Printed on 2/13/2026
For informational purposes only. This is not medical advice.
The QT interval on an electrocardiogram (ECG) represents the time for ventricular depolarization and repolarization. Because QT varies with heart rate, it must be corrected (QTc) for clinical interpretation. QT prolongation increases the risk of torsades de pointes, a potentially fatal arrhythmia. This calculator uses three well-established correction formulas.
Formula: Bazett: QTc = QT / √(RR), Fridericia: QTc = QT / ∛(RR)
Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about your health.
Normal QTc is generally considered ≤ 450 ms for men and ≤ 460 ms for women. QTc between 450–500 ms is borderline and should be monitored. QTc > 500 ms is considered significantly prolonged and carries a higher risk of dangerous arrhythmias.
Bazett's formula is the most commonly used and widely recognized. However, it overcorrects at high heart rates and undercorrects at low heart rates. Fridericia's formula is preferred when heart rate is outside the 60–100 bpm range. The Framingham formula uses a linear correction and may be more accurate across a wider range of heart rates.
QT prolongation can be caused by certain medications (antiarrhythmics, antibiotics, antipsychotics, antidepressants), electrolyte imbalances (low potassium, magnesium, or calcium), congenital long QT syndrome, heart disease, and other medical conditions. Always consult a healthcare provider for ECG interpretation.
The RR interval (in milliseconds) = 60,000 / heart rate (bpm). For example, a heart rate of 75 bpm gives an RR interval of 800 ms. This calculator automatically converts your heart rate to the RR interval needed for QTc calculations.