The QTc (corrected QT interval) normalizes the QT interval for heart rate. Prolonged QTc is a risk factor for torsades de pointes, a life-threatening ventricular arrhythmia associated with many medications.
Calculate QTc (Bazett's Formula)
How to Use the QTc Calculator
This QTc calculator uses Bazett's formula: QTc = QT ÷ √(RR). The RR interval (in seconds) = 60 ÷ heart rate.
QTc Thresholds
Normal: <440ms (men) / <460ms (women). Borderline: 440–470ms. Prolonged: >450ms (men) / >470ms (women). High risk: >500ms warrants immediate cardiology evaluation and potential medication discontinuation. A QTc above 500ms is associated with 2-3x increased risk of torsades de pointes.
FAQ
Is this QTc calculator free?
Yes, completely free with no signup required.
What is QTc and why does it matter?
QTc is the QT interval corrected for heart rate. The QT interval measures ventricular depolarization and repolarization. When QTc is prolonged (>450ms in men, >470ms in women), it indicates increased risk of torsades de pointes, a potentially fatal ventricular arrhythmia.
What is Bazett's formula?
QTc = QT / √(RR interval). The RR interval in seconds = 60 / heart rate. Bazett's formula is the most widely used correction despite over-correcting at high heart rates. Fridericia's formula (QT / RR^⅓) is more accurate at extremes of heart rate.
Which drugs prolong QT interval?
Many drugs prolong QT, including antiarrhythmics (amiodarone, sotalol), antipsychotics (haloperidol, ziprasidone), antibiotics (azithromycin, fluoroquinolones), antifungals (fluconazole), antidepressants (TCAs), and methadone. The crediblemeds.org database lists risk levels for hundreds of drugs.