Body Fat Calculator

What does this calculator do?

This tool estimates your body fat percentage from a few tape-measure circumferences using the US Navy method — the same tape-based approach the US military uses for body composition screening, and the most common circumference-based method used by web calculators. No skinfold calipers, scales, or DEXA scans required — just a height and a couple of tape measurements.

Formula:

• Men: %BF = 495 / (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450
• Women: %BF = 495 / (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450

All measurements (height, waist, neck, hip) go in as centimeters; the tool converts imperial (inch) input automatically. The formula comes from a body-density regression published by Hodgdon & Beckett (1984), converted to a percentage via the Siri equation.

Worked example: a man who is 180 cm tall with an 85 cm waist and a 38 cm neck: log10(85 − 38) ≈ 1.672, log10(180) ≈ 2.255. The denominator works out to about 1.062, so %BF = 495 / 1.062 − 450 ≈ 16.1% — squarely in the "Fitness" category.

How to use it

1. Select your sex — the formula and category thresholds differ for men and women.
2. Choose metric (cm) or imperial (inches) for your measurements.
3. Enter your height.
4. Enter your waist circumference, measured at navel level.
5. Enter your neck circumference, measured just below the larynx.
6. If you selected female, enter your hip circumference at its widest point (this field only appears for women — the men's formula doesn't use it).
7. Your body fat percentage, category, and category range appear instantly and update as you type. Copy or download the result as CSV.

Body fat percentage categories

These are the standard ACE (American Council on Exercise) classifications used to interpret the result:

Category	Men	Women
Essential Fat	2–5%	10–13%
Athletes	6–13%	14–20%
Fitness	14–17%	21–24%
Average	18–24%	25–31%
Obese	25%+	32%+

FAQ

How accurate is the Navy method? It's an estimate, typically within about 3-4 percentage points of more precise methods like DEXA scans for most body types. It's popular precisely because it needs no special equipment — just a tape measure — while still being more consistent than a simple BMI-based guess. Accuracy can drop for very lean or very heavy body types, or unusual body-fat distributions.

Why do men and women use different formulas? Men and women store fat differently — women's formula factors in hip circumference because women tend to carry more fat around the hips, while men's formula relies only on waist and neck. Using the wrong formula for your sex will produce a meaningfully inaccurate result.

Why does the tool say my measurements are invalid? For men, waist must be larger than neck; for women, waist plus hip must be larger than neck. These circumferences enter the formula as waist − neck (or waist + hip − neck), and that value must be positive — a measurement error or unit mismatch (mixing inches and centimeters) is the usual cause.

How should I measure waist, neck, and hip? Use a flexible tape measure, pulled snug but not compressed against the skin. Measure your waist at navel level, your neck just below the larynx (Adam's apple), and — for women — your hips at their widest point. Measure standing relaxed, not flexed, for consistency between measurements.

Can I use this to track progress over time? Yes — since it only needs a tape measure, it's easy to re-measure weekly or monthly. Keep your measurement technique consistent (same spots, same tension) so changes reflect real body composition shifts rather than measurement variance.

Privacy

All calculations run entirely in your browser (or via the API if you call it programmatically) — no measurements are uploaded or stored. This is an estimate only, not a substitute for a professional body composition assessment (DEXA, hydrostatic weighing, or a qualified clinician).