Body Fat Percentage Calculator (US Navy method)
Free body fat percentage calculator using the US Navy formula — enter height, neck, waist (and hip for women) in metric or imperial, get the % and the category.
Pick sex, unit system, and type in the measurements. The calculator returns body-fat % and the category according to standard ACE ranges. Measurements take 30 seconds with a tape measure.
How the formula works
The US Navy method is a regression equation that estimates body density from a few circumferences, then converts density to body-fat percentage:
Men: %BF = 86.010 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76
Women: %BF = 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387
Both expect inches; the calculator converts metric input internally so the output is the same number regardless of which unit you type.
Where to measure
| Site | Where exactly |
|---|---|
| Neck | Just below the Adam’s apple, tape level, looking forward. |
| Waist (men) | At the navel, relaxed, don’t suck in. |
| Waist (women) | At the narrowest point (usually 1–2” above the navel). |
| Hip (women) | At the widest point of the glutes, tape level. |
| Height | Without shoes, standing tall, against a wall. |
Use a soft tape measure (sewing tape works). Snug but not compressing. Same time of day, ideally morning before eating or drinking.
What the categories mean (ACE)
Men:
| % | Category |
|---|---|
| 2–5 | Essential fat (below survival floor for many) |
| 6–13 | Athlete (visible abs, trained physique) |
| 14–17 | Fit (defined, low body fat) |
| 18–24 | Average |
| 25+ | Obese (health-risk range) |
Women:
| % | Category |
|---|---|
| 10–13 | Essential fat |
| 14–20 | Athlete |
| 21–24 | Fit |
| 25–31 | Average |
| 32+ | Obese |
Women have higher essential-fat floors than men because of reproductive biology — sustaining body fat below ~12% in women is associated with hormonal disruption.
Limitations
Regression equations work well at the population center and drift at the edges. Very tall, very short, very muscular, or very lean people get less accurate results. The Navy method is consistent, though: track the same person over time and the change is meaningful even if the absolute number is off by a few points. Use it as a trendline tool, not as a clinical measurement.
Worked examples
-
5'10" man, 38" waist, 16" neck
Body fat: 23.0% — average for men (Navy method).
-
5'6" woman, 30" waist, 38" hips, 13" neck
Body fat: 27.9% — average for women (Navy method).
Frequently asked questions
How does the Navy method actually work?
It's a regression equation that **estimates** body density from a few easy-to-measure circumferences and then converts density to a body-fat percentage via the standard Siri equation. The Navy uses it because it requires only a tape measure — no calipers, no DEXA scan, no immersion tank. The published accuracy is within ±3–4% for most adults; outliers (very tall, very short, very lean, very obese) drift further from the truth.
Why does the formula differ for men and women?
Adult bodies distribute fat differently by sex — women carry more around the hips and thighs, men more abdominally. The two regression equations were fit on different populations to capture that. The women's formula adds **hip circumference** to the waist + neck terms; the men's doesn't.
Where exactly do I measure?
**Neck** — just below the larynx (Adam's apple for men), tape level, head looking forward. **Waist** — at the navel for men; at the *narrowest* point for women (often a couple inches above the navel). **Hip** (women) — at the widest point of the glutes, tape level. Stand relaxed, don't suck in. The tape should be snug but not compressing skin.
How accurate is this compared to other methods?
**DEXA scans** and **hydrostatic weighing** are the gold standards (±1%). **Skinfold calipers** in trained hands run ±3%. **Bioimpedance scales** are roughly ±5% with caveats around hydration. The **Navy method is ±3–4%** for average adults, and importantly it's **consistent**: changes over time are trustworthy even if the absolute number is off, because it always uses the same anatomy.
What are the categories based on?
Standard ACE (American Council on Exercise) ranges. **Men**: essential fat 2–5%, athlete 6–13%, fit 14–17%, average 18–24%, obese 25%+. **Women**: essential 10–13%, athlete 14–20%, fit 21–24%, average 25–31%, obese 32%+. Women have higher essential-fat floors because of reproductive biology — going below ~12% sustainably is a health risk.
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