Body fat calculator: body fat percentage from tape measurements
Estimate your body fat percentage with the US Navy tape method — three measurements, and an honest account of the error bars.
How the US Navy body fat method works
The US Navy circumference method was developed to screen personnel without laboratory equipment. It infers body fat from the relationship between height and a few circumferences: neck and waist for men, plus hips for women, because fat distribution differs enough that one equation cannot serve both.
It works because waist circumference tracks abdominal fat and neck circumference partly stands in for lean mass. It is regression, not measurement — the equation predicts what a lab would have found in someone with those numbers.
Taking the measurements properly
- Use a flexible tape, snug against the skin without compressing it.
- Measure first thing in the morning, before eating or training.
- Stand relaxed and breathe out normally. Holding the stomach in changes the result by several percentage points, which is more than the effect of a month of dieting.
- Take each measurement three times and use the middle value.
How accurate is a body fat percentage calculator
Against a DEXA scan, this method typically lands within about 3 to 4 percentage points, and it is least accurate at the extremes — very lean and very heavy people both get worse estimates. It also assumes an average fat distribution, so anyone who carries fat unusually will be misread in a consistent direction.
The useful consequence: a single reading is a rough figure, but the same person measuring the same way each month gets a reliable trend, and the trend is what any of this is for. Do not compare your number against a friend's from a different method.
Healthy body fat percentage ranges
Commonly cited healthy ranges are roughly 8 to 20% for men and 21 to 33% for women, with essential fat — the minimum compatible with normal physiology — at around 3% and 12% respectively. Women's higher figures are not a lack of discipline; they are reproductive physiology, and pushing far below them reliably disrupts menstrual function.
This is a calculator, not medical advice. The equations behind it describe populations and can be wrong about any individual. If you are pregnant, managing a health condition, taking medication, or recovering from an eating disorder, the number to act on is the one your doctor or a registered dietitian gives you.
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