Body Fat Calculator

It uses your waist and neck circumference (plus hip for women) along with height, plugged into a logarithmic equation developed by Hodgdon and Beckett in 1984 and validated against hydrostatic underwater weighing. This calculator uses the metric version of that formula — a widely copied version online is defined for inches, and feeding it centimetres instead overestimates body fat by several percentage points.

The US Navy method carries a standard error of roughly 3-4 percentage points against hydrostatic weighing, so a result of 16.9% is more honestly read as 'about 13-21%' than an exact figure. Results are also sensitive to measuring technique — a 2 cm difference in where you place the tape at the waist can shift the number by several points, so measuring the same way each time matters more than precision on any single reading.

The Navy method uses your actual body shape (circumference measurements), while the BMI-based Deurenberg equation only uses your BMI, age, and sex — it has no idea whether your weight is muscle or fat. That means it inherits every weakness BMI has, including misclassifying muscular people as carrying more fat than they do. Use it only as a fallback when you can't take circumference measurements.

They measure different things. Body fat percentage is how much of your weight is fat; waist-to-hip ratio describes where that fat is distributed. The WHO's 2008 expert consultation set 0.90 (men) and 0.85 (women) as thresholds for substantially increased risk from central fat specifically, because abdominal fat carries more cardiovascular and metabolic risk than fat carried on the hips — so it's possible to have a moderate body fat percentage and still cross the WHR risk threshold.

Not reliably. Circumference-based equations lose accuracy at the extremes — bodybuilders and very lean athletes fall outside the range the Navy equation was validated on, since it was originally developed using US military personnel who were fitter and younger than the general population. DEXA, hydrostatic weighing, or air displacement plethysmography are meaningfully more accurate if you're at either extreme.