How it's calculated
The U.S. Navy method comes from two 1984 Naval Health Research Center reports by Hodgdon and Beckett. They predict body density from tape measurements in centimeters, and the Siri equation turns density into percent body fat:
Women: density = 1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.22100 × log₁₀(height)
Body fat % = 495 ÷ density − 450
US inputs are converted to centimeters first, so both unit systems give the same answer.
Worked example
A man 70 in tall (177.8 cm) with a 34 in waist (86.36 cm) and a 15 in neck (38.1 cm):
= 1.0324 − 0.32121 + 0.34777 = 1.05896
Body fat = 495 ÷ 1.05896 − 450 = 17.4%
How to measure
- Use a flexible tape, snug but not pressing into the skin, and measure twice.
- Neck: just below the Adam's apple (larynx), with the tape sloping slightly down at the front.
- Waist: men at the navel; women at the narrowest point. Breathe out normally and don't pull in.
- Hips (women): around the widest part of the buttocks.
ACE body fat categories
| Category | Women | Men |
|---|---|---|
| Essential fat | 10–13% | 2–5% |
| Athletes | 14–20% | 6–13% |
| Fitness | 21–24% | 14–17% |
| Acceptable | 25–31% | 18–24% |
| Obesity | 32% and above | 25% and above |
These are the American Council on Exercise's labels. They're general fitness norms, not medical cut-offs.
The BMI-based estimate (less accurate)
For comparison, the page also shows the Deurenberg (1991) formula, which estimates body fat from BMI, age and sex only:
It can't see where your weight is, so it's usually further off than the tape method, especially for muscular people.
Limitations
- The Navy reports give a typical error of about 3.5 body fat percentage points for men, so treat the result as a range.
- The formulas were built on adult Navy personnel. They may be less accurate for older adults, very lean or very large bodies, and during pregnancy.
- Small tape errors at the waist or neck shift the result noticeably.
- Methods such as DXA scans are more precise if you need a measured value.
For anyone under 18, these calculators show a pointer to the CDC's BMI-for-age calculator and growth charts instead of adult results, because children and teens are still growing.
Sources
- Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men from body circumferences and height. Naval Health Research Center Report 84-11, 1984.
- Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy women from body circumferences and height. Naval Health Research Center Report 84-29, 1984.
- American Council on Exercise: Percent body fat calculator and norms
- Deurenberg P, Weststrate JA, Seidell JC. Body mass index as a measure of body fatness: age- and sex-specific prediction formulas. Br J Nutr 1991;65:105–114.
- Siri WE. Body composition from fluid spaces and density: analysis of methods (1961), in Techniques for Measuring Body Composition, National Academy of Sciences.