Fat Calculator

body fat % = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4 (sex: 1 for men, 0 for women)

BMI

Body mass index: weight in kilograms divided by height in metres squared (kg)

age

Age in whole years, 16 and over

sex

1 for men and 0 for women, which is where the 10.8-point difference comes from

Use it to estimate body fat from a BMI you already have, and to see how your fat mass and lean mass split. Use it to compare yourself with yourself over time rather than with a published table: the equation was fitted on a European population and comes with a standard error of about four percentage points. For a measurement rather than an estimate, skinfold calipers or a waist-based method will tell you more.

1. 175 cm, 75 kg, age 40, man

   1. BMI: 75 ÷ 1.75² = 24.5
   2. 1.20 × 24.5 = 29.4
   3. 0.23 × 40 = 9.2, and the sex term is −10.8 for a man
   4. 29.4 + 9.2 − 10.8 − 5.4 = 22.4%
   5. Fat mass: 75 × 0.224 = 16.8 kg, lean mass: 75 − 16.8 = 58.2 kg

   The age term adds 2.3 points per decade, so the same man at 60 would read 27.0% on unchanged height and weight.

2. The same numbers for a woman

   1. BMI is unchanged at 24.5
   2. 1.20 × 24.5 = 29.4 and 0.23 × 40 = 9.2 are both unchanged
   3. The sex term is 0 instead of −10.8
   4. 29.4 + 9.2 − 0 − 5.4 = 33.2%

   Four identical inputs, one different answer: 10.8 percentage points, or 8.1 kg of fat on the same body weight. That gap is a property of the equation, not a measurement of either person.

This is a prediction, not a measurement, and its error is large: the original study reported a standard error of about 4.1 percentage points, so a reading of 22% is compatible with a true value anywhere from roughly 18% to 26%. The equation was fitted on 1,229 adults in the Netherlands, and it systematically overestimates body fat in obese people and underestimates it in very lean ones. It was derived for adults aged 16 and over; children need a different equation with different coefficients. It says nothing about where the fat sits, and it can only be as good as the BMI going into it.

How does this fat calculator estimate body fat from BMI?

It uses the Deurenberg equation, a regression published in 1991: 1.20 times BMI, plus 0.23 times age, minus 10.8 if you are a man, minus 5.4. It was fitted on 1,229 adults whose body fat was measured, and it reproduces the finding that body fat rises with BMI and with age, and is higher in women at any given BMI.

What is a normal body fat percentage?

There is no single number. Published ranges differ by source and by the method used to measure the fat — calipers, DXA and bioimpedance all give different answers on the same person — and they all shift upward with age. That is why this page prints the estimate without a category. Add an error of about four percentage points and a reading that lands near any boundary cannot tell you which side of it you are on; only the direction of travel over months is worth reading.

Why is body fat percentage for women higher than for men at the same BMI?

Because that is what the data show. The equation encodes it as a fixed 10.8-point subtraction for men, so at the same BMI and age a woman's estimate is always 10.8 points higher. Some of it is essential fat — the minimum for reproductive and hormonal function is higher in women — and some is the average difference in muscle mass, since muscle is the tissue BMI cannot see.

Can I use this for a child or teenager?

No. Below 16 the original paper uses a different equation with different coefficients, and this calculator does not implement it: mixing the two would produce a number with no published basis. Children are assessed on age- and sex-specific percentile charts instead.

Is this more accurate than a skinfold measurement?

No, and it is not trying to be. A skinfold measurement reads a property of your body directly; this equation predicts it from BMI, age and sex, and inherits BMI's blind spot for muscle. Its advantage is that it needs no equipment and no one to take the measurement. If you have calipers, use them.

Why are fat mass and lean mass always adding up to my body weight?

Because lean mass is computed as the remainder. Fat mass is your weight times the unrounded body fat percentage, and lean mass is your weight minus that figure, so the two always sum exactly to your body weight. Rounding each one separately for display can make them appear to be a tenth of a kilogram off; they are not.

• Body mass index as a measure of body fatness: age- and sex-specific prediction formulas — British Journal of Nutrition, 1991 (Deurenberg, Weststrate and Seidell)
• Nutrition and Athletic Performance — joint position statement — Academy of Nutrition and Dietetics, Dietitians of Canada and American College of Sports Medicine, 2016