Body Fat Calculator
Enter your sex, height, neck, waist, and (for women) hip circumference to get an estimated body fat percentage using the US Navy method.
How It Works
This calculator uses the US Navy circumference method. For men:
body fat % = 495 / (1.0324 - 0.19077 x log10(waist - neck) + 0.15456 x log10(height)) - 450.
For women:
body fat % = 495 / (1.29579 - 0.35004 x log10(waist + hip - neck) + 0.221 x log10(height)) - 450.
All measurements go into the formula in centimeters, so imperial entries are converted
before the calculation runs.
Worked Example
A man who is 180 cm tall with a 38 cm neck and 90 cm waist: waist minus neck = 52 cm. Log10 of 52 is about 1.716, log10 of 180 is about 2.255. Plugging in: 495 / (1.0324 - 0.19077 x 1.716 + 0.15456 x 2.255) - 450 = 495 / (1.0324 - 0.3274 + 0.3487) - 450 = 495 / 1.0537 - 450, which works out to about 19.8 percent body fat.
Why Waist and Neck, Specifically
Notice that body weight never appears in the formula at all. Instead, the whole calculation rests on the gap between waist and neck circumference (plus hip for women). Neck size acts as a rough stand-in for someone's basic frame, since it barely changes as a person gains or loses fat. Waist size, by contrast, is one of the areas most sensitive to fat gain, especially abdominal fat. Subtracting neck from waist gives a number that reflects how much bigger your midsection is than your baseline frame, and that gap is what actually drives the result: a wider gap pushes the estimate up, a narrower one pulls it down. Height is folded in separately because the same waist-minus-neck gap means something different on someone 150 cm tall versus someone 200 cm tall.
This is also why measuring technique matters more here than in most calculators on this site. A waist measurement taken an inch higher or lower, or a tape pulled too tight, changes the gap the whole formula is built on.
Where This Method Falls Short
This formula does not measure fat directly, it estimates it from a handful of tape measurements using coefficients fitted to a reference population years ago. It has no way to account for how much of your weight is muscle versus fat, so two people with identical height, neck, and waist numbers get the identical body fat estimate, even if one of them is considerably more muscular than the other. In practice, this means the method tends to be less accurate at the extremes: it can overestimate body fat for very muscular people, whose larger frames and waist size come from muscle rather than fat, and it can be a poor fit for very lean people, whose proportions may not match the population the formula was built on. A lab method like a DEXA scan, which directly measures bone, lean tissue, and fat mass with imaging rather than inferring them from circumference, will generally be more accurate for anyone in those situations. For most people in an average range, though, this method is a reasonable, free way to estimate body fat and track it over time.
Frequently Asked Questions
How accurate is the US Navy body fat method?
It is generally accurate within about 3 to 4 percent of body fat compared to more precise methods like DEXA scans, for most body types. It tends to be less accurate for people who are very lean or who carry weight in unusual proportions.
Why does the formula only need three or four measurements?
The method was developed by the US Navy as a fast field measurement using tape-measure circumferences instead of equipment like calipers or scanners, trading some precision for speed and simplicity.
Where exactly should I measure my waist and neck?
Measure the neck just below the larynx, with the tape sloping slightly downward toward the front. Measure the waist at its narrowest point for men, or at the level of the belly button for women. Measure hips at their widest point. Keep the tape snug but not compressing the skin.
Why is hip measurement only used for women?
The original Navy formula was developed and validated separately for men and women, and the women's version includes hip circumference because fat distribution patterns differ between sexes.
Can I use inches instead of centimeters?
Yes. Choose inches from the unit selector and enter your measurements as usual. The calculator converts everything to centimeters internally before applying the formula, since the published coefficients are defined in metric units.
What should I do with my body fat percentage once I have it?
It is most useful as a baseline to track change over time using the same measuring technique, rather than as an exact clinical figure. Re-measure every few weeks under similar conditions to see a trend.