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Cardiovascular
QTc Interval (Bazett)
Correct the QT interval for heart rate using the Bazett formula to assess TdP risk.
When to use: QTc prolongation is a surrogate marker for risk of Torsades de Pointes (TdP), a potentially fatal ventricular arrhythmia. Evaluate QTc before prescribing QT-prolonging medications (antiarrhythmics, antiemetics, antipsychotics, fluoroquinolones, azithromycin), in hypokalemia/hypomagnesemia, hypothyroidism, and congenital long-QT syndromes. Repeat ECG after initiating or increasing dose of any QT-prolonging drug.
Calculator
Measured from start of QRS to end of T wave
Fill in all fields to see the result
Formula
QTc (Bazett) = QT (ms) / √RR (s)
RR (s) = 60 / HR (bpm)
Key Points for NEET PG
- Bazett formula: QTc = QT / √RR — most widely used despite overcorrection at extreme heart rates.
- Normal QTc: ≤440 ms in men, ≤460 ms in women; risk threshold for TdP is >500 ms.
- Women have intrinsically longer QTc due to lower testosterone — important for drug safety studies.
- Classic QT-prolonging drugs: sotalol, amiodarone, haloperidol, methadone, azithromycin, ondansetron, hydroxychloroquine.
- Congenital long-QT syndrome: LQT1 (KCNQ1, exertion-triggered), LQT2 (KCNH2, sudden noise), LQT3 (SCN5A, sleep/rest) — high-yield genetics for NEET PG.
References
Bazett HC. An analysis of the time-relations of electrocardiograms · Heart (1920)
Roden DM. Drug-induced prolongation of the QT interval · N Engl J Med (2004)
For educational purposes only. Not for clinical decision-making without professional oversight.