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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).

When to use

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.

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.

Inputs

Parameter	Type	Unit	Required	Description
`qt_ms`	number	ms	yes	Measured QT interval in milliseconds. Range: ≥ 150, ≤ 800
`heart_rate_bpm`	number	bpm	no	Heart rate in beats per minute (RR = 60000 / HR). Give this or rr_interval_ms. Range: ≥ 20, ≤ 300
`rr_interval_ms`	number	ms	no	Optional RR interval in ms, used instead of heart rate when given. Range: ≥ 200, ≤ 3000
`sex`	enum: male \| female		yes	Sex used to select the formula coefficients or thresholds.

Outputs

Output	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.

Example

QT 400 ms, 75 bpm, male: {"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: {"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

Machine access

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

Sources

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.

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