Corrected QT Calculator — QTc (Bazett's Formula)
Calculate the heart-rate-corrected QT interval (QTc) using Bazett's formula: QTc = QT / √(RR interval), where RR = 60 / heart rate (seconds). Normal QTc is below 450 ms for males and below 460 ms for females. A QTc above 500 ms carries significant risk of torsades de pointes arrhythmia. Enter QT interval (ms) and heart rate (bpm).
Medical Disclaimer
This tool is for educational purposes only. QT interval measurement requires a proper ECG and clinical expertise. Prolonged QTc requires evaluation by a cardiologist. Never make clinical decisions based solely on this calculator.
Corrected QT (QTc) Calculator
Calculate the heart-rate-corrected QT interval (QTc) using Bazett's formula. Normal QTc: male <450 ms, female <460 ms.
Typical range: 350–440 ms
Normal resting: 60–100 bpm
Frequently Asked Questions
What is the QTc and why is it important?
The QTc (corrected QT interval) is the QT interval on an ECG adjusted for heart rate. The raw QT interval shortens at faster heart rates and lengthens at slower rates. QTc standardizes this to allow meaningful comparison. Prolonged QTc (>450ms in men, >460ms in women) indicates risk of life-threatening arrhythmias, particularly torsades de pointes.
What is Bazett's formula for QTc?
Bazett's formula: QTc = QT / √(RR interval), where RR = 60 / heart rate (in seconds). For example: QT = 400ms, HR = 75 bpm → RR = 0.8s → QTc = 400 / √0.8 ≈ 447ms. Despite its known limitations at extreme heart rates, Bazett's formula remains the clinical standard.
What is a normal QTc interval?
Normal QTc values per AHA/ACC guidelines: Males: QTc < 440ms (normal), 440–449ms (borderline), ≥450ms (prolonged). Females: QTc < 450ms (normal), 450–459ms (borderline), ≥460ms (prolonged). A QTc >500ms is associated with significantly increased risk of torsades de pointes regardless of sex.
What causes prolonged QT interval?
Causes of prolonged QT include: congenital Long QT Syndrome (LQTS), medications (antiarrhythmics like sotalol, antibiotics like azithromycin, antipsychotics), electrolyte disorders (hypokalemia, hypomagnesemia, hypocalcemia), myocardial ischemia, hypothyroidism, hypothermia, and anorexia nervosa.
What is torsades de pointes?
Torsades de pointes (TdP) is a polymorphic ventricular tachycardia associated with prolonged QTc. On an ECG, the QRS complexes appear to twist around the baseline. TdP can degenerate into ventricular fibrillation and cause sudden cardiac death. It is more likely when QTc >500ms, with hypokalemia, bradycardia, or in females.
How do I measure the QT interval on an ECG?
The QT interval is measured from the beginning of the Q wave (or earliest deflection from baseline) to the end of the T wave (where the T wave returns to baseline). Measurement is typically done in leads II and V5. Use the longest QT interval found. Avoid measuring in leads with U waves or T wave abnormalities.
Does Bazett's formula overcorrect at fast heart rates?
Yes — Bazett's formula tends to overcorrect (overestimate QTc) at fast heart rates (>100 bpm) and undercorrect at slow heart rates (<60 bpm). Alternative formulas like Fridericia (QTc = QT / RR^(1/3)) or Framingham are more accurate at extreme heart rates. However, Bazett remains the standard in clinical practice.
Which medications prolong the QT interval?
Common QT-prolonging medications include: antiarrhythmics (sotalol, amiodarone, quinidine), antibiotics (azithromycin, clarithromycin, fluoroquinolones), antipsychotics (haloperidol, quetiapine, ziprasidone), antidepressants (citalopram, escitalopram), antifungals (fluconazole), and methadone. The website CredibleMeds.org maintains a comprehensive list.
About the Corrected QT (QTc) Calculator
About QT and QTc
The QT interval is a measurement on the electrocardiogram (ECG/EKG) that represents the total time for the ventricles to depolarize and repolarize — essentially the electrical cycle of a heartbeat. It is measured from the beginning of the Q wave to the end of the T wave.
The problem with raw QT interval measurement is that it varies with heart rate: faster heart rates shorten QT; slower rates lengthen it. The QTc (corrected QT) adjusts for this rate dependence, allowing meaningful comparison across different heart rates.
Bazett's Formula
QTc = QT / √(RR interval)
RR interval = 60 / Heart Rate (seconds)
Where:
- QT — Measured QT interval in seconds (or milliseconds)
- RR — Time between two consecutive R waves in seconds (= 60 / HR)
- QTc — Heart rate-corrected QT interval
Example 1: QT = 400ms, HR = 60 bpm → RR = 1.0s → QTc = 400/√1.0 = 400ms (normal)
Example 2: QT = 400ms, HR = 75 bpm → RR = 0.8s → QTc = 400/√0.8 ≈ 447ms (borderline male)
Normal QTc Values
| Sex | Normal | Borderline | Prolonged |
|---|---|---|---|
| Male | < 440 ms | 440–449 ms | ≥ 450 ms |
| Female | < 450 ms | 450–459 ms | ≥ 460 ms |
Thresholds vary slightly between guidelines (AHA, ACC, ESC). These values follow the AHA/ACC Task Force guidelines. A QTc > 500ms is associated with significantly increased risk of torsades de pointes (a dangerous ventricular arrhythmia).
QT Prolongation and Torsades de Pointes
Prolonged QTc increases the risk of torsades de pointes (TdP) — a potentially fatal ventricular tachyarrhythmia that can degenerate into ventricular fibrillation and sudden cardiac death.
- QTc > 500ms: Significantly elevated risk of TdP
- QTc > 600ms: Very high risk — consider urgent cardiology consultation
- Risk is amplified by hypokalemia, hypomagnesemia, bradycardia, and female sex
- ECG monitoring may be required when starting QT-prolonging medications
Common Causes of Prolonged QTc
Congenital
- Long QT Syndrome (LQTS)
- Andersen-Tawil syndrome
- Timothy syndrome
Medications
- Antiarrhythmics (sotalol, amiodarone)
- Antibiotics (azithromycin, fluoroquinolones)
- Antipsychotics (haloperidol, quetiapine)
- Antidepressants (citalopram)
Electrolyte Disorders
- Hypokalemia
- Hypomagnesemia
- Hypocalcemia
Other
- Myocardial ischemia
- Hypothyroidism
- Hypothermia
- Anorexia nervosa
Other QTc Correction Formulas
Bazett's formula is the most widely used but has limitations — it overcorrects at fast heart rates and undercorrects at slow rates. Alternative formulas include:
Despite its limitations, Bazett's formula remains the standard in clinical practice and most ECG machines, as most drug trial data and reference ranges were established using Bazett.
Medical Disclaimer
This QTc calculator is for educational and informational purposes only. QT interval measurement requires a proper 12-lead ECG with careful measurement technique. QTc prolongation requires evaluation by a qualified cardiologist. Never start or stop medications based solely on this calculator. If you have symptoms such as palpitations, syncope, or chest pain, seek emergency medical attention immediately.