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BMI to body fat percentage calculator
Short answer
The Deurenberg equation turns BMI into an estimated body fat percentage: body fat % = 1.2 × BMI + 0.23 × age − 10.8 × sex − 5.4 (sex = 1 for men, 0 for women). At a BMI of 25 and age 40, that gives 23.0% for a man and 33.8% for a woman. Individual results typically miss by about 4.1 percentage points either way.
Last reviewed 28 October 20261 published sourcesNot medical advice
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Uses your height, weight, age and sex. For ages 7 to 83, the range the equations were built on.
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| BMI | Women | Men |
|---|---|---|
| 20 | 27.8 | 17.0 |
| 25 | 33.8 | 23.0 |
| 30 | 39.8 | 29.0 |
| 35 | 45.8 | 35.0 |
Where the equations come from
Deurenberg, Weststrate and Seidell measured body fat by densitometry in 1,229 people aged 7 to 83 with BMIs from 13.9 to 40.9, and fitted separate equations for adults and for children aged 15 and under (Deurenberg, Weststrate & Seidell, 1991).
The adult equation explained 79% of the variation in body fat (standard error 4.1%); the children's equation explained 38% (standard error 4.4%). In obese people both slightly overestimated body fat (Deurenberg, Weststrate & Seidell, 1991).
Limits, and better options
BMI cannot tell muscle from fat, so this is a population average for your BMI, age and sex rather than a measurement. A tape-based estimate such as the body fat calculator or the skinfold body fat calculator uses more information about you. Check your BMI itself with the BMI calculator.
Body changes, tracked over time
Reps keeps your weight and body measurements from Apple Health next to your nutrition and training, so a trend is easy to see.
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Common questions
Can you work out body fat from BMI?
Only roughly. The Deurenberg adult equation explained 79% of the variation in measured body fat, with a standard error of 4.1 percentage points, so roughly one person in three is off by more than that.
Is there a version for children?
Yes. For ages 7 to 15, body fat % = 1.51 × BMI − 0.70 × age − 3.6 × sex + 1.4. It is much less precise: it explained only 38% of the variation, with a standard error of 4.4 points.
When is this estimate least reliable?
In people with obesity the equations slightly overestimated body fat. They also assume typical muscle for your BMI, so very muscular people will read too high, and they were built on BMIs from 13.9 to 40.9.
Sources
Figures on this page were checked against these publications on 28 October 2026.
- Deurenberg P, Weststrate JA, Seidell JC. Body mass index as a measure of body fatness: age- and sex-specific prediction formulas. British Journal of Nutrition. 1991;65(2):105–114. doi.org/10.1079/bjn19910073
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