Body Fat Calculator

How accurate is a body fat calculator?

Less accurate than the single number suggests, and that is true of every one of them, not just this one. The equation used here was published with a standard error of 4.1 percentage points. An estimate of 20% means the real figure is most likely somewhere between 16% and 24%. That is why this page reports a range alongside the number rather than a clean figure on its own.

Nothing you can do at home is much better. Skinfold callipers depend heavily on who is holding them, bathroom scales using bioelectrical impedance shift with how hydrated you are, and a 2023 study found that circumference-based methods failed to capture real body composition changes across eight weeks of training. DEXA is the practical gold standard, and it involves a clinic.

How is body fat percentage calculated here?

With the Deurenberg equation, which predicts body fat from BMI, age and sex. It was derived from 1,229 people whose body composition was measured by densitometry, and it explains about 79% of the variation in that sample.

1. 1. Work out BMI

   BMI = weight in kg ÷ (height in m)²

2. 2. Apply the Deurenberg equation

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

3. 3. State the error

   likely range = estimate ± 4.1 percentage points

The catch worth knowing about

This method starts from BMI, which means it inherits BMI's central weakness: it cannot tell muscle from fat. A heavily trained person with a high BMI will get a body fat estimate that is too high, for exactly the same reason their BMI reads too high.

If that describes you, the number here will be wrong in a predictable direction, and the guide on why BMI misreads muscular people explains why. Waist measurements and body composition scans are the honest alternatives.

What counts as a normal body fat percentage?

There is no single authority on this in the way the CDC defines the BMI categories, so the bands below are descriptive rather than diagnostic. Reference ranges differ between sources, and women carry more essential fat than men for physiological reasons.

Given the four-point error on the estimate, a result near a band edge could sit in either. Read the band as a rough position, not a verdict.

What should you do with the number?

Track the direction rather than the value. The error on any single estimate is large, but it is reasonably consistent, so a figure that falls over several months is telling you something real even if neither reading is exactly right.

If your goal is changing that number, the levers are calories and training rather than the measurement. The Calorie Calculator sets the target and the Macro Calculator sets the protein that helps preserve lean mass while you do it.