Body fat percentage describes what your weight is made of, which BMI cannot do. This calculator uses the circumference equations developed by Hodgdon and Beckett at the Naval Health Research Center in 1984 — the same approach the US military uses for body composition screening — so all you need is a flexible tape measure.
How to use the body fat calculator
- Choose US (inches) or Metric (centimeters) and select your sex.
- Enter your height, then your neck and waist circumferences. Women also enter hips.
- Optionally add your weight to split it into fat mass and lean mass.
- Read your body fat percentage and ACE category on the tape; the bar shows where you fall among the categories.
US Navy body fat formulas
All measurements are in inches, and log₁₀ is the common logarithm.
Fat mass is weight × %BF ÷ 100, and lean mass is the rest: muscle, bone, organs and water.
Worked example
A man who is 70 inches tall measures 15 inches at the neck and 34 inches at the navel, and weighs 180 lb.
Waist − neck = 34 − 15 = 19 in
%BF = 86.010 × log₁₀(19) − 70.041 × log₁₀(70) + 36.76 = 109.99 − 129.23 + 36.76 = 17.5%
Fat mass = 180 × 0.175 ≈ 31.5 lb; lean mass ≈ 148.5 lb. ACE category: fitness.
For a woman who is 65 inches tall with a 13-inch neck, 29-inch waist and 38-inch hips, waist + hips − neck = 54 inches and the equation gives about 27.3%, in the ACE average range.
ACE body fat categories
The American Council on Exercise publishes these general ranges for adults:
| Category | Women | Men |
|---|---|---|
| Essential fat | 10–13% | 2–5% |
| Athletes | 14–20% | 6–13% |
| Fitness | 21–24% | 14–17% |
| Average | 25–31% | 18–24% |
| Obese | 32% and up | 25% and up |
Women’s ranges sit higher because essential fat includes tissue tied to reproduction and hormones. The calculator treats each boundary as the start of the next category, so a man at 17.5% counts as fitness and one at 18.0% as average.
How accurate is the tape method?
Lab techniques such as DEXA scans, underwater weighing and air-displacement plethysmography measure body composition more directly, but they cost money and need equipment. Validation studies generally put circumference equations within about 3–4 percentage points of those methods for typical adults. Accuracy drops at the extremes: very lean, muscular people can be overestimated, and the formula may underestimate fat in people who carry much of it on the limbs. Because the equations depend on logarithms of differences, a half-inch error in the waist moves the result by about one percentage point.
Measuring tips
- Use a non-stretch tape, snug against the skin but not compressing it.
- Men measure the waist horizontally at the navel; women at the narrowest point between ribs and hips.
- Measure the neck just below the larynx with the tape angled slightly downward toward the front.
- Take each measurement two or three times and use the average. Measure at the same time of day, before eating, when tracking progress.
For a weight-for-height screen, compare your result with the BMI calculator; to see how lean mass affects calorie burn, try the Katch–McArdle option in the BMR calculator.
This estimate is informational only and is not medical advice. Body composition cannot diagnose a health condition; consult a healthcare professional for a personal assessment.
Frequently asked questions
How accurate is the US Navy body fat method?
Compared with laboratory methods, circumference equations like the Navy's usually come within about 3 to 4 percentage points for most adults. Errors grow for very lean or very heavy people and for anyone whose fat is distributed unusually. Consistent measuring matters more than the method if you want to track change.
Why does the formula for women include the hips?
Women tend to store more fat around the hips and thighs, so the waist alone misses much of it. The women's equation adds hip circumference to the waist and subtracts the neck, while the men's version uses waist minus neck.
What is a healthy body fat percentage?
The American Council on Exercise classes 21–24% as fitness and 25–31% as average for women, and 14–17% as fitness and 18–24% as average for men. Below about 10–13% for women or 2–5% for men is essential fat, the minimum needed for normal body function.
Can I enter measurements in centimeters?
Yes. Choose Metric and enter centimeters; the calculator converts them to inches before applying the Navy equations, which were published in inch form. The result is the same either way.
Why is this different from my bathroom scale's body fat reading?
Smart scales use bioelectrical impedance, which is sensitive to hydration, recent meals and exercise, so their readings can swing by several points in a day. Tape methods depend on measuring technique instead. Neither is a lab test, and the two often disagree by a few percentage points.