How to use the body Fat Calculator
- Choose your sex and your units.
- Enter your weight. It is only used to split your weight into fat mass and lean mass.
- Measure your height, neck and waist with a soft tape, keeping it level and snug but not compressing the skin. Women also measure the widest part of the hips.
- Measure each site two or three times and use the average.
- The result updates instantly as you type. There is no button to press.
- Use Copy result to copy the figures, or Copy link to share a link that reopens the body Fat Calculator with the same inputs.
How it works
The US Navy method estimates body fat percentage from a few tape-measure readings. It was developed by researchers at the Naval Health Research Center in the 1980s so that personnel could be assessed quickly without lab equipment, and it is still used by the US military today. Its logic is simple: the waist (and, for women, the hips) is where much body fat is stored, while the neck and height help account for overall frame size.
Compared with lab methods such as DEXA scans or hydrostatic weighing, the Navy formula is typically accurate to within about 3–4 percentage points for most people, which is similar to many consumer smart scales and far better than guessing. Accuracy depends heavily on measuring technique, so measure at the same time of day, before eating, and in the same places each time. Tracking the trend over weeks matters more than any single reading.
The category shown uses the ranges published by the American Council on Exercise. Women naturally carry more essential fat than men, related to hormones and reproduction, so their ranges are higher. If you enter your weight, the calculator also splits it into fat mass and lean mass, which includes muscle, bone, organs and water.
Formula
All measurements in centimetres; imperial inputs are converted at 2.54 cm per inch. The 495/… − 450 step is the Siri equation, which converts body density into body fat percentage.
Example
A man who is 178 cm tall with a 38 cm neck and an 85 cm waist: waist − neck = 47, so log₁₀(47) ≈ 1.672 and log₁₀(178) ≈ 2.250. The density term is 1.0324 − 0.19077 × 1.672 + 0.15456 × 2.250 ≈ 1.0612, and 495 ÷ 1.0612 − 450 ≈ 16.4% body fat, in the "fitness" range. At 80 kg that is about 13.1 kg of fat mass and 66.9 kg of lean mass.
Frequently asked questions
How accurate is the Navy body fat method?
Studies typically find it within about 3–4 percentage points of lab methods for most adults. It can be less accurate for very lean or very muscular people, and for people whose fat is stored mainly outside the waist.
Where exactly should I measure my waist?
For men, measure horizontally at the level of the navel. For women, measure at the narrowest part of the natural waist. Relax and breathe out gently before reading the tape.
What is a healthy body fat percentage?
Ranges vary by source and age, but roughly 10–20% for men and 18–28% for women is often considered healthy. Essential fat, the minimum needed for normal function, is about 2–5% for men and 10–13% for women.
Disclaimer: This calculator provides general estimates for education only and is not medical advice, diagnosis or treatment. Formulas are based on population averages and may not fit your body, age, pregnancy or health conditions. Talk to a doctor, midwife or registered dietitian before making decisions about your health.