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BMI Calculator
Computes body mass index (BMI) from weight and height and classifies it with the WHO adult categories, plus the healthy weight range for that height.
When to use
You need an adult's BMI, its WHO/CDC weight category, or the weight range that corresponds to a normal BMI (18.5–24.9) for a given height.
Do not use when: The person is under 20 years old (children need age- and sex-specific BMI percentiles), or you need body-fat percentage (use body-fat) or calorie needs (use bmr / tdee).
Formula
BMI = weight_kg / (height_cm / 100)²
BMI is a screening measure, not a diagnosis: it does not distinguish fat from muscle mass and thresholds were derived from European adult populations. WHO categories: <18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, ≥30 obesity (classes I–III at 30, 35, 40).
Inputs
| Parameter | Type | Unit | Required | Description |
|---|---|---|---|---|
weight_kg | number | kg | yes | Body weight in kilograms. Range: > 0, ≤ 700 |
height_cm | number | cm | yes | Standing height in centimetres. Range: ≥ 50, ≤ 300 |
Outputs
| Output | Type | Unit | Description |
|---|---|---|---|
bmi | number | kg/m² | Body mass index = weight / height². |
category | string | WHO adult BMI classification. | |
healthy_weight_min_kg | number | kg | Weight at BMI 18.5 for this height. |
healthy_weight_max_kg | number | kg | Weight at BMI 24.9 for this height. |
prime | number | BMI divided by the upper normal limit 25; values above 1 indicate excess weight. |
Example
70 kg, 175 cm: {"weight_kg":70,"height_cm":175} → {"bmi":22.86,"category":"Normal weight","healthy_weight_min_kg":56.7,"healthy_weight_max_kg":76.3,"prime":0.914}
95 kg, 180 cm: {"weight_kg":95,"height_cm":180} → {"bmi":29.32,"category":"Overweight (pre-obesity)"}
GET https://tttkmbb.com/api/v1/calculate/bmi?weight_kg=70&height_cm=175
Machine access
- API:
GET https://tttkmbb.com/api/v1/calculate/bmi(query parameters) orPOSTwith a JSON body{"inputs": {...}} - Schema: https://tttkmbb.com/api/v1/calculators/bmi · Markdown: https://tttkmbb.com/health/bmi.md · JSON definition: https://tttkmbb.com/health/bmi.json
- MCP: server
https://tttkmbb.com/mcp, toolcalculate_bmi - OpenAPI operationId:
calculate_bmi - Freshness:
static. Authentication: none. Rate limit: fair use (see rate limits).
Sources
- WHO – A healthy lifestyle: body mass index (standard)
- CDC – Adult BMI categories (standard)
FAQ
Is BMI accurate for athletes?
No. Muscular people can have a high BMI without excess fat, because BMI only uses weight and height. Use body-fat percentage for body composition.
Which thresholds are used?
WHO adult thresholds (18.5 / 25 / 30 / 35 / 40). Some Asian populations use lower action points (23 and 27.5) recommended by WHO for public-health action.
Does this work for children?
No. For ages 2–19 BMI must be interpreted with age- and sex-specific percentiles (CDC/WHO growth charts), which this calculator does not implement.
Related calculators
- BMR Calculator — Estimate resting calorie needs for the same weight and height.
- Body Fat Calculator — BMI does not measure body composition; the US Navy method estimates body-fat percentage.
- Ideal Weight Calculator — Compare with ideal-weight formulas (Devine, Robinson, Miller, Hamwi).