6.1 × (1.8 − 1.778) = 22.1.
What is a good FFMI?
There is no official FFMI scale like the BMI categories. What is known comes mainly from research in men. In the 1995 study by Kouri and colleagues, drug-free male athletes rarely had a normalized FFMI above about 25, while some steroid users went well beyond it. That figure is a research observation, not a hard limit for every person.
For women there is no widely accepted reference range, because the best-known studies were done in men. Women naturally carry less lean mass, so their FFMI is usually lower at the same training level. The most useful way to use FFMI is to track your own number over time as you train.
Worked examples
Example 1: a trained man
5 ft 10 in, 180 lb, 15% body fat.
- Lean mass 153.0 lb (69.4 kg)
- FFMI 22.0
- Normalized FFMI 22.1
- BMI 25.8
His BMI is in the overweight range, but his FFMI shows much of that weight is lean mass.
Example 2: a woman
5 ft 5 in, 140 lb, 25% body fat.
- Lean mass 105.0 lb
- FFMI 17.5
- Normalized FFMI 18.4
Shorter people get a larger upward adjustment when normalized.
Example 3: a very muscular man
5 ft 10 in, 220 lb, 10% body fat.
- Lean mass 198.0 lb
- FFMI 28.4
- Normalized FFMI 28.5
Well above the level rarely exceeded by drug-free men in the 1995 study. Check the body fat figure: an underestimate makes FFMI look higher.
FFMI vs BMI
BMI uses total weight, so a muscular person can look overweight. FFMI removes fat mass and looks only at lean tissue relative to height, which makes it more useful for people who lift weights. The trade-off is that FFMI depends on an accurate body fat percentage, which is harder to measure than weight.
Where an FFMI calculator helps
Tracking muscle gain
See whether you are adding lean mass over months of training.
Beyond BMI
Separate muscle from fat when BMI looks high.
Comparing builds
Normalized FFMI lets different heights compare fairly.
Setting goals
Plan lean mass targets for a bulk or recomposition.
Estimate your body fat percentage first, then compare with your BMI, or plan a lean gain with the macro calculator and protein intake calculator. This is general information, not medical advice.
FFMI calculator FAQ
What is FFMI?
Fat-free mass index: your lean body mass in kilograms divided by your height in meters squared.
How do I calculate FFMI?
Multiply your weight by (1 − body fat %) to get lean mass, then divide by your height in meters squared. For example, 69.4 kg at 1.778 m gives 22.0.
What is normalized FFMI?
FFMI adjusted to a height of 1.8 m by adding 6.1 × (1.8 − height in m), so people of different heights can be compared.
What is the natural FFMI limit?
In a 1995 study of male athletes, those who did not use steroids rarely exceeded a normalized FFMI of about 25. It is not a strict limit.
Is FFMI accurate?
It is only as accurate as your body fat figure. A few percentage points of error in body fat changes FFMI noticeably.
What is a good FFMI for women?
There is no widely accepted scale for women. Track your own FFMI over time rather than comparing with men.