How the body fat calculator works
The historical circumference method comes from Hodgdon and Beckett's 1984 Naval Health Research Center reports for men (84-11) and women (84-29). This calculator implements the density equations below, followed by the Siri conversion to estimated fat percentage. It does not implement current military eligibility or fitness-assessment rules.
men: D = 1.0324 - 0.19077 log10(waist - neck) + 0.15456 log10(height) women: D = 1.29579 - 0.35004 log10(waist + hip - neck) + 0.22100 log10(height) fat % = 495 / D - 450 (Siri conversion; lengths in cm)
Here D is estimated density in g/cm³, and log10 is the base-10 logarithm. Inch measurements are multiplied by 2.54 first. The metric circumference coefficients are also reproduced in Table 1 of Zhang and colleagues' 2001 thermal-model study.
The BMI comparison uses the adult equation in Deurenberg, Weststrate, and Seidell (1991): fat % = 1.20 BMI + 0.23 age - 10.8 sex - 5.4, where sex = 1 for men and 0 for women. BMI is weight in kilograms divided by height in metres squared. The paper gives a separate equation for children aged 15 and younger, which this calculator does not use.
Worked example
The defaults describe a 30 year old man, 70 inches tall, 180 pounds, with a 15 inch neck and a 34 inch waist. In centimeters that is a height of 177.8, neck of 38.1, and waist of 86.36, so waist minus neck is 48.26 cm.
log10(48.26) = 1.6836 log10(177.8) = 2.2499 D = 1.0324 - 0.19077 x 1.6836 + 0.15456 x 2.2499 = 1.0324 - 0.3212 + 0.3478 = 1.0590 fat % = 495 / 1.0590 - 450 = 17.4% fat mass = 180 lb x 0.174 = 31.4 lb lean mass = 180 lb - 31.4 lb = 148.6 lb BMI = 703 x 180 / 70^2 = 25.8, so the BMI estimate is 1.2 x 25.8 + 0.23 x 30 - 10.8 - 5.4 = 21.7%
The circumference estimate is about 17.4%, and the BMI estimate is about 21.7%. Both are predictions from the entered measurements. The calculated fat and lean masses inherit the uncertainty of the circumference estimate; lean mass is not a separate measurement of muscle.
Reference categories shown by the calculator
| Category | Women | Men |
|---|---|---|
| Essential fat | Below 14% | Below 6% |
| Athletes | 14% to below 21% | 6% to below 14% |
| Fitness | 21% to below 25% | 14% to below 18% |
| Average | 25% to below 32% | 18% to below 25% |
| Obese | 32% and above | 25% and above |
This table documents the fixed boundaries used for the result's legacy ACE labels. Classification uses the unrounded estimate. These labels do not establish health, fitness, or a recommended target. In particular, the lowest label is not a statement that every percentage below its boundary is safe.
Measurement sites and limitations
For the male equation, the waist input means abdominal circumference at the navel. For the female equation, it means the natural waist at the narrowest part of the abdomen, with hips measured around the greatest projection of the buttocks. These sites are described in the 2022 USARIEM body-composition study, pages 8–9. Tape placement and the measurements entered affect the estimate.
A population equation cannot provide a precise individual body-fat measurement. This calculator does not calculate an uncertainty interval, diagnose a condition, or determine a diet or exercise plan. If a result will inform a medical decision, ask a qualified health professional which assessment is appropriate. See the calculator methodology and limitations.