The Navy Method requires waist circumference (measured at navel level), neck circumference (just below the larynx), and for women, hip circumference (at the widest point). Use a flexible tape, not rigid. Measure twice and average.
Your result has a ±3–5% margin. Healthy ranges: men 10–20%, women 18–28%. Athletic: men 6–13%, women 14–20%. Essential fat (minimum for organ function): men 2–5%, women 10–13%.
Most measurement errors come from the waist: measure at navel level while standing relaxed — not sucking in. Measuring lower (at the hip) will underestimate body fat significantly.
Body fat percentage tells you composition; BMI tells you mass relative to height. A muscular person will show normal/athletic body fat with an overweight BMI. Always compare both metrics.
Track at the same time of day (morning, before eating) with consistent measurement positions. Single readings are unreliable — compare monthly averages of 3 consecutive morning measurements.

AI Integrations

Contract, discovery endpoints, and developer notes for agent use.

Always available for agents

Tool contract JSON

https://aifithub.io/contracts/body-fat-percentage-calculator.json

Stable input and output contract for this exact tool.

Human review

People can use the browser page to sense-check outputs and charts, but agents should still execute against the contract and discovery endpoints.

/agent-tools.json - machine-readable tool index.
/llms.txt - human-readable model discovery guide.
/.well-known/webmcp.json - WebMCP capabilities manifest.
/.well-known/ai-plugin.json - plugin-style discovery manifest.

{
  "tool": "body_fat",
  "gender": "male",
  "weight_kg": 80,
  "waist_cm": 85,
  "neck_cm": 38,
  "height_cm": 178
}

Open tool contract JSON Open AI integrations guide

Expand developer notes

Agent playbook

Resolve Body Fat Percentage Calculator from /agent-tools.json and open its contract before execution.
Validate inputs against the contract schema instead of scraping labels from the page UI.
Open the browser page only when a person wants to review charts, assumptions, or related tools.

Agent FAQ

Should ChatGPT, Claude, or another agent click through the UI?

No. Start with /agent-tools.json, then follow the tool's contract URL. The page UI is for human review, not parameter discovery.

When do tools show Quick and Advanced?

Every tool opens in Quick Start first. Advanced Controls keeps the same scenario, reveals more assumptions or diagnostics, and every tool keeps AI integrations inline below the instructions.

When should an agent still open the browser page?

Open it when a human wants to sense-check the output, review the chart, or keep exploring related tools after the calculation finishes.

Questions people usually ask

How accurate are body fat estimation methods?

DEXA scan: ±1-2% (gold standard, requires equipment). Hydrostatic weighing: ±2-3%. BodPod (air displacement): ±2-3%. Skinfold calipers (trained user): ±3-5%. Bioelectrical impedance (home scales): ±5-8%, highly variable with hydration. This calculator uses the Navy Method (circumference-based), typically ±3-5%.

What is a healthy body fat percentage?

For men: essential fat 2-5%, athletic 6-13%, fitness 14-17%, average 18-24%, obese 25%+. For women: essential fat 10-13%, athletic 14-20%, fitness 21-24%, average 25-31%, obese 32%+. Women require higher essential fat due to hormonal and reproductive function.

Why does my body fat percentage seem high even though I exercise?

Exercise alone rarely produces large fat loss without a calorie deficit. Cardio burns 200-400 calories per session — easily offset by diet. Resistance training adds muscle but may not reduce fat significantly without dietary control. Fat loss requires a sustained calorie deficit regardless of exercise type.

How quickly can I reduce body fat percentage?

Sustainable fat loss is 0.5-1% of body weight per week. A 180 lb person can lose roughly 1-1.5 lb per week on a 500-750 calorie deficit. Faster loss accelerates muscle loss. Body fat percentage dropping 1-2 points per month is excellent progress; 3+ months is needed for visible changes.

Related Resources

Learn the decision before you act

Every link here is tied directly to Body Fat Percentage Calculator. Use the explanation, formula, examples, and benchmarks to pressure-test the calculator output from first principles.

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Guides · 2 Checklists · 1 Comparisons · 2 Learn · 1 Tips · 1

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