Body Fat Percentage Calculator
Estimate body fat from a tape measure using the US Navy circumference method, and from height and weight using the Deurenberg BMI equation, both at once, with the arithmetic shown. The two disagree on the same body, which is the honest answer no single number gives you.
Every box feeds both methods at once. The Navy side reads height, neck and waist; the BMI side reads height, weight and age. Fill in all of them and the two estimates appear together. Typing the unit yourself, “80 kg”, “180 cm”, “5'10"”, always beats the toggle.
Circumference methods carry roughly ±3–4 percentage points of error against a DEXA scan. Track the trend over weeks rather than trusting a single reading.
The bands these categories come from
American Council on Exercise norms for men. Women carry roughly eight points more essential fat than men, which is why the two tables differ.
| Category | Body fat | This reading |
|---|---|---|
| Below essential fat | under 2% | |
| Essential fat | 2–5.9% | |
| Athletic | 6–13.9% | |
| Fitness | 14–17.9% | 16.9% |
| Average | 18–24.9% | |
| Obese | 25% and above |
Common questions
- How accurate is the Navy body fat method?
- The US Navy circumference equation is accurate to roughly ±3–4 percentage points against hydrostatic weighing for most adults, which is respectable for a tape measure and free. It is least accurate at the extremes, meaning very lean athletes and very high body fat, because it infers fat distribution from a small number of girths. Treat a single reading as a rough band, and track the direction of change over weeks rather than the absolute number.
- Why do the two methods give me different answers?
- Because they measure different things. The Navy method reads where your circumference actually sits; the Deurenberg BMI equation only knows your height, weight, age and sex, so it cannot tell muscle from fat. A muscular person gets a high BMI-derived estimate and a much lower tape estimate. When the two diverge sharply, the tape reading is usually closer, and the gap itself tells you something about your build.
- Where exactly do I measure my neck and waist?
- Neck: just below the larynx, tape sloping slightly down at the front. Waist: at the navel for men; at the narrowest point for women, plus hips at the widest point. Measure on bare skin, at the end of a normal exhale, with the tape snug but not compressing. Measurement error is the largest source of error in the result. A centimetre at the waist moves the answer by roughly a percentage point.
- What is a healthy body fat percentage?
- The commonly used bands are, for men: 2–5% essential, 6–13% athletic, 14–17% fitness, 18–24% average, 25%+ obese. For women, whose essential fat is higher: 10–13% essential, 14–20% athletic, 21–24% fitness, 25–31% average, 32%+ obese. These come from the American Council on Exercise and are population guidance, not a medical diagnosis.
- Why does the calculator refuse when my waist is smaller than my neck?
- The Navy formula takes the logarithm of waist minus neck. If that difference is zero or negative the logarithm is undefined, so there is no answer to give. In practice it means one of the two measurements is wrong. Most often the neck was measured over the widest part of the jaw rather than below the larynx. The tool names which measurement to re-take rather than returning a nonsense number.
- Does anything I type get uploaded?
- No. The whole calculation runs in your browser. There is no server to send it to, and you can confirm that in your browser's network tab. Body measurements are personal data, and the safest way to handle them is to never transmit them at all.
Estimates. Both formulae are accurate to roughly ±3–4 points against clinical measurement, and disagree with each other on the same body, which is why both are shown. Not a medical assessment.