Cardiology

QTc Interval Calculator

Calculate corrected QT interval using all 4 validated formulas simultaneously: Bazett, Fridericia, Framingham, and Hodges. Instant drug-induced QT prolongation risk classification, sex-specific normal ranges, and TdP risk assessment.

4 formulas simultaneously Sex-specific thresholds PDF export
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QTc Calculator
ms
Measure from start of QRS to end of T-wave. Use the longest QT (usually V5-V6). Enter a valid QT interval (200-700 ms).
bpm
Enter a valid heart rate (20-300 bpm).
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QTc Results
-
ms - QTc (Bazett, primary clinical reference)

All four formula results:

Bazett Most Used
-
ms
QTc = QT ÷ √RR
Fridericia EMA Preferred
-
ms
QTc = QT ÷ ∛RR
Framingham
-
ms
QTc = QT + 154×(1-RR)
Hodges
-
ms
QTc = QT + 1.75×(HR-60)

Normal QTc ranges (Bazett):

Male - Normal
< 440 ms
Female - Normal
< 460 ms
Borderline (both)
440-500 ms
High TdP risk
> 500 ms
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QTc interval correction

The QT interval shortens at faster heart rates and lengthens when the heart slows. QTc corrects for that, letting you compare intervals across different rates. Four formulas are widely used. None is perfect, and they diverge most at extreme heart rates.

Which formula to use

Bazett is everywhere. Drug labels quote it, hospital systems use it, and most clinicians know the thresholds. Its weakness is overcorrection at fast rates and undercorrection below 60 bpm. At normal heart rates (60-100 bpm) the difference between formulas is clinically negligible.

The EMA recommends Fridericia for formal drug safety studies. If you're evaluating drug-induced QT prolongation in a tachycardic patient, Fridericia gives a more reliable number.

Common QT-prolonging drugs

Antipsychotics (haloperidol, quetiapine), antidepressants (citalopram, tricyclics), antibiotics (azithromycin, moxifloxacin), antiemetics (ondansetron, domperidone), and antiarrhythmics (amiodarone, sotalol) all carry QT risk. Check CredibleMeds for drug-specific risk classification. Hypokalaemia and hypomagnesaemia compound the risk significantly.

Related tools

For AF anticoagulation monitoring alongside antiarrhythmic drugs, see CHA2DS2-VASc Score. For 10-year CVD risk, see Framingham Risk Score. For ACS risk, see TIMI Score and HEART Score. For electrolyte conversion (K in mmol/L vs mEq/L), see Lab Values Converter. For renal dosing of QT-prolonging drugs, see Creatinine Clearance.

References

  • Bazett HC. "An analysis of the time-relations of electrocardiograms." Heart. 1920;7:353-370.
  • Fridericia LS. "Die Systolendauer im Elektrokardiogramm." Acta Med Scand. 1920;53:469-486.
  • European Medicines Agency. ICH E14 Guideline: QT/QTc interval prolongation. EMA, 2005.
Clinical note: QTc calculation is a reference tool. Always interpret in full clinical context, including medications, electrolytes, and clinical status.
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