Body fat percentage tells you how much of your weight is fat versus lean tissue like muscle, bone, and water. It is often more useful than the scale alone, because two people at the same weight can have very different body compositions.
This body fat calculator uses the US Navy circumference method, which estimates body fat from a few tape measurements instead of skinfold calipers or expensive scans. It is quick, repeatable at home, and a practical way to track changes over time as you train and adjust your nutrition.
How the body fat % calculator works
The calculator uses the US Navy circumference (tape-measure) method. You measure a few body circumferences and your height, and the formula uses the base-10 logarithm (log10) of the differences between them.
For men, it needs neck, waist, and height:
%BF = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76
For women, it adds the hip measurement:
%BF = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387
In US units, all measurements are in inches. The method works because waist (and hip, for women) tends to grow with stored fat, while the neck stays relatively lean, so the difference between them tracks body fat. No calipers, scales, or scans are required, just a flexible tape measure and an accurate height. Measure on bare skin, keep the tape level and snug without compressing, and re-measure for consistency.
Worked example
A man with a 34-inch waist, 16-inch neck, and 70-inch height: waist − neck = 18, and log10(18) = 1.2553, so 86.010 × 1.2553 = 107.97. Then log10(70) = 1.8451, so 70.041 × 1.8451 = 129.23. Putting it together: 107.97 − 129.23 + 36.76 = 15.5% body fat. That places him in a fit, athletic range for men.
Things to keep in mind
- It is an estimate, not a direct measurement. Lab methods like DEXA or hydrostatic weighing are more accurate.
- Accuracy depends heavily on correct, consistent tape placement; a half-inch error at the waist can shift the result by several percent.
- The formula was developed on general adult populations and may be less accurate for very muscular athletes, pregnant women, or people with unusual fat distribution.
- It estimates total body fat percentage only and tells you nothing about where fat is stored or your metabolic health.
Frequently asked questions
How accurate is the US Navy body fat calculator?
For most people it lands within about 3 to 4 percentage points of a DEXA scan, which is good for a free tape-measure method. Accuracy improves when you measure carefully and use the same technique each time, so trends over weeks are more reliable than any single reading.
Where exactly do I measure my waist and neck?
Measure your neck just below the larynx, sloping slightly downward to the front. Men measure the waist at the navel; women measure at the narrowest point. Women also measure the hips at the widest point. Keep the tape level, snug, and on bare skin without compressing it.
Why do women need a hip measurement but men do not?
Women typically store more fat around the hips and thighs, so adding the hip circumference makes the estimate more accurate for female body composition. The US Navy formula reflects this with separate equations and constants for men and women.
What is a healthy body fat percentage?
General fitness ranges are roughly 10 to 20 percent for men and 18 to 28 percent for women, with athletes often lower. Essential fat is about 2 to 5 percent for men and 10 to 13 percent for women. These are guidelines, not medical thresholds, so consult a professional for personal advice.
Do I need calipers or a special scale for this?
No. The whole point of the US Navy method is that it needs only a flexible tape measure and your height. There are no calipers, smart scales, or scans involved, which makes it easy to repeat at home.
How often should I re-measure?
Every two to four weeks is plenty. Body fat changes slowly, and day-to-day fluctuations in water and food can muddy the picture. Measure at the same time of day under similar conditions to keep your comparisons fair.