Body Fat Calculator (US Navy Method)
How This Calculator Works
US Navy circumference method. Different formula for men and women — women's includes hip circumference because more body fat sits there.
All values in inches.
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
Lean body mass = weight × (1 − BF%/100)
Fat mass = weight × BF%/100Citation: Hodgdon JA, Beckett MB. "Prediction of percent body fat for U.S. Navy men/women from body circumferences and height." Naval Health Research Center Technical Report 84-29, San Diego, 1984.
What This Number Tells You
Body fat percentage is more useful than weight alone for tracking body composition. Two people at the same weight and height can look completely different — the lean one has more muscle and less fat. This calculator gives you a directional estimate; consistency of method matters more than absolute precision. Measure the same way every time.
Limitations
Accurate within roughly ±3–4% versus DEXA gold-standard. Less reliable at the extremes — very lean (under 6% men, under 12% women) or very heavy. Sensitive to measurement technique: a tape measure pulled tight reads tighter than relaxed; a different measurement site changes the answer. For best results, take the same measurements, in the same order, at the same time of day, and average a few tries.
Frequently Asked Questions
How does the US Navy method work?
It uses tape-measure circumferences and your height. The math is empirical — Hodgdon and Beckett at the Naval Health Research Center fit log-regression equations to body fat measured by hydrostatic weighing across thousands of sailors. The result: a formula that needs only a tape measure and a calculator, with about ±3–4% accuracy versus underwater weighing.
Where exactly do I measure?
Neck: just below the larynx, perpendicular to the long axis of the neck. Don't pull the tape tight. Waist (men): at the level of the navel. Waist (women): at the narrowest point above the hipbones. Hip (women): at the widest point of the buttocks. Stand relaxed; breathe normally; measure to the nearest 0.5 inch.
Why is the formula different for men and women?
Because women carry significantly more fat in the hips and thighs, which the waist circumference alone doesn't capture well. Adding the hip measurement to the women's formula fixes that. Without it, the calculation would systematically overestimate body fat in women — which is why early circumference methods didn't work well for women.
How accurate is this versus DEXA?
The Navy method is accurate within roughly 3–4% of body fat measured by hydrostatic weighing, which itself agrees with DEXA within similar margins. So if your real body fat is 18%, this calculator might say anywhere from 14% to 22%. Useful for tracking direction over time (is it going down?), less useful as an exact snapshot.
What's a healthy body fat percentage?
ACSM ranges (acceptable, not obese): men 18–24%, women 25–31%. 'Fitness' ranges: men 14–17%, women 21–24%. 'Athlete' ranges: men 6–13%, women 14–20%. Below those, you start losing essential fat (men under ~3%, women under ~12%) which can cause hormonal and reproductive problems.
Why is my body fat % different from my BMI suggestion?
Because they measure different things. BMI is just weight per height squared — it can't tell muscle from fat. A muscular person with 12% body fat can have a BMI in the 'overweight' range. A sedentary person with 30% body fat can have a 'normal' BMI. Body fat percentage is a more direct measure of body composition.
How often should I remeasure?
Every 2–4 weeks at most. Body fat changes slowly — a half percent in a couple weeks is meaningful progress. Daily or weekly measurements just capture noise (water, digestive content, where you're holding the tape). Take the same measurements at the same time of day, ideally morning, fasted, and average across a few attempts to reduce error.
Is this method reliable for very lean or very heavy people?
Less so. The formulas were derived from sailors, who skewed toward the average and athletic ranges. Very lean people (under ~6% men, ~12% women) often find the method overestimates their body fat. Very heavy people often find it underestimates, because waist circumference alone can't fully capture extreme fat distribution. DEXA or hydrostatic weighing give better data at the extremes.