What this calculator does
Body fat percentage tells you what fraction of your body weight is fat versus everything else (muscle, bone, organs, water). Two people at the same weight and height can sit at very different body fat percentages, one lean and muscular, one not. BMI doesn’t catch that distinction; body fat percentage does.
This calculator uses the US Navy’s circumference formula. It needs only a tape measure: height, neck, waist, and (for women) hip. Plug in the numbers, get an estimate accurate to within about 3 percentage points for most people. That’s not as precise as a DEXA scan but it’s good enough to track progress over weeks of training and dieting, and it costs $0 instead of $80.
How to measure correctly
The biggest source of error in this calculator isn’t the formula, it’s how people measure. Here’s the cheat sheet:
- Height: stand barefoot against a wall, no shoes. Round to the nearest 0.5 cm.
- Neck: measure just below the larynx (Adam’s apple). Tape should be snug, not pulling tight, not loose enough to slide. Look straight ahead while measuring.
- Waist: men measure at the navel. Women measure at the narrowest point of the torso, which is usually slightly above the navel. Don’t suck in.
- Hip: women only. Measure the widest point of the hips and butt. Stand with feet together.
Take each measurement twice. If they disagree by more than half a centimeter, take a third. Use the median.
How the formula works
The US Navy method was originally developed for the military to assess body composition without expensive equipment. It uses log-transformed circumferences:
- 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 math comes from a regression study correlating circumferences with hydrostatic weighing (the gold-standard underwater method that’s hard to do outside a lab). The formula isn’t perfect but it’s been validated on tens of thousands of people across body types.
What the percentages mean
The categories vary slightly between sources, but the standard ranges:
Men:
- 2-5%: essential fat, bodybuilder competition lean, not sustainable
- 6-13%: athletic
- 14-17%: fitness
- 18-24%: average
- 25%+: above average / clinically overweight
Women:
- 10-13%: essential fat
- 14-20%: athletic
- 21-24%: fitness
- 25-31%: average
- 32%+: above average / clinically overweight
Women carry essential fat that men don’t, reproductive function depends on it. Going below 14% body fat as a woman often disrupts menstruation and bone density.
When this calculator is wrong
The formula assumes a typical body shape. It overestimates fat in very muscular people because muscle adds to the waist circumference (lifters with thick obliques look “fatter” than they are). It underestimates fat in people with long torsos and short legs. Anyone with significant edema, post-pregnancy, or unusual proportions should treat the result as a rough ballpark, not a precise number.
For more accuracy, the next steps up are:
- Skinfold calipers ($15, takes practice but ±2% accuracy)
- Bioimpedance scales ($50-200, varies wildly, morning measurement after waking is most consistent)
- DEXA scan ($80-150 per scan, gold standard, ±1%)
Frequently asked questions
Why does my smart scale show a different number? Bioimpedance scales send a tiny current through your body to estimate fat. They’re sensitive to hydration, the same body can read 18% in the morning and 22% in the evening. Trust trends over single readings.
How fast can I drop body fat? Sustainable rate is about 0.5-1% per month for most people. Aggressive cuts of 2%+ per month usually mean losing muscle alongside fat. The calorie calculator pairs with this one, a 500 kcal/day deficit gives roughly 1 lb fat loss per week.
Should I aim for the lowest body fat possible? No. Below 6% (men) or 14% (women) is hard on hormones, mood, sleep, and athletic performance. The “fitness” range is the sustainable target for most people who want to look lean and stay healthy.
Can I use this for kids? The formula was validated on adults. Children’s body composition changes with growth, pediatricians use age-and-sex-specific charts instead.