This tool estimates your body fat percentage using the U.S. Navy circumference method, a widely used formula that relies on a tape measure rather than specialized equipment. Knowing your body fat percentage gives you a clearer picture of your body composition than scale weight alone, because two people at the same weight can carry very different amounts of muscle and fat. That makes it a practical reference for setting realistic fat-loss or muscle-building targets.
The U.S. Navy method uses the circumference of your neck, waist, and, for women, hips, along with your height. The core idea is that the ratio between these measurements tracks how much fat you carry. The equations are logarithmic. For men, body fat percentage is based on the log of (waist minus neck) and the log of height. For women, it is based on the log of (waist plus hip minus neck) and the log of height. In plain terms, a larger waist relative to neck and height pushes the estimate up, while a leaner midsection pulls it down.
Imagine a man who is 180 cm / 5 ft 11 in tall and 80 kg / 176 lb. He measures a waist of 85 cm / 33.5 in and a neck of 38 cm / 15 in. The method takes the difference between waist and neck, roughly 47 cm / 18.5 in, and compares it against his height. Feeding those numbers through the Navy equation returns a body fat estimate in the mid-teens, around 15 percent, which would place him in the fitness range. If his waist grew to 95 cm / 37.4 in with everything else unchanged, the estimate would climb by several percentage points, showing how sensitive the result is to waist size.
Select your sex, then enter your height, weight, waist circumference (measured horizontally at the navel), and neck circumference (measured just below the larynx). Women also enter hip circumference at the fullest point. Toggle between inches and centimeters or pounds and kilograms as needed, then press Calculate. Your result appears alongside five reference categories: Essential, Athletes, Fitness, Average, and Obese.
The Navy method is an estimate, not a clinical measurement. It works best for people with average builds and can drift for very muscular or very lean individuals, since it infers fat from shape rather than measuring it directly. Measurement technique matters a great deal, so a tape held at an angle or pulled too tight can swing your result by several points. Direct methods such as DEXA scans, hydrostatic weighing, or skinfold testing are more precise. Treat this number as a trend line to watch over weeks, not a diagnosis, and consult a physician or registered dietitian before making major changes.
Is the Navy method accurate? It gives a reasonable estimate for most people, usually within a few percentage points of lab methods. Use it as a reference point and track the direction of change over time rather than fixating on a single reading.
Where exactly do I measure my waist? Measure horizontally at the navel, not at the narrowest part of your torso, keeping the tape snug but not compressing the skin.
Why do women need a hip measurement? Women typically store fat differently, so the female equation includes the hip circumference to improve the estimate.
How often should I remeasure? Every two to four weeks is plenty, taken under the same conditions for a fair comparison.