Body fat calculator
Enter your height, neck and waist (and hips for women) to estimate body fat percentage with the US Navy formula, and see your fat and lean mass.
Your body fat
17.5%
Below essential fat for a man by the US Navy method
How to use it
- Pick your sex and units. The formula is different for men and women because women carry essential fat in different places.
- Measure your neck just below the larynx, and your waist at the navel (men) or the narrowest point (women), with the tape snug but not pulling. Women also measure the hips at the widest point.
- Enter your height and, if you want fat and lean mass in pounds or kilograms, your weight.
- Read the percentage and the category. Measure at the same time of day, before eating, and track the trend over weeks rather than the single number.
Worked example
A man 5 ft 10 in tall with a 15 in neck and a 34 in waist, weighing 175 lb.
Waist minus neck is 19 in. 86.010 × log10(19) − 70.041 × log10(70) + 36.76 = 110.0 − 129.2 + 36.8 = 17.5%, at the top of the fitness range. Fat mass 30.6 lb, lean mass 144.4 lb.
Two inches off the waist, everything else the same, gives 13.4%.
How this is calculated
The US Navy method estimates body fat from circumferences rather than weight, which is why it can tell a muscular person from a heavy one where BMI can't. For men it uses the waist minus the neck; for women, the waist plus the hips minus the neck. Those girths and the height go into a logarithmic formula that the Navy fitted to underwater weighing results, and the answer is a percentage of body weight that is fat. It's accurate to within about three to four percentage points for most adults when the tape is placed correctly, and it drifts for very lean or very heavy people and for anyone whose fat sits unusually. Metric measurements are converted to inches and the same formula applied. Fat mass is the percentage times your weight; lean mass is everything else, and it's the number to watch when you're dieting, because losing it is the failure mode.
Men: body fat % = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76
Women: body fat % = 163.205 × log10(waist + hips − neck) − 97.684 × log10(height) − 78.387
All measurements in inches; centimetres are converted at 2.54 cm to the inch.
fat mass = weight × body fat %, lean mass = weight − fat mass
Categories are the American Council on Exercise ranges.
Rates, sources and limits
An estimate for healthy adults from a formula, not a clinical measurement, and not medical advice. Anyone with a medical condition should use a clinician's assessment.
Last reviewed 23 Sep 2026 by the CalcRabbit team. Estimates only, not financial, tax or medical advice.
Frequently asked questions
How accurate is the tape method?
Studies put the Navy formula within about 3 to 4 percentage points of underwater weighing or a DEXA scan for most adults, which is comparable to consumer bioimpedance scales and better than BMI. It's least accurate at the extremes and for people with an unusual build. Its real strength is consistency: measured the same way each time, it tracks change well.
Where exactly do I measure?
Neck: just below the Adam's apple, tape sloping slightly downward at the front, not pressing in. Waist for men: horizontally at the navel, after a normal exhale, without sucking in. Waist for women: at the narrowest point of the torso. Hips for women: horizontally at the widest point of the buttocks. A cloth tape, snug but not tight.
What's a healthy body fat percentage?
For men, roughly 8% to 20% covers the athletic-to-average range, with essential fat around 2% to 5%. For women, 14% to 31%, with essential fat at 10% to 13%. The categories shown are the American Council on Exercise's. Below essential fat is dangerous, not impressive.
Why does it say to check my measurements?
The formula takes the logarithm of the waist minus the neck, so if the waist isn't bigger than the neck there's no answer. That almost always means one of them was entered in the wrong unit, or the neck was measured too low.
Is this better than a smart scale?
Different, and often steadier. Bioimpedance scales swing with hydration, meals and the time of day; a tape measure doesn't. Neither is a lab measurement. For tracking a diet or a training block, pick one method, use it the same way each week, and watch the trend.