QTc Calculator – Corrected QT Interval

The QT interval on an electrocardiogram (ECG) represents the total duration of ventricular depolarization and repolarization — from the beginning of the QRS complex to the end of the T wave. Because the QT interval naturally shortens at faster heart rates and lengthens at slower rates, a raw QT measurement cannot be compared between patients or across time without correcting for heart rate. The corrected QT interval (QTc) removes this rate dependency and provides a standardized measure that reflects the underlying electrical properties of the ventricular myocardium. A prolonged QTc is an important cardiac risk marker. It indicates delayed ventricular repolarization, which creates a window of vulnerability during which an ectopic impulse can trigger life-threatening arrhythmias, most notably Torsades de Pointes — a polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation and sudden cardiac death. QTc prolongation can be congenital (long QT syndrome) or acquired, arising from electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia), medications, cardiac disease, or other conditions. This calculator supports three widely used correction formulas. The Bazett formula (QTc = QT ÷ √RR) was the first to be widely adopted and remains the most frequently used in clinical practice. However, it tends to over-correct at fast heart rates and under-correct at slow heart rates. The Fridericia formula (QTc = QT ÷ RR^(1/3)) uses a cube root correction that performs more accurately across a broader range of heart rates, particularly at elevated rates above 80–90 bpm. The Framingham formula (QTc = QT + 154 × (1 − RR)) uses a linear correction derived from a large population study and is considered one of the most accurate across the full heart rate spectrum encountered in clinical practice. The RR interval used in the formulas represents the time between two successive ventricular beats, expressed in seconds. When heart rate is entered in bpm, the RR interval in seconds is calculated as 60 divided by heart rate. When entering the RR interval directly from ECG measurements, the value in milliseconds is divided by 1000 to obtain seconds. Generally accepted QTc thresholds are: below 440 ms is considered normal for most adults; 440–460 ms is borderline and warrants attention, particularly in the context of medications or electrolyte changes; above 460 ms is considered prolonged and increases arrhythmia risk, with values above 500 ms carrying substantial risk. Some guidelines use 450 ms for men and 460 ms for women as the upper limit of normal. Always apply QTc interpretation alongside the full clinical picture, medication history, and electrolyte status. This calculator is for educational and clinical support purposes only. QTc interpretation requires integration with the full clinical context and should be performed by qualified healthcare professionals.