Body Fat Calculator

Body fat percentage tells you what portion of your body weight is fat versus everything else — muscle, bone, water, and organs. Unlike BMI, which only looks at weight relative to height, this measure directly reflects body composition, so two people with the same BMI can have very different body fat percentages depending on how much muscle they carry.

This calculator uses the US Navy circumference method: a formula built from your height plus a few tape-measure readings (neck and waist for men; neck, waist, and hip for women). It was developed for the US Navy's own body composition standards specifically because it's accurate enough for practical use without calipers or a DEXA scan — just a flexible tape measure and a few centimeters of precision.

Your result includes a category — Essential Fat, Athletic, Fitness, Average, or Obese — based on the American Council on Exercise's sex-specific classification bands. These are descriptive ranges, not a diagnosis: body fat naturally varies with age, training history, and genetics. For the most reliable trend over time, measure at the same time of day, in the same conditions, and track changes over weeks rather than reacting to any single reading.

A worked example

Take a man who is 180 cm tall with a 38 cm neck and an 85 cm waist. The male formula uses waist minus neck: 85 − 38 = 47 cm. Take the base-10 logarithm of that (log₁₀47 ≈ 1.6721) and of the height (log₁₀180 ≈ 2.2553), then plug both into the denominator: 1.0324 − 0.19077 × 1.6721 + 0.15456 × 2.2553 ≈ 1.0620. Divide 495 by that and subtract 450: 495 ÷ 1.0620 − 450 ≈ 16.1% body fat — landing in the "Fitness" category on the ACE scale (13.1–17% for men).

A common misconception

A single body fat percentage often gets read as more precise than it actually is. The Navy circumference method is generally within about 3–4% of a DEXA scan for people in the roughly 15–30% body fat range it was originally validated on — a real but imperfect level of agreement, not lab-grade precision. It's also easy to compare your number directly to someone else's without accounting for how differently fat is distributed between individuals — two people at the same body fat percentage can look quite different depending on where that fat sits.

Where the standard method is less reliable

The Navy method was developed and validated on a population resembling active-duty military personnel — generally fit, within a normal weight range, and mostly under 50 — so its accuracy drops for people well outside that profile. It tends to underestimate body fat in people with obesity, and can be thrown off by atypical fat distribution or unusually high muscle mass, since it infers body fat purely from a few circumference measurements rather than measuring tissue directly. DEXA scanning, by contrast, images bone, fat, and lean tissue separately and is accurate to within about 1–2%, which is why it — not a tape-measure formula — is considered the clinical gold standard when precision genuinely matters.

How to use your result

A single reading is less useful than a trend. Because the Navy method depends on precise circumference measurements, small differences in where or how snugly you place the tape can shift the result by a percentage point or two — measuring at the same time of day, in the same way, and tracking the number over weeks smooths out that noise and shows real change far more reliably than any one measurement.

Body fat vs. BMI

BMI and body fat percentage answer different questions from the same starting point. BMI only needs weight and height and says nothing about what that weight is made of; body fat percentage, estimated here from actual circumference measurements, describes body composition directly. Two people can share an identical BMI and land in very different body fat categories — which is exactly the gap this calculator is built to fill, particularly for anyone whose BMI reading doesn't match how they actually look or feel.