Body Fat Percentage Calculator

The US Navy developed this method to screen thousands of sailors without lab equipment. It predicts body density from circumference measurements, then converts that to a fat percentage. All lengths go into the formula in inches (this calculator converts metric inputs for you).

Men: BF% = 86.010 × log10(waist - neck) - 70.041 × log10(height) + 36.76

Women: BF% = 163.205 × log10(waist + hip - neck) - 97.684 × log10(height) - 78.387

Worked example: a 5'10" man (70 inches) with a 15-inch neck and a 34-inch waist. The waist minus neck difference is 19 inches, so the formula gives 86.010 × log10(19) - 70.041 × log10(70) + 36.76, which comes out to about 17.5% body fat. At 180 lb, that is roughly 31.5 lb of fat mass and 148.5 lb of lean mass, right at the top of the ACE fitness range.

The BMI tab uses the Deurenberg formula instead: BF% = 1.20 × BMI + 0.23 × age - 10.8 × sex - 5.4, where sex is 1 for men and 0 for women. It needs no tape measure, but it inherits all the weaknesses of BMI, so treat it as a rough second opinion.

DEXA is the practical gold standard for body composition. Validation studies put the Navy method within about 3 to 4 percentage points of DEXA on average, with larger errors for very lean or very heavy people. That beats most consumer bioimpedance scales, which swing with hydration, and it costs nothing.

The real strength of a tape measurement is consistency. If your Navy estimate drops from 24% to 21% over two months of training, fat is coming off, even if a DEXA scan would put the absolute numbers a little higher or lower. Measure the same spots, the same way, at the same time of day, and trust the trend more than any single reading.

The American Council on Exercise publishes these widely used category ranges. Women carry more essential fat than men because of hormone production and reproductive function, so every category sits higher for women.

Essential fat is the floor your body needs to function, not a goal. Competitive physique athletes dip near it briefly for a show and pay for it in hormones, sleep, and recovery. For most people, the fitness and average ranges are where health and sustainability meet.

• Neck:wrap the tape just below the larynx (Adam's apple), sloping slightly downward toward the front. Don't flare your neck muscles.
• Waist (men): measure at navel level, at the end of a normal exhale. No sucking in.
• Waist (women): measure at the narrowest point of the torso, usually just above the navel.
• Hips (women): measure at the widest point around the hips and glutes, feet together.
• Keep the tape level, snug against bare skin, and never tight enough to dent the skin.
• Take each measurement two or three times and use the average. Measure in the morning before eating for the most repeatable numbers.