What this calculates
Enter your height, weight and a few tape measurements — neck and waist, plus hips for women — and this estimates your body fat percentage by the US Navy method, splits your weight into fat and lean mass, and shows where you sit on the American Council on Exercise (ACE) chart.
The formula
Men (centimetres)
%fat = 495 ÷ (1.0324 − 0.19077 × log(waist − neck) + 0.15456 × log(height)) − 450
Women (centimetres)
%fat = 495 ÷ (1.29579 − 0.35004 × log(waist + hip − neck) + 0.22100 × log(height)) − 450
Fat and lean mass
fat mass = weight × %fat; lean mass = weight − fat mass"log" is the base-10 logarithm. Inches are converted to centimetres first (× 2.54). The method was published by Hodgdon and Beckett for the US Navy in 1984.
Worked example
At 17.2% he is in the ACE fitness range for men (14–17%). A woman of 165 cm with a 33 cm neck, 76 cm waist and 98 cm hips comes out at 28.9%, in the average range for women (25–31%).
When to use it, and the mistakes to avoid
Use it to track progress while losing weight or building muscle — the trend over weeks says more than any single reading — or to get a fuller picture than BMI gives.
The mistakes that cost the most:
- Measuring the waist in the wrong place. Men measure at the navel, women at the narrowest point; a few centimetres changes the result by several percent.
- Pulling the tape tight or sucking in. Keep it snug and stand relaxed.
- Mixing units. Every measurement must be in the same unit as the height.
- Comparing across methods. Smart scales, callipers and this formula all differ; compare a method only with itself.
- Chasing a very low number. Below essential fat (about 5% for men, 13% for women) is unhealthy, not fitter.
FAQ
How accurate is the US Navy body fat method?
For most people it lands within about 3 to 4 percentage points of a laboratory measurement, which is close for a tape measure. It is less reliable for very muscular people and those with a lot of abdominal fat. Measuring the same way each time matters more than the absolute figure.
How do I take the measurements?
Use a soft tape, snug but not pressing into the skin, and measure standing, relaxed, not sucking in. Neck: just below the Adam's apple. Waist: at the navel for men, at the narrowest point for women. Hips (women): at the widest point. Take each twice and use the average.
What is a healthy body fat percentage?
On the American Council on Exercise chart, men are in the fitness range at 14-17% and average at 18-24%; women are in the fitness range at 21-24% and average at 25-31%. Above 25% for men or 32% for women counts as obese. Essential fat - the minimum the body needs - is 2-5% for men and 10-13% for women.
Why do women have more body fat than men?
Women need more essential fat for hormones and reproduction, so every category on the chart sits about 8 points higher for women. That is also why the women's formula uses the hips as well.
What is lean body mass?
Everything that is not fat - muscle, bone, organs and water. A man of 80 kg with 17.2% body fat carries about 13.8 kg of fat and 66.2 kg of lean mass. When losing weight, the aim is to lose fat while keeping lean mass.
Is body fat percentage better than BMI?
It tells you more, because BMI cannot tell muscle from fat: a muscular person can have an 'overweight' BMI and a low body fat. But BMI needs no tape measure and is more repeatable. The two together give a better picture than either alone.