QTc Calculator
Calculate corrected QT interval (QTc) from QT and heart rate using Bazett, Fridericia, Framingham, and Hodges formulas. Free QTc calculator with risk assessment and charts.
About This Calculator
The QTc (Corrected QT Interval) Calculator is an essential tool for healthcare professionals, cardiologists, and medical students to assess ventricular repolarization from an ECG tracing. The QT interval represents the time from ventricular depolarization to repolarization, and since it varies with heart rate, correction formulas normalize it to a standard rate of 60 bpm for consistent clinical interpretation.
This calculator computes QTc using four validated formulas: Bazett (QTc = QT / √RR), Fridericia (QTc = QT / ∛RR), Framingham (QTc = QT + 0.154[1-RR]), and Hodges (QTc = QT + 1.75[HR - 60]). Each formula provides different rate correction characteristics — Bazett is the clinical standard but overcorrects at tachycardia, while Fridericia and Framingham offer more reliable correction across a broader heart rate range. The Framingham formula, derived from the Framingham Heart Study, is considered by many experts to provide the best rate correction and mortality prediction.
QT prolongation is a significant risk factor for torsades de pointes, a polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation. Common causes include inherited Long QT Syndrome, QT-prolonging medications (such as amiodarone, sotalol, erythromycin, and certain antipsychotics), electrolyte disturbances (hypokalemia, hypocalcemia, hypomagnesemia), myocardial ischemia, and bradycardia. The calculator provides sex-specific interpretation thresholds — normal QTc is ≤430 ms for men and ≤450 ms for women, with prolongation defined as >450 ms for men and >470 ms for women.
Regional Notes
Global: QTc assessment is universally used in cardiology. The Bazett formula is the most commonly applied worldwide, endorsed by the American Heart Association (AHA) and European Society of Cardiology (ESC) guidelines. The same normal value thresholds apply across all regions. Clinicians should be aware that different ECG machines may use different correction formulas, and manual verification with a standardized formula is recommended for critical clinical decisions.
Frequently Asked Questions
What is QTc and why is it important?
QTc is the corrected QT interval, a heart electrical measurement adjusted for heart rate. It is important because QT prolongation increases the risk of ventricular arrhythmias including torsades de pointes and sudden cardiac death. A normal QTc is ≤430 ms for men and ≤450 ms for women.
Which QTc formula is most accurate?
Bazett's formula (QTc = QT / √RR) is the most widely used but tends to overcorrect at high heart rates and undercorrect at low heart rates. The Framingham and Fridericia formulas provide more consistent rate correction across a range of heart rates according to studies in the Journal of the American Heart Association.
What is a normal QTc value?
For adult males, a normal QTc is ≤430 ms, borderline is 431-450 ms, and prolonged is >450 ms. For adult females, normal is ≤450 ms, borderline is 451-470 ms, and prolonged is >470 ms. These thresholds follow standard cardiology guidelines.
What causes a prolonged QT interval?
QT prolongation can be caused by genetic disorders (Long QT Syndrome), certain medications (antiarrhythmics, macrolide antibiotics, antipsychotics), electrolyte imbalances (hypokalemia, hypocalcemia, hypomagnesemia), heart disease, bradycardia, hypothermia, and hypothyroidism.
Can I use this calculator for diagnosis?
No. This calculator is for educational and reference purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a healthcare professional for interpretation of ECG findings.
How do I measure QT interval on an ECG?
Find the beginning of the QRS complex and the end of the T wave. Measure the distance between these points. At standard paper speed of 25 mm/s, one small box (1 mm) equals 0.04 s (40 ms) and one large box (5 mm) equals 0.2 s (200 ms). Use the lead with the clearest T wave ending, typically lead II or V5.
What is the difference between QT and QTc?
QT is the raw measured interval from QRS start to T wave end on an ECG. QTc is the heart-rate-corrected version. Since QT shortens with faster heart rates and lengthens with slower rates, QTc normalizes the value to a standard heart rate of 60 bpm, allowing comparison across different heart rates.