Body Fat from BMI
This is the estimate to use when you have neither a tape measure nor a scale that reads composition. At the same BMI, older bodies carry more fat, and women read about 11 points higher than men.
Your numbers
Body fat
21.9%
Fat mass
16.2kg
Lean mass
57.8kg
BMI
25
Formula
Body fat = 1.20 × BMI + 0.23 × Age − 10.8 × Sex (M1 / F0) − 5.4
It overestimates for muscular people. If you have a tape measure, the neck-and-waist method is more accurate.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Weight (kg) (kg)The second weight you are comparing against. | 74 | 1 and up |
| Height (cm) (cm)Height in centimetres; converted to metres inside the formula. | 172 | 50 and up |
| AgeAge in completed years — it shifts by one depending on the birthday. | 35 | 5 ~ 100 |
| Sex (M1 / F0)Where the formula splits by sex — enter 1 for male, 0 for female. | 1 | 0 ~ 1 |
Step by step
Quick reference table
Results when only Weight (kg) (kg) changes and everything else stays put.
| Weight (kg) (kg) | Body fat (%) | Fat mass (kg) | Lean mass (kg) |
|---|---|---|---|
| 37 | 6.9 | 2.5 | 34.5 |
| 55.5 | 14.4 | 8 | 47.5 |
| 74 | 21.9 | 16.2 | 57.8 |
| 111 | 36.9 | 40.9 | 70.1 |
| 148 | 51.9 | 76.8 | 71.2 |
What each result means
| Result | At default values |
|---|---|
| Body fat (%)The share of body weight that is fat. | 21.9 |
| Fat mass (kg)How much of your weight is fat, in kilograms. | 16.2 |
| Lean mass (kg)Everything that is not fat — muscle, bone, water. | 57.8 |
| BMIA build index from height and weight alone; it cannot tell muscle from fat. | 25 |
Common mistakes
It overestimates for muscular people. If you have a tape measure, the neck-and-waist method is more accurate.
Glossary
- Weight (kg)
- The second weight you are comparing against.
- Height (cm)
- Height in centimetres; converted to metres inside the formula.
- Age
- Age in completed years — it shifts by one depending on the birthday.
- Sex (M1 / F0)
- Where the formula splits by sex — enter 1 for male, 0 for female.
- Body fat
- The share of body weight that is fat.
- Fat mass
- How much of your weight is fat, in kilograms.
- Lean mass
- Everything that is not fat — muscle, bone, water.
- BMI
- A build index from height and weight alone; it cannot tell muscle from fat.
Frequently asked questions
Q. How is Body Fat from BMI calculated?
Body fat = 1.20 × BMI + 0.23 × Age − 10.8 × Sex (M1 / F0) − 5.4 — This is the estimate to use when you have neither a tape measure nor a scale that reads composition. At the same BMI, older bodies carry more fat, and women read about 11 points higher than men.
Q. Can you walk through an example?
With Weight (kg) 74kg, Height (cm) 172cm, Age 35, Sex (M1 / F0) 1, the answer is Body fat 21.9%.
Q. What do I need to enter?
Enter Weight (kg), Height (cm), Age, Sex (M1 / F0). 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) 37 → Body fat (%) 6.9 and Weight (kg) (kg) 148 → Body fat (%) 51.9. 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?
It overestimates for muscular people. If you have a tape measure, the neck-and-waist method is more accurate.
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