Body Fat Estimate from BMI (Deurenberg)

Understanding the body fat from BMI calculator — concept and principle

If you know only your weight, height, age and sex, you can still get a rough idea of your body fat percentage. Deurenberg and colleagues derived an equation from adults whose body fat had been measured by underwater weighing, and it remains one of the most cited ways to turn BMI into an estimated fat percentage. This calculator applies that equation and also shows your BMI.

BMI and age push the estimate up, being male and the constant pull it down: 21.24% here.

How it is calculated

BMI = weight (kg) ÷ height (m)².
Estimated body fat (%) = 1.2 × BMI + 0.23 × age − 10.8 × sex − 5.4, where sex = 1 for men and 0 for women.
Age is in years. The equation was developed for adults.

The principle

BMI rises with both fat and muscle, so on its own it cannot tell them apart. The Deurenberg equation adds two pieces of information that change body composition at a given BMI. First, sex: at the same BMI, women on average carry about 10.8 percentage points more fat than men, which is the size of the sex coefficient. Second, age: with the years, muscle mass tends to fall and fat to rise even if weight stays the same, so each year adds 0.23 points. The coefficients come from a regression, the best-fitting straight line through the measured data, so the result describes an average person with your BMI, age and sex.

Worked example

Inputs: male, age 35, weight 75 kg, height 175 cm.
BMI = 75 ÷ 1.75² = 75 ÷ 3.0625 = 24.49.
1.2 × 24.49 = 29.39; 0.23 × 35 = 8.05.
Body fat = 29.39 + 8.05 − 10.8 × 1 − 5.4 = 21.24%.

Each point is the calculator re-run with only “Height (cm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Height (cm)	Estimated body fat
43.8	462.05%
87.5	109.4%
131	44.09%
175	21.24%
219	10.66%
262	4.91%
350	-0.8%

How to read the result

Where does your result fall?

For a 35-year-old man, about 21% falls in the "average" range of common fitness classifications (for example ACE: 18–24% for men, 25–31% for women). Because the equation knows nothing about your muscles, a strength athlete with a BMI of 27 may be estimated above 20% while actually being much leaner. The prediction error reported for the equation is about 4 percentage points, so read the result as a range of roughly ±4 points. For better individual estimates use circumference methods, skinfolds, bioelectrical impedance or DEXA.

Common mistakes

Entering sex as 1 for women; in this formula 1 means male.
Using the adult equation for children and adolescents. Deurenberg published a separate equation for them.
Treating the estimate as a measurement and tracking small weekly changes with it.

Keep in mind: This is a population estimate, not a diagnosis. It overestimates fat in muscular people and may underestimate it in older adults who have lost muscle. Talk to a doctor if you are concerned about your weight or health.

Frequently asked questions

Why does my body fat go up with age when my weight stays the same?

The equation reflects the typical loss of muscle and gain of fat with age; it adds 0.23 points per year.

Is this more accurate than a bathroom scale with body fat?

Both are estimates. Home impedance scales measure something about you, but are affected by hydration.

Does the formula work for Asian people?

Studies have found that Asian adults often have more body fat at the same BMI, so the equation may underestimate it.

Sources: 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 65 (1991) 105–114

Checked 8 Nov 2026. Deurenberg, Weststrate & Seidell, Br J Nutr 1991.

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