Body Fat from BMI (Deurenberg)

If you have a scale and a tape measure but no calipers, the Deurenberg equation turns a plain BMI reading into a rough body-fat estimate. The formula was published in 1991 and is still widely used because it needs nothing but height, weight, age and sex: body fat percentage equals 1.20 times BMI, plus 0.23 times age, minus 10.8 for men, minus 5.4.

Two details explain why this formula exists at all. First, BMI alone systematically overestimates fat in the young and underestimates it in the old, because body composition drifts with age even when weight is stable. Second, women carry more essential fat than men at the same BMI, which the sex term corrects. The 0.23-per-year age term is doing real work: a 20-year-old and a 60-year-old with identical BMI will differ by roughly nine percentage points of estimated fat.

The accuracy is modest. Reported error is around four to five percentage points against a DEXA reference, and the equation was built on a general population rather than athletes. Very muscular people will be read as fatter than they are, and people with very little muscle will be read as leaner, because BMI cannot see the difference between a kilogram of muscle and a kilogram of fat. Use it when a tape measure is not handy, or as a second opinion alongside the tape method — if the two agree, you can be reasonably confident in the ballpark.

The most honest way to use this number is comparatively. Log it every couple of weeks under the same conditions and watch the slope. If estimated body fat is falling while body weight stays flat, you are almost certainly trading fat for muscle, which is exactly the outcome most people are training for.

This calculator returns estimates for general information only. It is not medical advice, diagnosis or treatment, and it cannot replace an assessment by a qualified health professional. Individual results vary, and figures such as calorie needs, body-fat percentage and heart-rate zones are approximations based on population averages rather than measurements of you.