Body Fat Calculator

Your bathroom scale tells you how much you weigh. It cannot tell you how much of that weight is fat versus muscle, bone, and water. Two people can weigh the same amount but have completely different health profiles — a 180 lb person at 12% body fat carries about 22 lb of fat and 158 lb of lean tissue, while a 180 lb person at 28% body fat carries 50 lb of fat and 130 lb of lean tissue. Same number on the scale. Very different bodies.

The U.S. Navy method estimates this split using circumference measurements at specific anatomical sites. The logic is geometric: fat accumulates predictably around the waist and hips, while the neck circumference serves as a proxy for overall frame and muscularity. By comparing the fat-storage sites against a height-adjusted baseline, the formula produces a body fat estimate without any body density measurement or water immersion required.

The math uses logarithmic scaling to model the non-linear relationship between circumference and fat percentage. A one-inch increase in waist circumference does not mean the same thing at 28 inches as it does at 40 inches — fat density and distribution change with total adiposity. The log transformation captures this curve and is why the formula stays reasonably accurate across a wide body composition range, from lean athletes to individuals with obesity.

This tool is appropriate when you want a quick, free body composition estimate without gym equipment or a medical appointment. It works well for tracking changes over weeks and months — as long as you measure at the same sites under the same conditions each time, the trend is reliable even if the absolute number has some error. It is also the right tool for setting a concrete fat loss target when you want to preserve lean mass.

Do not rely on this result for medical decisions. If you are being treated for a metabolic condition, preparing for bariatric surgery, or your physician needs body composition data for dosing or risk assessment, a DEXA scan or hydrostatic weighing provides the required precision. The Navy method can also lose accuracy at the extremes — very lean individuals below 8% body fat and individuals with very large abdominal circumferences both fall outside the range where the formula was validated. Results in those cases should be treated as rough estimates only.

This calculator is also not appropriate as the sole diagnostic tool for eating disorders or for setting body fat targets below medically safe minimums. Essential fat serves real physiological functions — hormone regulation, nerve protection, organ cushioning. Targets below 5% for men or 12% for women should only be pursued under medical supervision.

The most common mistake is measuring at the wrong anatomical site. For men, the waist measurement should cross the navel — not the narrowest point of the torso. Measuring above the navel pulls the number down artificially, producing an optimistic result. For women, the hip measurement must capture the widest circumference of the hips and buttocks, not just the hip bones. Measuring too high understates the hip value and overstates body fat.

A second frequent error is measuring over clothing or holding the tape too loosely. The tape should sit flat against the skin with light tension — not tight enough to compress tissue, but snug enough that a finger cannot slide underneath. Loose measurements overstate circumference and inflate the body fat estimate by 2-3 percentage points, which is the difference between the fitness and average categories for many people.

The third mistake is treating the result as more precise than it is. People enter numbers to two decimal places and then anchor on a result like 19.3% as if it were laboratory-verified. The Navy formula has a standard error of around 3-4 percentage points, meaning 19.3% really means somewhere between 15% and 23%. Use the category — Athletic, Fitness, Average — as your primary signal. Use the percentage as a tracking tool over time, where consistent measurement technique makes the trend meaningful even if the absolute number has uncertainty.

The male formula is: Body Fat % = 86.010 x log10(waist - neck) - 70.041 x log10(height) + 36.76

The female formula is: Body Fat % = 163.205 x log10(waist + hip - neck) - 97.684 x log10(height) - 78.387

All measurements must be in inches. If you measure in centimeters, divide each circumference by 2.54 before applying the formula.

To find fat mass: multiply body weight by (body fat percentage divided by 100). To find lean mass: subtract fat mass from total weight. To find the weight needed to reach a target body fat percentage while preserving lean mass, use: target body weight = lean mass / (1 - target body fat fraction). The fat to lose is then the difference between current weight and target body weight. This approach assumes lean mass stays constant — a reasonable approximation for fat loss with adequate protein intake.

The Navy formula assumes that circumference measurements capture fat distribution accurately, but this breaks down for people with high muscle mass in the torso. A powerlifter with a large neck and only a moderately large waist will appear leaner than they are because the formula interprets the large neck as a non-fat frame marker. Conversely, someone with significant visceral fat but relatively small external waist circumference — common in individuals of East or South Asian descent — may be underestimated. If your result contradicts visible evidence or performance metrics, consider it a lower bound rather than a point estimate.