What is body fat percentage?
Body fat percentage is the share of your total body weight that is fat. Unlike BMI, it separates fat from muscle, bone and water, so two people with the same height and weight can have very different results. This calculator estimates it from a few tape-measure readings using the US Navy circumference method.
You enter your sex, height, neck and waist in centimeters, plus hips if you're female. The result is shown to one decimal place.
The US Navy formula
The method was developed by J. A. Hodgdon and M. B. Beckett at the Naval Health Research Center in 1984 to give the Navy a quick body fat screen that needed only a tape measure. Their equations predict body density from circumferences, and the result is turned into a body fat percentage with the Siri equation (%BF = 495 ÷ density − 450).
In the form that takes measurements in centimeters:
- Men: %BF = 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450
- Women: %BF = 495 ÷ (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450
log10 is the base-10 logarithm. The bigger your waist is relative to your neck, the higher the estimate. Taller people get a slightly lower estimate for the same waist-to-neck difference.
Worked example
A man who is 178 cm tall with a 38 cm neck and an 86 cm waist:
- Waist minus neck: 86 − 38 = 48 cm, and log10(48) = 1.6812
- log10(178) = 2.2504
- Density: 1.0324 − 0.19077 × 1.6812 + 0.15456 × 2.2504 = 1.0595
- Body fat: 495 ÷ 1.0595 − 450 = 17.2%
A woman who is 165 cm tall with a 33 cm neck, 76 cm waist and 100 cm hips: waist + hip − neck = 143 cm, the density works out to 1.0314, and body fat is 495 ÷ 1.0314 − 450 = 29.9%.
How to measure
Accuracy depends almost entirely on the tape measure. Small errors in the waist reading change the result noticeably.
- Measure in the morning, before eating, standing relaxed.
- Height: barefoot, against a wall.
- Neck: just below the larynx, with the tape sloping slightly down toward the front.
- Waist: men at the level of the navel; women at the narrowest point of the abdomen. Breathe out normally and don't pull your stomach in.
- Hips (women): around the widest part of the buttocks, with feet together.
- Take each measurement two or three times and use the average.
If your tape is in inches, the length converter will convert to centimeters (1 inch = 2.54 cm).
How it compares with other methods
| Method | What it needs | Notes |
|---|---|---|
| US Navy circumference | Tape measure | Free and repeatable; an estimate from population data |
| Skinfold calipers | Calipers and a trained tester | Depends heavily on the tester's skill |
| Bioelectrical impedance | Smart scale or handheld device | Affected by hydration, meals and exercise |
| DEXA scan | Clinic or lab | Commonly used as a reference method in research |
Limits
- The equations were built from a military population, so they fit some body types better than others.
- Results for very muscular, very lean or very heavy people are less reliable.
- The calculator rejects a waist that isn't larger than the neck (men) or a waist plus hip that isn't larger than the neck (women), since the formula can't work with those values.
- Use it for trends. Measuring the same way every few weeks tells you more than any single reading.
Related calculators
- BMI calculator: the height-and-weight screen that body fat percentage complements.
- Ideal weight calculator: a reference weight for your height.
- Calorie calculator: daily calories to lose fat or build muscle.
- BMR calculator: your resting calorie burn.
This calculator is for general information and doesn't replace advice from a healthcare professional.
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