The US Navy method uses neck, waist, and (for women) hip circumference plus height — no scale needed, just a flexible measuring tape.
The pre-filled example — male, 170 cm tall, a 38 cm neck, and an 85 cm waist — runs through the same formula as the FAQ example below, just in metric: bodyFat% = 86.010 × log10(85−38) − 70.041 × log10(66.93) + 36.76 ≈ 17.9%, landing in the "Fitness" category, just under the line into "Average." Converting the centimeter measurements to inches first (170 cm ≈ 66.93 in) is exactly what this calculator does internally — the formula itself always runs in inches, regardless of which unit is entered.
Body fat does not scale in direct proportion to the waist-minus-neck gap — each additional inch around the waist matters slightly less than the one before it, which is the kind of diminishing relationship a logarithm captures naturally. That is also why the formula is fitted from real circumference and body-composition data (the US Navy's original validation study) rather than derived from first principles the way a geometric formula like area or volume would be — a statistical approximation tuned to fit a population, not an exact physical law.
Two people with identical height, neck, and waist measurements can still carry meaningfully different amounts of visceral fat, muscle mass, or bone density, none of which this formula distinguishes — it only sees the outside shape. That is the trade-off for a method needing nothing more than a tape measure: convenience and reasonable trend-tracking accuracy, not laboratory precision. For the same measurements checked against military body-composition standards instead of general fitness categories, see the Army Body Fat Calculator; once a body fat percentage is known, the Lean Body Mass Calculator converts it directly into pounds or kilograms of fat versus everything else.
Typically within about 3-4% of more precise clinical methods like DEXA scans or hydrostatic (underwater) weighing. That's more than accurate enough for tracking trends over time using nothing but a flexible tape measure, though it's not a substitute for a proper medical-grade body composition test if precision matters for your situation.
Measure your neck just below the larynx (Adam's apple), your waist level with the navel, and — for women — hips at their widest point. Use a flexible cloth or vinyl tape, keep it snug against the skin without pulling tight or compressing it, and measure standing normally rather than flexing muscles or sucking in your stomach, since either will throw the result off.
Body fat distribution patterns differ meaningfully by sex — men tend to store more visceral fat around the abdomen, while women store proportionally more around the hips and thighs. The US Navy method accounts for this by using a waist-minus-neck relationship for men and a waist-plus-hip-minus-neck relationship for women, each with its own set of fitted constants, rather than applying one formula to both.
BMI is calculated from height and weight alone, so it can't tell muscle from fat — a heavily muscled person can register as "overweight" on BMI despite having very low body fat. Body fat percentage estimates the fat portion directly from body measurements, making it a more informative number for body composition specifically. See the BMI Calculator for that quicker screening number, or the Ideal Weight Calculator for a target weight range by height.
For men: roughly 2-5% is classed as essential fat, 6-13% is typical of athletes, 14-17% a general fitness range, 18-24% average, and 25%+ falls into the obese category. For women: 10-13% is essential fat, 14-20% athletes, 21-24% fitness, 25-31% average, and 32%+ obese. Essential fat is the minimum your body needs for normal hormonal and organ function — going below it isn't a fitness goal, it's a health risk.
Worked example: a man 70 inches tall with a 15-inch neck and a 34-inch waist: bodyFat% = 86.010 × log10(34−15) − 70.041 × log10(70) + 36.76 = 86.010 × 1.2788 − 70.041 × 1.8451 + 36.76 ≈ 17.51% — squarely in the "Fitness" category.
For the specific standard used by the US military, see the Army Body Fat Calculator. Entering weight in the other unit? The kg to pounds converter gives the exact figure rather than the usual rough doubling.