Body Fat Calculator

Estimate your body fat percentage with nothing more than a tape measure, using the circumference equations developed for the US Navy. You also get fat and lean mass, and an honest idea of the margin of error.

The equations were built from data on biological sex.

Just below the larynx (Adam's apple), tape sloping slightly down to the front.

Men: around the navel. Women: at the narrowest point. Relax, don't suck in.

Around the widest part of the buttocks. Ignored for men.

Estimated body fat
28.9%
Average
Category
Average
Typical error range (± 1 standard error)
25.2% to 32.6%
Fat mass
46 lb
Lean mass
114 lb
  • Categories for women (American Council on Exercise): below 14% may be dangerously low; 25% to 31% is typical for non-athletes; 32% or more is in the obesity range.
  • Tape-measure estimates can be several percentage points off for an individual. Use the same tape, the same spots and the same time of day to track change over time.
  • An estimate for information only, not medical advice.

How the Navy method works

In the 1980s, researchers at the Naval Health Research Center measured hundreds of sailors with a tape and compared the results with body fat measured by underwater weighing. Their equations (Hodgdon and Beckett, 1984) predict body density from the logarithms of a few circumferences and height, and body fat percentage follows from density. The Navy adopted them to check body fat standards, and the Air Force has used slightly modified versions.

Men need height, neck and waist (measured around the navel). Women also need hip circumference, and their waist is measured at the narrowest point.

How accurate is it?

Against underwater weighing, the original equations had a standard error of about 3.5 percentage points for men and 3.7 for women, according to a National Academies review of military body composition methods. In plain terms, if the calculator says 25%, your true figure could easily be anywhere from about 21% to 29%.

Errors are likely to be larger for people whose build differs a lot from the military groups the equations were developed in, such as very muscular or very heavy people. Measuring technique matters too: half an inch at the waist can move the result by about half a point to a full percentage point.

Lab methods such as DEXA scans, underwater weighing and air displacement (Bod Pod) are more accurate but cost money. For tracking progress at home, consistency matters more than the absolute number.

Reading your result

The American Council on Exercise considers 18 to 24% body fat typical for men who aren't athletes and 25 to 31% typical for women, with 25% or more for men and 32% or more for women in the obesity range. It warns that below about 6% for men or 14% for women may be dangerously low.

The ranges differ by sex because women naturally carry more body fat than men. If your number is high, a waist-to-height ratio is another quick check of the fat around your middle, which is the type most linked to heart disease and type 2 diabetes.

The US Navy equations (measurements in cm)

Men: body fat % = 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450Women: body fat % = 495 ÷ (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450
  • log10 = base-10 logarithm
  • Inches are converted to cm by multiplying by 2.54
  • The denominator is body density; 495 ÷ density − 450 is the Siri equation

Example

  1. A woman 5 ft 5 in tall (165.1 cm) with a 13.5 in neck (34.29 cm), 30 in waist (76.2 cm) and 39 in hips (99.06 cm).
  2. Waist + hip − neck = 76.2 + 99.06 − 34.29 = 140.97 cm. log10(140.97) = 2.1491; log10(165.1) = 2.2177.
  3. Density = 1.29579 − 0.35004 × 2.1491 + 0.221 × 2.2178 = 1.0336.
  4. Body fat = 495 ÷ 1.0336 − 450 = 28.9%, in the 25 to 31% range typical for women who aren't athletes.
  5. At 160 lb, that is about 46 lb of fat and 114 lb of lean mass.

Frequently asked questions

How accurate is the US Navy body fat calculator?

Within about 3.5 to 3.7 percentage points of underwater weighing for a typical person, based on the original studies. Errors can be larger for very lean, very muscular or very heavy people.

Where exactly do I measure my waist?

For the Navy equations, men measure around the navel and women at the narrowest part of the torso. Keep the tape level and snug but not compressing the skin, and measure after breathing out normally.

What is a healthy body fat percentage?

The American Council on Exercise lists 18 to 24% as typical for non-athlete men and 25 to 31% for women, and treats 25% (men) or 32% (women) and above as obesity. Very low levels, under about 6% for men or 14% for women, can be risky.

Why does the formula need hip measurements for women?

Women tend to store more fat around the hips and thighs, so the women's equation adds hip circumference to the waist to capture it.

Is body fat percentage better than BMI?

It says more about what your weight is made of, since BMI can't tell muscle from fat. But tape-measure estimates have their own errors, so it's best used alongside BMI and waist size rather than instead of them.

Sources

Last reviewed for 2026. How we calculate.