QTc Calculator
Calculate the corrected QT interval (QTc) using Bazett, Fridericia, Framingham, and Hodges formulas. Enter QT interval and heart rate for instant QTc calculation.
Inputs
Common QT-prolonging drugs: Amiodarone, sotalol, dofetilide, procainamide, haloperidol, ondansetron, methadone, fluoroquinolones, azithromycin, citalopram/escitalopram. Always check CredibleMeds.org for a complete list.
Disclaimer: For educational purposes only. Not a substitute for clinical judgment.
About This Tool
What Is QTc and How Do You Calculate It?
The QT interval on an electrocardiogram represents the total duration of ventricular depolarization and repolarization. Because the QT interval naturally shortens at higher heart rates, a heart-rate correction is needed for meaningful interpretation. The corrected QT interval (QTc) adjusts the measured QT for heart rate, allowing comparison against standard normal values. Prolonged QTc is associated with increased risk of Torsades de Pointes (TdP), a potentially fatal polymorphic ventricular tachycardia.
QTc Calculation Formulas: Bazett vs Fridericia
Four formulas are commonly used: Bazett's formula (QTc = QT/√RR) is the most widely cited but is known to overcorrect at high heart rates and undercorrect at low heart rates. Fridericia's formula (QTc = QT/∛RR) provides better accuracy across a wider range of heart rates and is now preferred by the FDA for drug safety studies. The Framingham (linear regression-based) and Hodges (linear correction) formulas offer additional alternatives. In clinical practice, Bazett remains standard for most purposes, but Fridericia should be used when heart rate is outside 60–100 bpm.
What Is a Normal QTc and When Is It Prolonged?
QTc prolongation is a critical safety concern in both congenital long QT syndromes and acquired QT prolongation from medications, electrolyte abnormalities, or structural heart disease. Over 100 commonly prescribed medications can prolong the QT interval. A QTc > 500 ms or an increase of > 60 ms from baseline is considered high risk for TdP. Risk is amplified by concurrent hypokalemia, hypomagnesemia, bradycardia, female sex, advanced age, and heart failure.
🔑 Clinical Pearls
- QTc > 500 ms = high risk for Torsades de Pointes — reassess all QT-prolonging medications immediately.
- A ΔQTc > 60 ms from baseline is clinically significant even if absolute QTc is still < 500 ms.
- Bazett overcorrects at HR > 100 bpm — use Fridericia for tachycardic patients.
- Always check and correct K⁺ (> 4.0 mEq/L) and Mg²⁺ (> 2.0 mg/dL) in patients on QT-prolonging drugs.
Key References
- Bazett HC. An analysis of the time-relations of electrocardiograms. Heart. 1920;7:353-370.
- Fridericia LS. The duration of systole in an electrocardiogram. Acta Med Scand. 1920;53:469-486.
- Drew BJ, et al. Prevention of Torsade de Pointes in hospital settings. Circulation. 2010;121(8):1047-1060.
Formula last verified: February 2026