# Waist-to-Height Ratio Calculator

> Divides waist circumference by height and classifies the ratio using the 'keep your waist to less than half your height' guidance (NICE 2022).

- Calculator id: `waist-to-height-ratio` · Category: Health (`health`) · Tool name: `calculate_waist_to_height_ratio`
- Canonical page: https://tttkmbb.com/health/waist-to-height-ratio · This document: https://tttkmbb.com/health/waist-to-height-ratio.md · JSON definition: https://tttkmbb.com/health/waist-to-height-ratio.json

## Purpose

Divides waist circumference by height and classifies the ratio using the 'keep your waist to less than half your height' guidance (NICE 2022).

**Use when:** You want a simple central-adiposity screen that predicts cardiometabolic risk better than BMI alone.

**Do not use when:** The person is under 18 or pregnant.

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `waist_cm` | number | cm | required | Waist measured midway between the lowest rib and the top of the hip bone. (min 30, max 300) |
| `height_cm` | number | cm | required | Height in centimetres. (min 50, max 300) |

## Output

| Field | Type | Unit | Description |
| --- | --- | --- | --- |
| `ratio` | number |  | waist / height (same units). |
| `category` | string |  | Risk band: below 0.4 low/very slim, 0.4–0.49 healthy, 0.5–0.59 increased risk, 0.6 and above high risk. |
| `max_healthy_waist_cm` | number | cm | Waist circumference at which the ratio would reach 0.5. |

## Formula

`ratio = waist_cm / height_cm`

## Data Sources

- NICE guideline NG246 (2025) Overweight and obesity management – waist-to-height ratio — https://www.nice.org.uk/guidance/ng246 (standard, retrieved 2026-09-23)
- Ashwell M, Gibson S (2016) Waist-to-height ratio as an indicator of early health risk, BMJ Open 6:e010159 — https://bmjopen.bmj.com/content/6/3/e010159 (peer_reviewed, retrieved 2026-09-23)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/waist-to-height-ratio?waist_cm=…&height_cm=…`
- `POST https://tttkmbb.com/api/v1/calculate/waist-to-height-ratio` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/waist-to-height-ratio · OpenAPI operationId `calculate_waist_to_height_ratio` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "waist-to-height-ratio", "inputs": {…}}`

## Example

- Waist 85 cm, height 175 cm: inputs `{"waist_cm":85,"height_cm":175}` → `{"ratio":0.486,"category":"Healthy","max_healthy_waist_cm":87.5}`
- Waist 100 cm, height 170 cm: inputs `{"waist_cm":100,"height_cm":170}` → `{"ratio":0.588,"category":"Increased risk"}`

```
GET https://tttkmbb.com/api/v1/calculate/waist-to-height-ratio?waist_cm=85&height_cm=175
```

## Limitations

The person is under 18 or pregnant. All values are computed from the formula above; no measurement or live data is involved.

## FAQ

**Why is 0.5 the threshold?**

Meta-analyses show cardiometabolic risk rises clearly once the waist exceeds half the height, across sexes and ethnic groups, which is why NICE adopted it.

## Related

- [BMI Calculator](https://tttkmbb.com/health/bmi.md) — Combine with BMI for a fuller screening picture.
- [Body Fat Calculator](https://tttkmbb.com/health/body-fat.md) — Estimate body-fat percentage from circumferences.