Quick answer
FFMI = lean mass (kg) ÷ height (m)². A person weighing 80 kg at 15% body fat and 180 cm has 68 kg of lean mass, so FFMI = 68 ÷ 1.8² = 21.0. The normalized value adds a height correction.
Enter weight, height and body fat percentage. The calculator returns FFMI, normalized FFMI and lean mass.
Related tools: Lean Body Mass Calculator: Estimate lean body mass with the Boer formula. BMI Calculator: Find your Body Mass Index (BMI) and weight status. Calorie Calculator: Daily calories to maintain, lose or gain weight.
How to use the FFMI Calculator
- Enter weight (kg) (above 0).
- Enter height (cm) (above 0).
- Enter body fat (%) (between 0 and 80).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Weight (kg) | A number | > 0 |
| Height (cm) | A number | > 0 |
| Body fat | A number, in % | ≥ 0, ≤ 80 |
Formula and method
Formula
FFMI = lean mass (kg) ÷ height (m)²; normalized = FFMI + 6.1 × (1.8 − height in m)
IEEE-754 double precision; shown to the decimals indicated.
The normalized value adjusts for height as in Kouri et al. Informational estimate, not medical advice. Talk to a qualified health professional about your own situation.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Weight (kg) = 80; Height (cm) = 180.0; Body fat = 15.
| Result | Value |
|---|---|
| FFMI | 21 |
| Normalized FFMI | 21 |
| Lean mass | 68 kg |
Example 2
Inputs: Weight (kg) = 65; Height (cm) = 170.0; Body fat = 22.
| Result | Value |
|---|---|
| FFMI | 17.5 |
| Normalized FFMI | 18.2 |
| Lean mass | 50.7 kg |
Example 3
Inputs: Weight (kg) = 90; Height (cm) = 185.0; Body fat = 12.
| Result | Value |
|---|---|
| FFMI | 23.1 |
| Normalized FFMI | 22.8 |
| Lean mass | 79.2 kg |
The first line is the headline result (fFMI). Below it the fFMI Calculator lists normalized FFMI and lean mass, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
Why FFMI and not BMI
BMI cannot tell muscle from fat, so a muscular person can be classed 'overweight'. FFMI removes fat mass first. The normalized version adds 6.1 × (1.8 − height in metres) to compare people of different heights; at exactly 1.80 m the two numbers match, as the example shows.
Accuracy depends on body fat
The input is only as good as the body fat estimate. Skinfolds, scales and calipers have errors of several percentage points, and a 3 point error shifts lean mass by roughly 2.4 kg here. Values around 18 to 20 are typical for untrained men, while very high values are rare in natural lifters, but no single threshold is definitive.
How the result changes with weight
Holding every other input at the first worked example, this table changes only weight. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Weight | FFMI | Normalized FFMI | Lean mass |
|---|---|---|---|
| 40 | 10.5 | 10.5 | 34 kg |
| 80 | 21 | 21 | 68 kg |
| 160 | 42 | 42 | 136 kg |
| 320 | 84 | 84 | 272 kg |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Height (cm): entering "0" shows "Height (cm) must be greater than 0." beside that field and no result.
- Weight (kg): entering "0" shows "Weight (kg) must be greater than 0." beside that field and no result.
Choosing between the FFMI Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Lean Body Mass Calculator](/fitness/lean-body-mass-calculator/) | units, sex and weight | Lean body mass |
| [BMI Calculator](/health/bmi-calculator/) | units, weight and height | Your BMI |
| [Calorie Calculator](/health/calorie-calculator/) | units, sex and weight | Maintenance calories |
| [BMR Calculator](/health/bmr-calculator/) | units, sex and weight | Basal metabolic rate |
| [Pregnancy Due Date Calculator](/health/pregnancy/) | first day of last period | Estimated due date |
Assumptions and limitations
- Body-fat estimates are uncertain
- Not a diagnosis
This calculator gives general information based on published formulas. It is not medical advice, and it cannot account for your health history. Talk to a qualified clinician about decisions that affect your health.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for height (cm) and weight (kg) to confirm this.
Frequently asked questions
What is a good FFMI?
Roughly 18 to 20 is average for men, and 14 to 17 for women.
Why is normalized FFMI different?
It adjusts for height so taller and shorter people are comparable.
Can FFMI prove steroid use?
No; any such claim goes beyond what an index can show.
What does the FFMI Calculator calculate?
Fat-free mass index and its height-normalized value. It returns FFMI, Normalized FFMI and Lean mass.
What formula does the FFMI Calculator use?
FFMI = lean mass (kg) ÷ height (m)²; normalized = FFMI + 6.1 × (1.8 − height in m). The normalized value adjusts for height as in Kouri et al.
What do I need to enter in the fFMI Calculator?
You enter Weight (kg), Height (cm) and Body fat (%).
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Body-fat estimates are uncertain.
What are the limits of this calculator?
Body-fat estimates are uncertain. Not a diagnosis.
What is an example of a fFMI result?
With weight (kg) 80, height (cm) 180.0 and body fat 15, the fFMI is 21.
Where does the method come from?
The method follows: Kouri EM et al., Fat-free mass index in users and nonusers of anabolic-androgenic steroids, Clin J Sport Med 1995;5:223.
Can I rely on this calculator for a decision?
This calculator gives general information based on published formulas. It is not medical advice, and it cannot account for your health history. Talk to a qualified clinician about decisions that affect your health. For this calculator in particular: Body-fat estimates are uncertain.
Related calculators
- Lean Body Mass Calculator — Estimate lean body mass with the Boer formula.
- BMI Calculator — Find your Body Mass Index (BMI) and weight status.
- Calorie Calculator — Daily calories to maintain, lose or gain weight.
- BMR Calculator — Estimate basal metabolic rate with the Mifflin–St Jeor and revised Harris–Benedict equations.
- Height Calculator — Convert height between centimeters and feet-and-inches.
Sources
- Kouri EM et al., Fat-free mass index in users and nonusers of anabolic-androgenic steroids, Clin J Sport Med 1995;5:223
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.