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QTc Calculator (Corrected QT Interval)

Calculate Corrected QT Interval using Bazett, Fridericia, Framingham, and Hodges formulas. Essential cardiology tool for arrhythmia risk assessment.

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About

The QT interval on an electrocardiogram (ECG) represents the duration of ventricular depolarization and repolarization. However, this duration naturally fluctuates with heart rate; faster heart rates shorten the QT, and slower rates lengthen it. To assess cardiac risk accurately, the QT interval must be "corrected" (QTc) to a standard heart rate of 60 bpm.

Prolonged QTc is a significant biomarker for the risk of Torsades de Pointes, a potentially fatal ventricular arrhythmia. This tool computes QTc using four distinct validated methods, as no single formula is perfect for all heart rate ranges. Bazett is standard but over-corrects at high rates, while Fridericia and Framingham are often preferred for tachycardia or bradycardia contexts.

cardiology qt interval qtc ecg arrhythmia bazett fridericia

Formulas

Where QT is measured in seconds and RR is the interval between beats in seconds (60/HR).

Bazett: QTc = QT√RR
Fridericia: QTc = QTRR1/3
Framingham: QTc = QT + 0.154(1 − RR)
Hodges: QTc = QT + 1.75(HR − 60)

Reference Data

GenderNormal (ms)Borderline (ms)Prolonged / High Risk (ms)
Men< 430430 - 450> 450
Women< 450450 - 470> 470
Children< 440440 - 460> 460
Critical Risk--> 500 (Torsades risk)

Frequently Asked Questions

Bazett's formula is the most widely used in clinical practice and software, but it is known to over-correct at high heart rates (>100 bpm) and under-correct at low rates (<60 bpm). Fridericia or Framingham are statistically more accurate at altering heart rates. Always follow your institution's clinical guidelines.
Measure from the onset of the QRS complex to the end of the T wave. It is best to measure in Lead II or V5. Ensure you measure multiple beats and average them if sinus arrhythmia is present.
Causes include congenital Long QT Syndrome (LQTS), electrolyte imbalances (hypokalemia, hypomagnesemia), and many medications (antiarrhythmics, antibiotics like macrolides, and antipsychotics).