QT Interval Calculator

How to Use This Calculator

1. Enter the measured QT interval in milliseconds from your ECG.
2. Enter the patient's heart rate in bpm.
3. Select gender (upper normal limits differ: 450 ms male, 460 ms female).
4. Use the Extended tabs to compare Bazett, Fridericia, and Hodges–Framingham formulas.
5. Use the Professional tab to add drug class and electrolyte risk factors.

Formula

Example

Frequently Asked Questions

• What is a normal QTc interval? ▼
  A normal QTc is generally <450 ms in males and <460 ms in females using the Bazett formula. Values between 450-500 ms are considered borderline prolonged. A QTc ≥500 ms significantly increases the risk of torsades de pointes (TdP), a potentially fatal arrhythmia.
• What is the Bazett formula and why is it most commonly used? ▼
  The Bazett formula corrects the QT interval for heart rate: QTc = QT ÷ √(RR interval in seconds). It is the most widely used formula in clinical practice and on ECG machines. However, it overcorrects at high heart rates (>90 bpm) and undercorrects at low rates, which is why Fridericia and Hodges formulas are sometimes preferred.
• Which drugs most commonly cause QT prolongation? ▼
  High-risk drugs include Class Ia antiarrhythmics (quinidine, procainamide), Class III antiarrhythmics (sotalol, amiodarone), antibiotics (azithromycin, fluoroquinolones), antipsychotics (haloperidol, quetiapine), and certain antidepressants (citalopram, tricyclics). The CredibleMeds database at crediblemeds.org maintains the most comprehensive QT drug risk list.
• What electrolyte abnormalities worsen QT prolongation? ▼
  Hypokalemia (low potassium), hypomagnesemia (low magnesium), and hypocalcemia (low calcium) all prolong the QT interval and amplify drug-induced TdP risk. Correcting electrolytes is a critical part of managing QT prolongation.
• What is torsades de pointes? ▼
  Torsades de pointes (TdP) is a type of polymorphic ventricular tachycardia associated with QT prolongation. It can cause syncope and may degenerate into ventricular fibrillation. Risk is highest when QTc >500 ms, especially combined with QT-prolonging drugs and electrolyte abnormalities.

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