About this tool
This body fat calculator estimates the percentage of your weight that is fat from a few tape measurements, using the circumference method developed for the US Navy. Men enter height, neck, and waist; women add the hip measurement. The result is placed in the American Council on Exercise (ACE) categories (essential fat, athletes, fitness, average, obese), and if you enter your weight the page splits it into fat mass and lean mass and shows the weight you would need to reach other body fat levels while keeping lean mass constant. For comparison it also shows the Deurenberg estimate, which predicts body fat from BMI, age, and sex, so you can see how a tape measure and a scale differ. The method needs only a flexible tape measure, takes a minute, and is accurate enough to track progress over weeks and months, which makes it useful for anyone training to lose fat or gain muscle who wants something better than BMI without a DEXA scan or calipers.
How it works
The US Navy method (Hodgdon and Beckett, 1984) fits body density to circumferences and height, then converts density to fat percentage with the Siri equation. In centimeters with base-10 logarithms the combined formulas are: men, body fat % = 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450; women, body fat % = 495 ÷ (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450. Measurements are taken with the tape snug but not compressing the skin: the neck just below the larynx, the waist at the navel for men and at the narrowest point for women, and the hips at their widest. Fat mass is weight × body fat % and lean mass is the remainder; the target-weight table divides current lean mass by (1 − target fat %). The Deurenberg comparison (1991) is body fat % = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4, where sex is 1 for men and 0 for women. Inches and pounds are converted to centimeters and kilograms before calculation.
Frequently asked questions
How accurate is the US Navy body fat method?
Against hydrostatic weighing, the reference method it was calibrated to, the Navy formula has a standard error of about 3.5 percentage points for men and 4 for women, similar to skinfold calipers in trained hands and better than most consumer bioimpedance scales. Its weakness is people with unusual proportions, such as very muscular necks or fat carried mainly on the hips, where it can drift by more. Measure at the same time of day, with the same tape, and watch the trend rather than any single reading.
What body fat percentage is healthy?
The ACE ranges used here are: essential fat 2–5% for men and 10–13% for women, athletes 6–13% and 14–20%, fitness 14–17% and 21–24%, average 18–24% and 25–31%, and obese above 25% for men and 32% for women. Women carry more essential fat for hormonal and reproductive reasons. Body fat also rises modestly with age at the same weight, and very low levels, below the essential range, are unhealthy for anyone who is not competing.
Why does the Deurenberg BMI estimate differ from the tape measurement?
The Deurenberg formula only knows your BMI, age, and sex, so it assumes an average body composition for those inputs; it cannot tell a muscular person from an overfat one at the same weight and height. The Navy method reads waist and neck size directly, which respond to fat rather than muscle. When the two disagree, the tape measurement is usually closer for athletic people and the BMI estimate closer for sedentary people with average builds.
How do I measure correctly?
Use a flexible cloth or fiberglass tape, hold it level and snug against bare skin without denting it, and read to the nearest 0.5 cm or quarter inch. Neck: just below the Adam's apple, sloping slightly downward at the front. Waist: men at the navel, women at the narrowest point between ribs and hips, after a normal exhale, stomach relaxed. Hips (women): around the widest part of the buttocks. Take each measurement two or three times and use the average.