Body Fat Calculator
Estimate your body fat percentage using the US Navy circumference method. Enter your sex, height, and neck/waist (and hip, for women) circumference measurements to get an instant estimate — no scale or caliper needed.
How It's Calculated
Formula
\text{Male: } BF\% = \frac{495}{1.0324 - 0.19077 \times \log_{10}(waist - neck) + 0.15456 \times \log_{10}(height)} - 450
\text{Female: } BF\% = \frac{495}{1.29579 - 0.35004 \times \log_{10}(waist + hip - neck) + 0.22100 \times \log_{10}(height)} - 450This calculator uses the US Navy circumference method (Hodgdon & Beckett, 1984), the same body-composition screening formula the US Navy itself uses, to estimate body fat percentage from a few tape-measure readings instead of calipers, a scale, or a DEXA scan. Male and female bodies distribute fat differently, so the two sexes use different equations built from different regression studies: the male formula uses waist and neck circumference plus height, while the female formula adds hip circumference into the mix. All measurements are converted internally to inches, since the published formula constants were calibrated in that unit — enter your measurements in either centimeters or inches and the result comes out the same either way. Because this is a circumference-based estimate rather than a direct measurement of body composition, tape measurement technique matters a lot: keep the tape snug but not compressing the skin, keep it level and parallel to the floor, and measure at the same spot each time for consistent tracking. This is an estimate for general informational and educational purposes, based on a published formula — it is not a medical diagnosis, and it is not a substitute for individualized advice from a qualified healthcare provider. This formula was validated primarily on a military population, so it may be less accurate for body types that differ substantially from that population.
Worked Examples
Male example: neck 15 in, waist 34 in, height 70 in
- Compute waist − neck = 34 − 15 = 19
- log10(19) = 1.278754, log10(70) = 1.845098
- Denominator = 1.0324 − 0.19077×1.278754 + 0.15456×1.845098 = 1.073631
- Body Fat % = 495 / 1.073631 − 450 = 461.052 − 450 = 11.05%
Female example: neck 13 in, waist 38 in, hip 44 in, height 64 in
- Compute waist + hip − neck = 38 + 44 − 13 = 69
- log10(69) = 1.838849, log10(64) = 1.806180
- Denominator = 1.29579 − 0.35004×1.838849 + 0.221×1.806180 = 1.051264
- Body Fat % = 495 / 1.051264 − 450 = 470.852 − 450 = 20.85%
Frequently Asked Questions
Why do men and women use different body fat formulas here?
Men and women tend to carry and distribute body fat differently — women naturally carry more essential fat and store proportionally more of it around the hips. The Navy's 1984 regression studies were run separately for each sex, producing two different sets of constants and, for women, an equation that also factors in hip circumference. Using the male equation on a female measurement set (or vice versa) would produce an inaccurate result, which is why sex determines which formula this calculator applies.
Why isn't hip circumference used for men?
The original Navy research found that for men, waist and neck circumference (combined with height) were the statistically significant predictors of body fat percentage — adding hip circumference didn't materially improve the male regression model's accuracy. For women, hip circumference did add meaningful predictive value, so it's part of the female equation only. This calculator still shows a Hip Circumference field for every user, since the form doesn't change based on sex, but that value has no effect on a male result.
How is this different from BMI?
BMI is just weight divided by height squared — it can't distinguish muscle mass from fat mass, so a muscular person can register as "overweight" despite low body fat. This circumference method estimates body fat percentage directly from measurements that correlate with actual adiposity, giving a meaningfully different (and usually more informative) picture of body composition than BMI alone.
How can I get the most accurate measurement?
Use a flexible, non-stretchy tape measure, and keep it snug against the skin without compressing it or pulling it tight. Keep the tape perfectly horizontal (a helper makes this much easier for waist and hip). Measure at the same time of day for repeat measurements — first thing in the morning, before eating, tends to be most consistent. Take each measurement two or three times and use the average, since even an off-level tape or a slightly wrong landmark can shift the result by several percentage points.