Body Fat Percentage Calculator — No Calipers Needed

Body fat percentage answers the question a scale cannot: how much of your weight is fat, and how much is everything else. The calculator below estimates it from weight, height, age and sex — no calipers, no scan, no appointment. Treat the output as a range rather than a reading.

The equation doing the work

The estimate uses the Deurenberg formula, published in the British Journal of Nutrition in 1991 and still the standard way to convert BMI into a fat percentage:

BF% = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4, with sex scored as 1 for men and 0 for women.

It was fitted on 1,229 people whose composition was measured by densitometry, and it explains about 79 percent of the variation in fatness. The thirty-year retrospective by its own author is candid about the rest: the correlation between BMI and fat ranges from 0.37 to 0.92 depending on age and sex, and later work concluded that ethnic-specific criteria are needed. A result of 22 percent here means “somewhere in the low twenties”, not 22.0.

Nobody official will hand you a target

Most body fat pages open with a tidy five-row chart and never say where it came from. Here is where it came from: fitness organizations. As a 2010 letter in Mayo Clinic Proceedings put it, neither the World Health Organization nor any major scientific society studying obesity has defined a normal value for body fat percentage.

The most defensible American reference is the National Athletic Trainers’ Association position statement on safe weight practices, published in the Journal of Athletic Training in 2011:

	Men	Women
Essential fat	about 3%	about 12%
Low reference, adults	5%	12%
Healthy range	10–22%	20–32%
Low reference, adolescents	7%	14%

Essential fat is not spare tissue. It is lipid inside bone marrow, the nervous system and the organs, plus sex-specific stores in women. Below the low reference, reproductive and hormonal function is what gives way first. The band label the calculator prints above — essential, athletes, fitness, acceptable — is a fitness-industry convention; the table is the sourced reference to read your number against.

Where the estimate breaks

Because the formula begins with BMI, it inherits BMI’s blind spot. Dense muscle is mass, and mass reads as fat, so a trained lifter usually lands several points too high. If you carry serious muscle, the circumference-based Navy body fat calculator is the better instrument — it measures where fat actually collects instead of inferring it from weight.

The formula is also silent on the variable that carries most of the medical risk: location. The NHLBI flags a waist over 40 inches (102 cm) in men and over 35 inches (89 cm) in women as raising the risk of heart disease and type 2 diabetes, regardless of what any percentage says. Two people at 24 percent, one carrying it at the waist and one at the hips, are not in the same position.

What to do with the number

Run it again in six weeks under the same conditions and compare the two. A four-point standard error makes one reading nearly meaningless and a trend genuinely informative. To see the other half of the same measurement — the mass that is not fat — use the lean body mass calculator. For how the healthy band shifts across the decades, see ideal body fat percentage by age, and for why this number and BMI disagree so often, read BMI vs body fat percentage.

This content is informational and does not replace evaluation by a health professional.