How the body fat calculator works
This calculator uses the US Navy circumference method, developed by Hodgdon and Beckett at the Naval Health Research Center in 1984 and still used for Navy fitness assessments. It needs only a tape measure: height, neck, and waist for men, plus hips for women. The formula estimates body density from those circumferences, then converts density to a percentage of fat with Siri's equation. Because the fit was done in centimeters, inch measurements are converted first (1 inch = 2.54 cm).
men: D = 1.0324 - 0.19077 log10(waist - neck) + 0.15456 log10(height) women: D = 1.29579 - 0.35004 log10(waist + hip - neck) + 0.22100 log10(height) fat % = 495 / D - 450 (Siri 1956; all lengths in cm)
The BMI line underneath uses a separate estimate from Deurenberg, Weststrate, and Seidell (1991): fat % = 1.20 BMI + 0.23 age - 10.8 sex - 5.4, with sex = 1 for men and 0 for women. It is shown for comparison only; because BMI cannot distinguish muscle from fat, the Navy result is the one to track.
Worked example
The defaults describe a 30 year old man, 70 inches tall, 180 pounds, with a 15 inch neck and a 34 inch waist. In centimeters that is a height of 177.8, neck of 38.1, and waist of 86.36, so waist minus neck is 48.26 cm.
log10(48.26) = 1.6836 log10(177.8) = 2.2499 D = 1.0324 - 0.19077 x 1.6836 + 0.15456 x 2.2499 = 1.0324 - 0.3212 + 0.3478 = 1.0590 fat % = 495 / 1.0590 - 450 = 17.4% fat mass = 180 lb x 0.174 = 31.4 lb lean mass = 180 lb - 31.4 lb = 148.6 lb BMI = 703 x 180 / 70^2 = 25.8, so the BMI estimate is 1.2 x 25.8 + 0.23 x 30 - 10.8 - 5.4 = 21.7%
At 17.4% he lands in the ACE fitness band. The BMI estimate is four points higher because, at a BMI of 25.8, the regression assumes an average build; a narrower waist relative to the neck is exactly what the tape measure picks up and BMI cannot.
Body fat categories (ACE)
| Category | Women | Men |
|---|---|---|
| Essential fat | 10 to 13% | 2 to 5% |
| Athletes | 14 to 20% | 6 to 13% |
| Fitness | 21 to 24% | 14 to 17% |
| Average | 25 to 31% | 18 to 24% |
| Obese | 32% and above | 25% and above |
Women carry more essential fat than men because of the fat in breast tissue and around reproductive organs, which is why every band sits roughly eight to ten points higher. These ranges come from the American Council on Exercise and are guidelines for adults rather than clinical cut-offs; age, ethnicity, and sport all shift what is normal.
Measuring well
The formula is only as good as the tape work. Measure on bare skin with a flexible tape, standing relaxed, and read to the nearest quarter inch or half centimeter. Take each site twice and average; if the two readings differ by more than an inch, take a third. Measure at the same time of day for repeat checks, ideally in the morning before eating, since the waist can move an inch over a day. A one inch waist error shifts the result by roughly two percentage points for men and one and a half for women, so consistency matters more than any single reading.
Navy method versus other methods
| Method | Typical error | Needs |
|---|---|---|
| DEXA scan | 1 to 2 points | Clinic visit, X-ray scanner |
| Underwater weighing | 1 to 2 points | Lab tank, breath hold |
| Skinfold calipers (7 site) | 3 to 4 points | Calipers, a trained tester |
| US Navy tape method | 3 to 4 points | A tape measure |
| Bioimpedance scale | 4 to 8 points | A scale; very hydration sensitive |
| BMI estimate | 5 or more points | Height and weight only |
For most people the tape method is the best trade between accuracy and effort. Its weakness is people at the extremes: very lean athletes with thick necks read low, and people who carry fat mainly on the hips and thighs (rather than the waist) can read low as well. If the number matters for a medical decision, a DEXA scan is the reference.