How body fat is estimated
The US Navy method, published by Hodgdon and Beckett in 1984, estimates body fat from your height and the size of your neck and waist, plus your hips if you're female. All measurements are in centimetres.
For men the formula is 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450. For women it's 495 ÷ (1.29579 − 0.35004 × log10(waist + hips − neck) + 0.221 × log10(height)) − 450.
Your weight isn't used for the percentage. It's only used to split your weight into fat mass (weight × body fat percentage) and lean mass (the rest).
Worked examples
A man 178 cm tall weighing 80 kg, with a 38 cm neck and an 88 cm waist, has an estimated body fat of 18.7%. That's 15.0 kg of fat mass and 65.0 kg of lean mass.
A woman 165 cm tall weighing 65 kg, with a 33 cm neck, a 75 cm waist and 100 cm hips, has an estimated body fat of 29.4%. That's 19.1 kg of fat mass and 45.9 kg of lean mass.
How accurate is it?
This is an estimate. Circumference methods are typically within a few percentage points of lab measurements, but they can be further out for very lean or very muscular people and for people who carry fat in unusual places.
Small measuring errors make a noticeable difference, especially at the waist. Measure on bare skin, keep the tape level and snug without pressing in, and take the average of two or three readings. Measure at the same time of day if you're tracking changes.
This calculator is not medical advice. If you're concerned about your weight or body composition, talk to your GP.
Inputs for the body fat calculator
Sex
The method uses a different formula for men and women. Choosing female adds the hips measurement.
Height
Your height in centimetres, without shoes.
Weight
Your weight in kilograms. It's only used to work out fat mass and lean mass, not the percentage.
Neck
Measure around your neck just below the larynx (Adam's apple), with the tape sloping slightly downwards at the front.
Waist
Men measure at the navel. Women measure at the narrowest point. Breathe out normally and don't pull your stomach in. For men, the waist must be bigger than the neck.
Hips
Measure around the widest part of your hips and buttocks. Only shown if you choose female.
CALCULATOR QUESTIONS
Body fat calculator questions
What is a healthy body fat percentage?
There's no single agreed range. Women naturally carry more body fat than men, and healthy levels change with age. Use the result to track changes over time, and ask your GP if you want to know what's right for you.
Why is my result different from my smart scales?
Smart scales use a small electrical current, which is affected by how hydrated you are. The tape-measure method uses body shape instead. Both are estimates, so they rarely agree exactly.
Why doesn't my weight change the percentage?
The US Navy formula uses only height and circumferences. Your weight is used afterwards to turn the percentage into kilograms of fat and lean mass.
Why do I get an error?
The formula needs your waist to be bigger than your neck (or waist plus hips for women). If the measurements give a result outside a plausible range, check them and measure again.