FFMI (Fat-Free Mass Index)
BMI cannot separate muscle from fat, so running the same calculation on fat-free weight turns it into a muscle index. A correction to a 1.8 m reference removes most of the height bias.
Your numbers
FFMI
21.6
Lean mass
68kg
Fat mass
12kg
What it means
21.6 is the range of a consistently trained body.
Formula
FFMI = Lean mass ÷ (Height (cm) ÷ 100) ² + 6.1 × (1.8 − Height (cm) ÷ 100)
A wrong body-fat input carries straight through. Bioimpedance readings swing several points in a day with hydration.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Weight (kg) (kg)The second weight you are comparing against. | 80 | 1 and up |
| Height (cm) (cm)Height in centimetres; converted to metres inside the formula. | 178 | 50 and up |
| Body fat (%)The share of body weight that is fat. | 15 | 1 ~ 60 |
Step by step
Quick reference table
Results when only Weight (kg) (kg) changes and everything else stays put.
| Weight (kg) (kg) | FFMI | Lean mass (kg) | Fat mass (kg) |
|---|---|---|---|
| 40 | 10.9 | 34 | 6 |
| 60 | 16.2 | 51 | 9 |
| 80 | 21.6 | 68 | 12 |
| 120 | 32.3 | 102 | 18 |
| 160 | 43 | 136 | 24 |
What each result means
| Result | At default values |
|---|---|
| FFMIFat-free weight over height squared — a muscle index. | 21.6 |
| Lean mass (kg)Everything that is not fat — muscle, bone, water. | 68 |
| Fat mass (kg)How much of your weight is fat, in kilograms. | 12 |
Common mistakes
A wrong body-fat input carries straight through. Bioimpedance readings swing several points in a day with hydration.
Glossary
- Weight (kg)
- The second weight you are comparing against.
- Height (cm)
- Height in centimetres; converted to metres inside the formula.
- Body fat
- The share of body weight that is fat.
- FFMI
- Fat-free weight over height squared — a muscle index.
- Lean mass
- Everything that is not fat — muscle, bone, water.
- Fat mass
- How much of your weight is fat, in kilograms.
Frequently asked questions
Q. How is FFMI (Fat-Free Mass Index) calculated?
FFMI = Lean mass ÷ (Height (cm) ÷ 100) ² + 6.1 × (1.8 − Height (cm) ÷ 100) — BMI cannot separate muscle from fat, so running the same calculation on fat-free weight turns it into a muscle index. A correction to a 1.8 m reference removes most of the height bias.
Q. Can you walk through an example?
With Weight (kg) 80kg, Height (cm) 178cm, Body fat 15%, the answer is FFMI 21.6.
Q. What do I need to enter?
Enter Weight (kg), Height (cm), Body fat. The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Weight (kg) (kg) moves the answer to Weight (kg) (kg) 40 → FFMI 10.9 and Weight (kg) (kg) 160 → FFMI 43. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
A wrong body-fat input carries straight through. Bioimpedance readings swing several points in a day with hydration.
Related calculators
These calculators help you understand health figures; they do not replace diagnosis or prescription. See a clinician if you have symptoms.