US Navy tape-measure estimate
US Navy tape-measure estimate
BMI uses only height and weight, so it cannot distinguish muscle from fat. A muscular athlete and a sedentary person of the same height and weight receive the same BMI despite entirely different body composition. Body fat percentage measures the thing people actually care about.
This calculator uses the US Navy circumference method, which estimates body fat from tape measurements. It requires no equipment beyond a flexible tape measure and takes about a minute.
Measurement error is the largest source of error in this method, so technique matters more than the formula.
Neck: just below the larynx, with the tape sloping slightly downward at the front. Do not flare the shoulders or tense the neck.
Waist: for men, at the navel. For women, at the narrowest point of the torso. Measure after a normal exhale — do not suck in, and do not push the tape into the skin.
Hips (women): at the widest point of the buttocks.
Keep the tape level and snug without compressing. Measure at the same time of day, ideally in the morning before eating, and take each measurement twice. A half-inch difference at the waist shifts the result by roughly a full percentage point.
The Navy method uses logarithmic regression equations:
Men: 86.010 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76
Women: 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387
The women's formula includes hip circumference because fat distribution differs, and using waist alone would systematically misestimate. Both formulas work by inferring how much of your girth is fat rather than frame.
Commonly cited ranges, though these vary by source and population:
Women carry more essential fat than men for physiological reasons including hormone production. Comparing the two sets of numbers directly is not meaningful.
The Navy method typically falls within about 3–4 percentage points of DEXA scanning — useful for tracking but not precise. It was developed on military populations and tends to be less accurate at the extremes: very lean, very heavy, or unusually proportioned individuals will see larger deviations.
Because it infers fat from circumference, it cannot distinguish a thick waist caused by fat from one caused by muscular obliques or a broad frame. Nor can it detect visceral fat specifically, which is the type most associated with metabolic risk.
The practical implication: use it to track your own trend over time rather than to establish an exact figure. A reading that moves from 22% to 19% over three months is meaningful even if neither number is exactly right, because the measurement error is consistent in the same direction.
DEXA scan is the practical gold standard, giving fat, lean mass, and bone density with regional breakdown. Costs money and requires a facility.
Hydrostatic weighing and Bod Pod are accurate but less accessible.
Skinfold calipers are accurate in trained hands and unreliable in untrained ones.
Bioelectrical impedance scales are convenient and heavily affected by hydration — readings can swing several percentage points between morning and evening on the same day.
For most purposes, consistent tape measurements plus progress photographs tell you more about change over time than an expensive one-off measurement.
Typically within 3-4 percentage points of a DEXA scan for average builds. It is less accurate for very lean, very heavy, or unusually proportioned individuals. It is best used to track change over time rather than to establish an exact figure.
Commonly cited fitness ranges are around 14-17% for men and 21-24% for women. Essential fat - the minimum needed for physiological function - is roughly 2-5% for men and 10-13% for women.
BMI uses only height and weight, so it cannot tell muscle from fat. A muscular person can register as overweight by BMI while carrying low body fat. Body fat percentage measures composition directly.
Every two to four weeks is enough. Body fat changes slowly, and measuring more often mostly captures measurement noise and day-to-day fluid shifts rather than real change.