Explanation
Body-fat regression estimate
This calculator evaluates a published population regression based on BMI and age. It is a rough estimate, not a body-composition measurement, diagnosis or fitness goal.
Choose the question and enter values
Choose the published sex-specific regression and enter adult weight, height and age. The source uses male and female coefficients. The input BMI must remain inside the calculator’s documented research-range gate.
Body fat % = 1.2 × BMI + 0.23 × A − 10.8 × S − 5.4, with BMI = 10000 × w ÷ h² (w in kg, h in cm), A in years, S = 1 for the male and 0 for the female regression.
| w | Body weight |
|---|---|
| h | Height |
| A | Age |
Male regression at BMI 20
| Body weight | 80 |
|---|---|
| Height | 200 |
| Age | 30 |
| Result | 14.7% |
80 kg × 10000 ÷ (200 cm)² = BMI 20. Male regression: 1.2 × 20 + 0.23 × 30 − 5.4, minus a further 10.8 for the male regression, gives 14.7%.
Female regression for the same measurements
| Body weight | 80 |
|---|---|
| Height | 200 |
| Age | 30 |
| Result | 25.5% |
80 kg × 10000 ÷ (200 cm)² = BMI 20. Female regression: 1.2 × 20 + 0.23 × 30 − 5.4, minus a further 10.8 for the male regression, gives 25.5%.
Male regression at BMI 25
| Body weight | 100 |
|---|---|
| Height | 200 |
| Age | 40 |
| Result | 23% |
100 kg × 10000 ÷ (200 cm)² = BMI 25. Male regression: 1.2 × 25 + 0.23 × 40 − 5.4, minus a further 10.8 for the male regression, gives 23%.
Diagram of this result
The bar splits the entered body weight by the predicted percentage into estimated fat mass and fat-free mass. It is a regression estimate with an error of about four percentage points, not a body-composition measurement.
The exact numbers and their units are stated in the visible text. Predictions and scenario inputs are not medical measurements or prescriptions.
Common mistakes
Muscularity and individual body composition can make a BMI-based estimate misleading. Do not read small decimal differences as measured changes in body fat.
Assumptions and limits
Adult ages 20–83; BMI 13.9–40.9, weight 20–350kg, height 100–250cm. The input range is not an individual validity guarantee. Not for children, pregnancy, clinical care or performance targets. Published regression error is about four body-fat percentage points; errors can be greater for individuals. Educational information only, not medical advice, diagnosis or an individualized nutrition plan.
Why can this differ from a scan or other method?
It predicts body fat from indirect measurements. A scan, skinfold assessment or other method measures different information and has its own uncertainty.
Method and references
Deurenberg P, Weststrate JA, Seidell JC, 1991, Body mass index as a measure of body fatness: age- and sex-specific prediction formulas: https://pubmed.ncbi.nlm.nih.gov/2043597/ . The adult equation uses BMI, age and the published male/female coefficient; reported standard error of estimate is 4.1 body-fat percentage points.