How to use the Body Fat Calculator
- Choose your units and select male or female; women also enter a hip measurement.
- Measure your neck, waist and (for women) hips with a flexible tape, following the hints under each box.
- Enter your height, and your weight if you want fat mass and lean mass.
- Read your estimated body fat percentage and where it falls in the category table.
Features
- U.S. Navy circumference equations for men and women
- Metric or imperial input, converted automatically
- Fat mass and lean body mass from your weight
- American Council on Exercise categories with your band highlighted
- Measuring tips for each site to keep results consistent
- Private: measurements never leave your browser
How the U.S. Navy method works
Laboratory methods such as underwater weighing or DEXA scans measure body composition directly, but they need special equipment. In the 1980s, researchers at the Naval Health Research Center (Hodgdon and Beckett) developed equations that predict body density from height and a few circumference measurements, so that body fat could be checked with just a tape measure.
The idea is simple. Fat collects around the waist (and, in women, the hips), while the neck mostly reflects frame size and muscle. The larger your waist is compared with your neck and height, the higher your estimated body fat.
The formulas
With all measurements in centimeters:
- Men: body fat % = 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450
- Women: body fat % = 495 ÷ (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450
The part in parentheses estimates body density, and "495 ÷ density − 450" is the Siri equation that turns density into a fat percentage. Inches are converted to centimeters first, so the result agrees with the inch-based version of the formula to within a fraction of a percent. Fat mass is your weight × body fat %, and lean mass is the rest: muscle, bone, organs and water.
Worked example
A man who is 178 cm tall with a 38 cm neck and an 86 cm waist has waist − neck = 48 cm. The density term is 1.0324 − 0.19077 × log10(48) + 0.15456 × log10(178) ≈ 1.0324 − 0.3207 + 0.3478 = 1.0595. Then 495 ÷ 1.0595 − 450 ≈ 17.2% body fat, in the fitness range. At 80 kg that is about 13.8 kg of fat and 66.2 kg of lean mass.
How to measure correctly
- Use a flexible tape that does not stretch. Keep it level and snug against bare skin without pressing in.
- Neck: just below the larynx (Adam's apple), with the tape sloping slightly downward toward the front.
- Waist, men: horizontally at navel level, at the end of a normal breath out.
- Waist, women: at the narrowest part of the torso, usually a little above the navel.
- Hips, women: around the widest part of the buttocks with your feet together.
- Measure each site two or three times and use the average, at the same time of day whenever you compare results.
Body fat categories
The table in the calculator uses ranges published by the American Council on Exercise (ACE):
| Category | Women | Men |
|---|---|---|
| Essential fat | 10–13% | 2–5% |
| Athletes | 14–20% | 6–13% |
| Fitness | 21–24% | 14–17% |
| Average | 25–31% | 18–24% |
| Obese | 32% and above | 25% and above |
Some fat is essential for hormones, nerves and protecting organs, and women naturally need more of it than men. A very low reading is not a goal in itself.
How accurate is it?
Tape-measure equations give estimates. For most people they land within a few percentage points of laboratory methods, but they can be further off for very lean, very muscular or very heavy people, and a measuring error of a centimeter or two noticeably changes the answer. They are most useful for tracking your own trend with the same tape and technique. For another angle, compare with the BMI Calculator. Results from this calculator are estimates for general information and are not medical advice.
Frequently asked questions
How accurate is the U.S. Navy body fat method?
For most people it comes within a few percentage points of laboratory methods. Accuracy drops for very muscular, very lean or very heavy people, and careful, consistent measuring matters more than anything else.
Why do women need a hip measurement?
Women tend to store more fat on the hips and thighs, so the women’s equation adds the hip circumference to the waist to capture it. The men’s equation uses only the waist and neck.
What is a healthy body fat percentage?
ACE classes 25–31% as average for women and 18–24% for men, with lower "fitness" and "athlete" ranges below that. Essential fat is about 10–13% for women and 2–5% for men.
Why is my result different from my smart scale?
Smart scales use bioelectrical impedance, which is affected by hydration, meals and exercise. The tape method uses body shape instead. Neither is exact, so compare each method only with itself over time.
Do I need to enter my weight?
No. Body fat percentage comes from height and circumferences alone. Weight is only used to split your body into fat mass and lean mass.