Use the free Body Fat Percentage Calculator below: your data never leaves your browser — all work happens on your device, and nothing is uploaded to any server.
What This Calculator Does
A body fat percentage calculator estimates how much of your weight is fat and how much is everything else: muscle, bone, water, and organs. Two people can weigh the same and look very different if one carries more muscle and the other carries more fat. That is why body fat percentage is often more useful than weight alone. This tool uses the US Navy method, which estimates body fat from a few tape measurements instead of expensive scans. The Navy developed it as a quick field test for large groups of service members.
For men, the formula uses height, neck circumference, and waist circumference. For women, it also uses hip circumference, because fat distribution differs. The math uses logarithms of the measurements, which is why the tool handles the calculation for you. Typical healthy ranges often quoted for men are roughly 10 to 20 percent, and for women roughly 18 to 28 percent, though athletes sit lower and older adults sit higher. These ranges are guides, not rules, and the Navy method itself can be off by about 3 to 4 percentage points for an individual.
How to Use It
Choose your sex, then measure with a soft tape. Measure your neck just below the Adam’s apple, keeping the tape snug but not tight. Measure your waist at the navel, relaxed, after breathing out normally. Do not suck in your stomach. Women also measure hips at the widest point. Type all measurements in centimetres and click Calculate. Measure in the morning, before eating, for the most consistent results. Re-measure every two to four weeks rather than daily, because small tape errors can swamp real weekly change.
The Formula, Explained
The men’s formula is 495 divided by a density estimate, minus 450. The density estimate is 1.0324 minus 0.19077 times the log of waist minus neck, plus 0.15456 times the log of height. The women’s formula uses 1.29579 minus 0.35004 times the log of waist plus hip minus neck, plus 0.22100 times the log of height. The logs work with any consistent unit, so centimetres are fine. The key idea is simple: a waist that is large compared with the neck (and hips for women) points to more body fat, while a taller frame adjusts the estimate down.
A Worked Example
Take a man who is 170 centimetres tall, with a 38-centimetre neck and an 85-centimetre waist. His waist minus neck is 47. The formula gives roughly 19 percent body fat. If his waist grows to 95 centimetres with the same neck and height, the estimate rises to roughly 27 percent. That shows why waist change matters so much: the same person, ten centimetres larger at the waist, moves from a typical range into a higher one. For a woman, wider hips raise the estimate less than a wider waist does, which matches how the formula treats fat distribution.
Common Mistakes
The most common mistake is measuring the waist while sucking in, which can flatter the result by several percent. Another is pulling the tape tight over clothes; measure on bare skin. A third is comparing a Navy estimate directly with a DEXA scan and assuming one is wrong. They measure different things with different errors. Use one method consistently and watch the trend. If your waist is shrinking and your strength is steady, your body composition is almost certainly improving, whatever the exact percentage says.
Frequently Asked Questions
How accurate is the Navy method? For groups it is decent, but for one person expect an error of a few percentage points. Can I use it if I am very muscular? Very muscular people may get a flattering result, because a thick neck lowers the estimate. What is a good target? Aim for a range that supports your health and performance, not the lowest number you can reach. Extremely low body fat can disrupt hormones, immunity, and mood.
Educational estimate only. This is not medical advice. If you are concerned about your weight or body composition, talk to a health professional.
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