Corrected QT Interval (QTc

Clinical Electrophysiology: QT Interval Correction & Arrhythmia Safety

The QT interval on an ECG represents total ventricular electrical activity (depolarization and repolarization). Because the QT interval naturally shortens as heart rate increases and lengthens as heart rate slows, it must be corrected (QTc) for meaningful clinical comparison:

• The Bazett Equation Limitations: Published in 1920, Bazett's formula remains the default output on most automated hospital ECG machines. However, it overcorrects at high heart rates (producing false-positive prolonged QTc in tachycardia) and undercorrects in bradycardia.

• Fridericia Formula Superiority: The Fridericia cube-root formula is recommended by the FDA, AHA, and European Medicines Agency (EMA) during pharmaceutical clinical trials because it maintains uniform accuracy across tachycardia and bradycardia.

• The 500 ms Critical Threshold: A QTc exceeding 500 ms is the universally recognized critical threshold for Torsades de Pointes (TdP), a fatal polymorphic ventricular tachycardia that can rapidly degenerate into ventricular fibrillation.

• High-Alert QT-Prolonging Drugs: Common culprits include antiarrhythmics (Amiodarone, Sotalol), antibiotics (Azithromycin, Levofloxacin), psychotropics (Haloperidol, Citalopram, Quetiapine), and antiemetics.

How to Use This Calculator

Enter your Biological Sex — select from Female, Male, Raw QT Interval on ECG (milliseconds - ms) (e.g. 420 ms (Start of Q wave to end of T wave)), Heart Rate (BPM - Beats per Minute) (e.g. 75 BPM (or enter RR interval below)), and Or Direct RR Interval (ms) [Optional] (e.g. 800 ms (Leave blank to use Heart Rate)). Results update instantly — no submit button needed. Use metric or imperial units as labeled. For the most accurate result, measure height and weight in the morning without shoes.

The Formula

Corrected QT Interval (QTc) formulas (QT and RR intervals in seconds):

Bazett Formula: QTc = (QT) / (√(RR))

Fridericia Formula: QTc = (QT) / (√[3]RR)

Framingham Formula (Sagie): QTc = QT + 0.154 × (1 - RR)

Hodges Formula: QTc = QT + 1.75 × (HR - 60)

RR Interval (seconds) = (60) / (Heart Rate (BPM))

Worked Example

For example, evaluating an ECG from a female patient with raw QT interval 420 ms at heart rate 75 BPM: The RR interval is . Bazett QTc is . Fridericia QTc is .

Understanding Your Result

Health calculations based on standard clinical formulas give a population-reference estimate — individual variation exists. Compare your result to the reference ranges provided using validated sources (WHO, CDC, NIH, or specialty clinical guidelines). A single measurement is a starting point; meaningful health trends emerge from tracking the same metric over time under consistent conditions. For values outside reference ranges, consult a qualified healthcare provider before making changes to diet, exercise, or medication.