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Clinical — Cardiology
QTc Calculator
Enter the QT interval and heart rate to get the corrected QT (QTc) by the Bazett and Fridericia formulas.
QTc (Bazett)
—
QT corrected for heart rate.
| QTc (Fridericia) | — |
|---|---|
| RR interval | — |
| Interpretation Bazett | — |
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Enter your figures above to see the step-by-step working.
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How the QTc Calculator Works
The QT interval on an ECG shortens as heart rate rises, so it is corrected to a standard rate to be comparable — this is the QTc. The calculator first finds the RR interval (60 divided by the heart rate), then applies the Bazett correction (QT divided by the square root of RR) and the Fridericia correction (QT divided by the cube root of RR).
Bazett is the traditional formula but over-corrects at fast rates and under-corrects at slow rates; Fridericia is often preferred at rate extremes. A prolonged QTc raises the risk of dangerous arrhythmias. Thresholds vary, and men and women differ, so interpret alongside the clinical picture.
Bazett: QTc = QT ÷ √RR · Fridericia: QTc = QT ÷ ∛RR
Frequently Asked Questions
What is a normal QTc?
Roughly, a QTc up to about 440 ms is normal, 440–460 ms is borderline, and above 460 ms is prolonged, with over 500 ms carrying higher risk. Thresholds differ slightly for men and women and between guidelines.
Bazett or Fridericia — which is better?
Bazett is the most familiar but distorts at very fast or slow heart rates. Fridericia corrects more reliably at rate extremes. Many clinicians check both; this tool shows each.
Why correct the QT interval at all?
Because the raw QT interval naturally shortens as heart rate increases. Correcting to a standard rate lets you compare QT across different heart rates and detect true prolongation.
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