QTc Calculator
Calculate the corrected QT interval using four standard formulas.
Medical Disclaimer: This tool is for educational purposes only. It does not replace professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider.
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What Is the Corrected QT Interval (QTc)?
The QT interval on an electrocardiogram (ECG) measures the time from the start of the Q wave to the end of the T wave, representing the total duration of ventricular depolarization and repolarization. Because the QT interval naturally shortens at faster heart rates, clinicians use correction formulas to normalize it — producing the corrected QT interval (QTc).
Why Does QTc Matter?
A prolonged QTc interval (typically >450 ms in males, >470 ms in females) is associated with an increased risk of Torsades de Pointes, a potentially fatal ventricular arrhythmia. Monitoring QTc is critical when prescribing certain medications (antiarrhythmics, antibiotics, antipsychotics) and in patients with electrolyte imbalances or congenital long QT syndrome.
The Four Standard Formulas
- Bazett (1920): QTc = QT / √RR — The most widely used formula, but tends to overcorrect at high heart rates and undercorrect at low heart rates.
- Fridericia (1920): QTc = QT / RR1/3 — Considered more accurate than Bazett at extreme heart rates; preferred in many clinical trials.
- Framingham (1992): QTc = QT + 0.154 × (1 − RR) — A linear correction derived from the Framingham Heart Study population data.
- Hodges (1983): QTc = QT + 1.75 × (HR − 60) — A linear formula using heart rate directly instead of the RR interval.
Normal QTc Ranges
Normal QTc values are generally considered to be ≤440 ms for males and ≤460 ms for females. Values between 440–470 ms (males) or 460–480 ms (females) are borderline, and values above 500 ms are associated with significant arrhythmia risk regardless of sex.
Frequently Asked Questions
What is a normal QTc interval?
A normal QTc interval is generally ≤440 ms for males and ≤460 ms for females. Values between 440–470 ms (males) or 460–480 ms (females) are borderline. A QTc above 500 ms significantly increases the risk of dangerous arrhythmias like Torsades de Pointes.
Which QTc formula should I use?
Bazett's formula is the most widely used in clinical practice, but it tends to overcorrect at high heart rates. Fridericia is preferred in many clinical trials and regulatory submissions. Framingham and Hodges offer linear corrections that may be more accurate at extreme heart rates. When in doubt, compare all four results.
What medications can prolong the QT interval?
Many drug classes can prolong QT, including antiarrhythmics (amiodarone, sotalol), antibiotics (azithromycin, fluoroquinolones), antipsychotics (haloperidol, quetiapine), antidepressants (citalopram), and antiemetics (ondansetron). Always check CredibleMeds.org for a comprehensive list.
Can Apple Watch measure QT interval?
Apple Watch records a single-lead ECG but does not calculate QT or QTc intervals natively. HeartLab analyzes Apple Watch ECG recordings to measure QT intervals and calculate QTc using all four standard formulas — Bazett, Fridericia, Framingham, and Hodges.
What causes a prolonged QTc?
Prolonged QTc can be caused by medications, electrolyte imbalances (low potassium, magnesium, or calcium), congenital long QT syndrome (LQTS), heart disease, hypothermia, or bradycardia. It's important to identify the cause because prolonged QTc increases the risk of sudden cardiac events.
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