Body Fat Calculator
Estimate adult body fat with the classic Navy circumference equation and compare with a BMI-based estimate, including fat mass and lean mass.
This calculator compares two adult body-fat estimates: a historical circumference equation and the Deurenberg BMI equation. It also estimates fat mass and fat-free mass from the tape result. These models use population relationships, so disagreement between them is expected and neither is a direct measurement of body fat.
How to use this tool
- Choose your sex and enter your age.
- Measure height, neck and waist — and hips for women — in inches or centimeters.
- Enter your weight to get fat mass and lean mass.
- Compare the circumference and BMI estimates, remembering that neither gives an individual diagnosis.
Formula
Men: %BF = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76 · Women: %BF = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387
Measurements are in inches (centimeters are converted first). Men measure the waist horizontally at the navel and women at the narrowest point; hips go around the widest part of the buttocks and the neck just below the larynx. The BMI-based figure uses the Deurenberg equation: %BF = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4, with sex = 1 for men and 0 for women. The original Deurenberg sample spanned BMI 13.9–40.9 and ages up to 83; those limits are guards against extrapolation, not an accuracy guarantee.
Example
A 35-year-old man, 70 in tall and 180 lb, with a 15 in neck and a 34 in waist: %BF = 86.010 × log10(19) − 70.041 × log10(70) + 36.76 = 17.5%. That is about 31.5 lb of fat and 148.5 lb of lean mass. His BMI of 25.8 gives a Deurenberg estimate of 22.8%.
Frequently asked questions
How accurate is the Navy tape method?
Circumference equations estimate group relationships. Individual errors can be substantial and no fixed accuracy is guaranteed. Consistent tape placement and repeated readings reduce measurement error but do not remove model error.
Why do the two estimates differ?
The BMI method cannot tell muscle from fat, so it overestimates body fat in muscular people and can underestimate it in people with little muscle. The tape method responds to where fat is carried, especially around the waist.
What is a healthy body fat percentage?
A percentage alone does not establish health. Age, health, body shape and measurement technique affect interpretation. This tool intentionally gives no diagnostic category or individual target. The BMI estimate is withheld outside the original study age/BMI envelope.
Sources
This tool gives an estimate for general information only. It is not financial, tax, legal or medical advice — check with a qualified professional before you rely on the result.
Last updated: 2026-10-30