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Body Fat Calculator

Estimates body-fat percentage from height and neck, waist (and hip for women) circumferences using the US Navy method, and classifies the result with ACE categories.

When to use

You want a tape-measure estimate of body-fat percentage and fat/lean mass without calipers or a DEXA scan.

Do not use when: High accuracy is required (DEXA, hydrostatic weighing or BIA are better), or the person is a child or pregnant.

Formula

Men: %fat = 495 / (1.0324 − 0.19077·log10(waist − neck) + 0.15456·log10(height)) − 450. Women: %fat = 495 / (1.29579 − 0.35004·log10(waist + hip − neck) + 0.22100·log10(height)) − 450. All lengths in cm.

Inputs

Parameter	Type	Unit	Required	Description
`sex`	enum: male \| female		yes	Biological sex used to select the formula coefficients.
`height_cm`	number	cm	yes	Height in centimetres. Range: ≥ 50, ≤ 300
`waist_cm`	number	cm	yes	Waist measured at the navel (men) or narrowest point (women). Range: ≥ 30, ≤ 300
`neck_cm`	number	cm	yes	Neck measured just below the larynx. Range: ≥ 15, ≤ 100
`hip_cm`	number	cm	no	Hip at the widest point. Required for women, ignored for men. Range: ≥ 30, ≤ 300
`weight_kg`	number	kg	no	Optional body weight, used to report fat mass and lean mass in kg. Range: > 0, ≤ 700

Outputs

Output	Type	Unit	Description
`body_fat_percent`	number	%	Estimated body-fat percentage (US Navy formula).
`category`	string		ACE body-fat category for the given sex.
`fat_mass_kg`	number	kg	Body fat in kilograms (only when weight_kg is given).
`lean_mass_kg`	number	kg	Fat-free mass in kilograms (only when weight_kg is given).

Example

Male, 175 cm, waist 85, neck 38, 70 kg: {"sex":"male","height_cm":175,"waist_cm":85,"neck_cm":38,"weight_kg":70} → {"body_fat_percent":16.9,"category":"Fitness","fat_mass_kg":11.9,"lean_mass_kg":58.1}

Female, 165 cm, waist 75, neck 33, hip 95: {"sex":"female","height_cm":165,"waist_cm":75,"neck_cm":33,"hip_cm":95} → {"body_fat_percent":26.9,"category":"Average"}

GET https://tttkmbb.com/api/v1/calculate/body-fat?sex=male&height_cm=175&waist_cm=85&neck_cm=38&weight_kg=70

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/body-fat (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/body-fat · Markdown: https://tttkmbb.com/health/body-fat.md · JSON definition: https://tttkmbb.com/health/body-fat.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="body-fat"
OpenAPI operationId: calculate_body_fat_percentage
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

FAQ

How accurate is the US Navy method?

Roughly ±3–4 percentage points versus hydrostatic weighing for most adults; less accurate at very high or very low body fat.

Where exactly do I measure?

Neck just below the larynx, sloping slightly downward; waist horizontally at the navel (men) or the narrowest point (women); hips at the widest point (women).

Related calculators

Lean Body Mass Calculator — Formula-based lean mass without circumference measurements.
BMI Calculator — Compare with the simpler BMI screening measure.