# QTc Calculator

> Corrects a measured QT interval for heart rate with the Bazett, Fridericia, Framingham and Hodges formulas and flags prolongation against the sex-specific thresholds (450 ms men, 460 ms women).

- Calculator id: `qtc-interval` · Category: Medical & Clinical (`medical`) · Tool name: `calculate_qtc_interval`
- Canonical page: https://tttkmbb.com/medical/qtc-interval · This document: https://tttkmbb.com/medical/qtc-interval.md · JSON definition: https://tttkmbb.com/medical/qtc-interval.json

## Purpose

Corrects a measured QT interval for heart rate with the Bazett, Fridericia, Framingham and Hodges formulas and flags prolongation against the sex-specific thresholds (450 ms men, 460 ms women).

**Use when:** You have a QT interval in ms and a heart rate (or RR interval) from an ECG and need the rate-corrected QTc or a prolongation check.

**Do not use when:** The rhythm is irregular (atrial fibrillation) or QRS is wide (bundle branch block, paced rhythm), where standard QTc formulas are unreliable.

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `qt_ms` | number | ms | required | Measured QT interval in milliseconds. (min 150, max 800) |
| `heart_rate_bpm` | number | bpm | optional | Heart rate in beats per minute (RR = 60000 / HR). Give this or rr_interval_ms. (min 20, max 300) |
| `rr_interval_ms` | number | ms | optional | Optional RR interval in ms, used instead of heart rate when given. (min 200, max 3000) |
| `sex` | enum: male \| female |  | required | Sex used to select the formula coefficients or thresholds. |

## Output

| Field | Type | Unit | Description |
| --- | --- | --- | --- |
| `rr_interval_ms` | number | ms | RR interval used for the corrections. |
| `qtc_bazett_ms` | number | ms | QT / √RR (RR in s). |
| `qtc_fridericia_ms` | number | ms | QT / RR^(1/3). |
| `qtc_framingham_ms` | number | ms | QT + 154 × (1 − RR). |
| `qtc_hodges_ms` | number | ms | QT + 1.75 × (HR − 60). |
| `threshold_ms` | number | ms | 450 ms for men, 460 ms for women (AHA/ACCF/HRS 2009). |
| `interpretation` | string |  | Normal, prolonged or markedly prolonged (>500 ms), judged on Bazett at 60–100 bpm and Fridericia outside that range. |

## Formula

`RR (s) = 60 / heart_rate_bpm (or rr_interval_ms / 1000). Bazett: QTc = QT / √RR. Fridericia: QTc = QT / RR^(1/3). Framingham: QTc = QT + 154 × (1 − RR). Hodges: QTc = QT + 1.75 × (heart_rate − 60).`

Bazett over-corrects at fast rates and under-corrects at slow rates, so Fridericia is used for the interpretation when heart rate is outside 60–100 bpm; QTc above 500 ms is associated with a higher risk of torsades de pointes. Results are informational only and not a substitute for clinical judgement or medical advice.

## Data Sources

- Rautaharju PM et al. (2009) AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram, part IV: the ST segment, T and U waves, and the QT interval, Circulation 119:e241-e250 — https://pubmed.ncbi.nlm.nih.gov/19228821/ (standard, retrieved 2026-09-24)
- Fridericia LS (1920, reprinted 2003) The duration of systole in an electrocardiogram in normal humans and in patients with heart disease, Ann Noninvasive Electrocardiol 8:343-351 — https://pubmed.ncbi.nlm.nih.gov/14516292/ (peer_reviewed, retrieved 2026-09-24)
- Wikipedia – QT interval (Bazett 1920 and other correction formulas) — https://en.wikipedia.org/wiki/QT_interval (reference, retrieved 2026-09-24)

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/qtc-interval?qt_ms=…&sex=…`
- `POST https://tttkmbb.com/api/v1/calculate/qtc-interval` 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/qtc-interval · OpenAPI operationId `calculate_qtc_interval` 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": "qtc-interval", "inputs": {…}}`

## Example

- QT 400 ms, 75 bpm, male: inputs `{"qt_ms":400,"heart_rate_bpm":75,"sex":"male"}` → `{"rr_interval_ms":800,"qtc_bazett_ms":447,"qtc_fridericia_ms":431,"qtc_framingham_ms":431,"qtc_hodges_ms":426,"threshold_ms":450,"interpretation":"Normal (Bazett QTc 447 ms ≤ 450 ms for men)"}`
- QT 440 ms, 100 bpm, female: inputs `{"qt_ms":440,"heart_rate_bpm":100,"sex":"female"}` → `{"qtc_bazett_ms":568,"qtc_fridericia_ms":522,"qtc_framingham_ms":502,"qtc_hodges_ms":510,"interpretation":"Markedly prolonged (Bazett QTc 568 ms > 500 ms)"}`

```
GET https://tttkmbb.com/api/v1/calculate/qtc-interval?qt_ms=400&heart_rate_bpm=75&sex=male
```

## Limitations

The rhythm is irregular (atrial fibrillation) or QRS is wide (bundle branch block, paced rhythm), where standard QTc formulas are unreliable. Bazett over-corrects at fast rates and under-corrects at slow rates, so Fridericia is used for the interpretation when heart rate is outside 60–100 bpm; QTc above 500 ms is associated with a higher risk of torsades de pointes. Results are informational only and not a substitute for clinical judgement or medical advice. All values are computed from the formula above; no measurement or live data is involved.

## FAQ

**Which formula is best?**

Bazett is the most widely used and the basis of most reference thresholds, but Fridericia and Framingham track risk better at heart rates outside 60–100 bpm; drug-safety studies usually report Fridericia.

**How should QT be measured?**

From the start of the QRS to the end of the T wave in lead II or V5, averaged over several beats; automated ECG readings can mis-measure the T-wave end.

## Related

- [Heart Rate Zone Calculator](https://tttkmbb.com/health/heart-rate-zones.md) — Heart-rate context for exercise ECGs.