Calculating your body fat percentage (circumference method)

Calculating your body fat

The calculator asks for height, neck circumference and abdominal circumference. For women, it also asks for hip circumference. The percentage shown is an estimate, rounded to one decimal place.

Women, report 84-29, measurements in centimetres. Density = −0.35004 × log10(abdomen + hips − neck) + 0.22100 × log10(height) + 1.29579. The abdomen in this equation is abdomen I: the smallest circumference, roughly halfway between the tip of the breastbone and the navel. The neck is measured just below the larynx. The report converts density with the Siri formula (1961): percentage = 100 × (4.95 / density − 4.50).

Men, report 84-11: density = −0.19077 × log10(abdomen − neck) + 0.15456 × log10(height) + 1.0324, then the same conversion. The male abdomen is taken at navel level. The PDF of this report could not be opened on the day of writing (access denied). The coefficients are those of the equation published under this report number, of the same form as the women's equation read in report 84-29.

How to take your measurements

Tape horizontal, on bare skin, without pulling tight. Neck: just below the larynx, looking straight ahead, shoulders relaxed. Man: abdomen at the navel, at the end of a normal exhalation. Woman: smallest abdominal circumference (abdomen I of the report), and hips at the widest point of the buttocks.

A difference of one centimetre changes the result. Taking the same measurement twice and keeping the same time of day from one day to the next limits that noise.

Interpreting the result

The women's report gives a correlation of 0.85 with hydrostatic weighing, and a standard error of 0.00796 g/cm³, i.e. 3.72 percentage points. A chapter of the National Research Council indicates a standard error of about 3.5 points for the Navy circumference equations.

This page publishes no “normal percentage” table by age. No such range was read in these reports. The Navy's administrative thresholds (for example 26 % and 36 % in older instructions) are internal rules, not a health target.

For a man who trains, the calculator gives no “athletic” target. A low or high percentage is discussed in context, not against an invented threshold.

The other methods: skinfolds, bioimpedance, DEXA

Skinfolds (callipers) measure the thickness of fat under the skin at a few points, then an equation estimates the percentage. It requires a trained operator.

Bioimpedance sends a very weak current. Water, muscle and fat do not conduct in the same way. Hydration, a meal or exercise shift the reading. Vita works this way, with 8 electrodes. It is not a medical device.

DEXA (absorptiometry) is a reference measurement in clinical settings. A scale or a tape measure does not replace it.

Accuracy of each method

The circumference method is simple and reproducible if the circumferences are taken in the same place. Its published error is several points: 27 % and 31 % can describe the same person depending on the day and the tape.

None of these methods, on this page, is presented as accurate to the decimal. The figure serves to follow an order of magnitude, not to diagnose.

Frequently asked questions

Sources

• Hodgdon JA, Beckett MB, Prediction of percent body fat for U.S. Navy women, report 84-29, Naval Health Research Center, 1984
• Hodgdon JA, Beckett MB, Prediction of percent body fat for U.S. Navy men, report 84-11, Naval Health Research Center, 1984
• Body Composition Assessment guide, U.S. Navy, January 2023 (measurement sites)
• National Research Council, Body Composition and Physical Performance, Navy circumference equations

Read also

• Limits of bioimpedance
• Vita connected scale
• BMI calculator