Short answer
BMI = weight (kg) ÷ height (m)² and cannot separate muscle from fat. Body fat percentage separates them but needs measurements and has its own error. The Deurenberg formula estimates body fat from BMI, age and sex.
1. Side by side
| BMI | Body fat percentage | |
|---|---|---|
| Inputs | Height and weight | Tape or skinfold measurements |
| Separates fat from muscle | No | Yes, approximately |
| Main weakness | Ignores body composition | Measurement error |
2. The BMI-based estimate (Deurenberg)
Body fat % = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4, where sex is 1 for men and 0 for women. It is a rough population estimate.
| Person | BMI | Estimate |
|---|---|---|
| Man, 30, 178 cm, 80 kg | 25.2 | 21.0% |
| Woman, 30, BMI 23 | 23.0 | 29.1% |
Two people with the same BMI can have different amounts of body fat, which is why the formula is shown only for comparison.
3. Which should you use?
- Use BMI as a quick screening number.
- Use body fat percentage, measured the same way each time, to follow body composition.
- Look at both along with waist measurements and advice from a clinician.
Check both with the BMI Calculator and the Body Fat Calculator.
4. Frequently asked questions
Is body fat percentage better than BMI?expand_more
It answers a different question: it separates fat from lean mass, but its estimates have error too.
How do you calculate BMI?expand_more
Weight in kilograms divided by height in metres squared.
Can BMI estimate body fat?expand_more
Roughly, with the Deurenberg formula, which also uses age and sex.
Why can the same BMI mean different body fat?expand_more
BMI cannot distinguish muscle from fat.