Printed on 3/17/2026
For informational purposes only. This is not medical advice.
The QT interval on an electrocardiogram (ECG) represents the time for ventricular depolarization and repolarization. Because QT varies with heart rate, it must be corrected (QTc) for clinical interpretation. QT prolongation increases the risk of torsades de pointes, a potentially fatal arrhythmia. This calculator uses three well-established correction formulas.
Formula: Bazett: QTc = QT / √(RR), Fridericia: QTc = QT / ∛(RR)
Input your sex, the measured QT interval in milliseconds, and the heart rate in beats per minute from the ECG strip or monitor.
The calculator applies Bazett, Fridericia, and Framingham correction formulas, adjusting the raw QT for heart rate to produce QTc values.
Compare your QTc against sex-specific normal ranges. Values above 450ms (men) or 460ms (women) warrant attention; values above 500ms indicate significant risk.
Prescribing physicians, pharmacists
Check baseline QTc before starting medications known to prolong QT — antiarrhythmics, fluoroquinolones, antipsychotics, certain antidepressants.
Cardiologists, intensivists
Monitor QTc during treatment with QT-prolonging drugs. Guidelines recommend checking ECG at baseline, after dose changes, and periodically during therapy. For critically ill patients on multiple medications, use [APACHE II](/tools/apache-ii-score) or [SOFA Score](/tools/sofa-score) for comprehensive risk assessment.
Emergency physicians, electrophysiologists
Evaluate patients with unexplained syncope or seizures for congenital or acquired long QT syndrome by calculating QTc from their ECG.
Hospitalists, nephrologists
Check QTc in patients with hypokalemia, hypomagnesemia, or hypocalcemia, as these electrolyte abnormalities can prolong the QT interval. Use [Corrected Calcium](/tools/corrected-calcium) to assess true calcium status in hypoalbuminemic patients.
Genetic counselors, cardiologists
Screen family members of patients with congenital long QT syndrome. First-degree relatives have a 50% chance of inheriting the condition.
Anesthesiologists, surgeons
Review QTc before procedures requiring anesthetic agents that may prolong QT, especially in patients with cardiac history or electrolyte concerns. Assess cardiovascular risk with [ASCVD Risk Calculator](/tools/ascvd-risk) or [Framingham Risk Score](/tools/framingham-risk) for patients with cardiac history.
Bazett's formula overcorrects at heart rates above 100 bpm and undercorrects below 60 bpm. When heart rate is outside 60-100, Fridericia or Framingham formulas are more accurate. Many FDA drug trials now require Fridericia.
The QT interval is best measured in leads II or V5 where the T wave is usually most clearly defined. Avoid leads where the T wave is flat, biphasic, or obscured by U waves.
If the U wave merges with the T wave (common with hypokalemia), measure to the end of the U wave. If T and U are clearly separated, measure to the end of T only. Document which approach you used.
QT varies beat-to-beat, especially in atrial fibrillation. Measure QT from 3-5 consecutive beats and average them. In AFib, some experts recommend measuring the longest QT in a 10-second strip.
A QTc above 500 ms is associated with significantly increased risk of torsades de pointes. This is the threshold at which most guidelines recommend discontinuing or avoiding QT-prolonging medications.
For bundle branch block or ventricular pacing, the QRS is artificially widened. Consider using the JTc interval (QT minus QRS, then rate-corrected) instead, as the JT portion represents repolarization time alone.
If QTc is prolonged and the patient has low potassium or magnesium, correct the electrolytes before making treatment decisions. Repleting K+ to 4.0-4.5 mEq/L and Mg2+ to 2.0 mg/dL may normalize the QTc.
The CredibleMeds database (crediblemeds.org) maintains a list of drugs that prolong QT, cause torsades, or interact with other QT-prolonging agents. Check it before prescribing combinations of concerning medications.
In suspected congenital long QT syndrome, a QTc >470 ms in males or >480 ms in females is diagnostic (in the absence of secondary causes). Lower thresholds apply if symptoms or family history are present.
A single QTc is a snapshot. If monitoring a patient on QT-prolonging therapy, serial ECGs (baseline, 2-4 hours post-dose for some drugs, and steady state) provide a more complete picture of the QT effect.
QTc correction formulas: Bazett (1920) is most widely used but has known rate-dependence limitations. Fridericia (1920) is preferred by FDA for drug trials. Normal QTc thresholds (≤450ms men, ≤460ms women) are based on population studies. The QTc >500ms threshold for torsades risk is supported by AHA/ACC/HRS guidelines (2017) and multiple epidemiological studies.
Your QTc result is measured in milliseconds (ms). A QTc ≤ 450 ms in men or ≤ 460 ms in women is generally considered normal. Values between 450–500 ms are borderline prolonged and warrant monitoring — particularly if the patient is on QT-prolonging medications. A QTc > 500 ms is significantly prolonged and is associated with a markedly increased risk of torsades de pointes, a life-threatening ventricular arrhythmia.
This calculator displays results from multiple correction formulas. Bazett's formula is the most widely referenced but tends to overcorrect at high heart rates. If results differ significantly between formulas, the Fridericia or Framingham value may be more reliable, especially at heart rates outside the 60–100 bpm range.
QTc calculation is essential whenever prescribing or monitoring medications known to prolong the QT interval, including many antiarrhythmics (sotalol, amiodarone), antibiotics (fluoroquinolones, azithromycin), antipsychotics (haloperidol, ziprasidone), and antidepressants (citalopram). It is also important when evaluating patients with unexplained syncope, seizures, or a family history of sudden cardiac death, which may suggest congenital long QT syndrome.
Routine QTc assessment is standard in critical care, pre-operative evaluations for certain surgeries, and whenever electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia) are present or suspected.
QTc correction formulas are mathematical approximations and no single formula is accurate across all heart rates. Bazett's formula is unreliable at heart rates below 60 or above 100 bpm. Even Fridericia and Framingham formulas have reduced accuracy at extreme heart rates.
Accurate QTc calculation depends on precise QT interval measurement, which can be challenging — the end of the T wave is often difficult to define, especially when U waves are present, the T wave is notched, or the tracing is noisy. Inter-observer variability in QT measurement can be 20–40 ms.
QTc should not be interpreted in isolation. Atrial fibrillation with irregular RR intervals makes QTc calculation unreliable. Bundle branch blocks and ventricular pacing also widen the QRS complex and artificially prolong the measured QT interval. In these cases, the JTc interval (QT minus QRS duration, corrected for rate) may be more appropriate.
Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about your health.
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