What is a normal QTc?

Normal QTc is generally considered ≤ 450 ms for men and ≤ 460 ms for women. QTc between 450–500 ms is borderline and should be monitored. QTc > 500 ms is considered significantly prolonged and carries a higher risk of dangerous arrhythmias.

Which QTc formula should I use?

Bazett's formula is the most commonly used and widely recognized. However, it overcorrects at high heart rates and undercorrects at low heart rates. Fridericia's formula is preferred when heart rate is outside the 60–100 bpm range. The Framingham formula uses a linear correction and may be more accurate across a wider range of heart rates.

What causes QT prolongation?

QT prolongation can be caused by certain medications (antiarrhythmics, antibiotics, antipsychotics, antidepressants), electrolyte imbalances (low potassium, magnesium, or calcium), congenital long QT syndrome, heart disease, and other medical conditions. Always consult a healthcare provider for ECG interpretation.

How do I find the RR interval from heart rate?

The RR interval (in milliseconds) = 60,000 / heart rate (bpm). For example, a heart rate of 75 bpm gives an RR interval of 800 ms. This calculator automatically converts your heart rate to the RR interval needed for QTc calculations.

What is torsades de pointes?

Torsades de pointes ('twisting of the points') is a polymorphic ventricular tachycardia associated with QT prolongation. It can degenerate into ventricular fibrillation and cause sudden cardiac death. Risk increases significantly when QTc exceeds 500 ms.

What medications most commonly prolong QT?

Common QT-prolonging drugs include: antiarrhythmics (sotalol, amiodarone, dofetilide), antibiotics (fluoroquinolones, azithromycin, erythromycin), antipsychotics (haloperidol, ziprasidone, thioridazine), antidepressants (citalopram, escitalopram), and methadone. Always check drug interactions.

Should I stop a medication if QTc is prolonged?

It depends on the degree of prolongation, the indication for the drug, and available alternatives. QTc >500 ms or an increase >60 ms from baseline typically warrants discontinuation. Borderline prolongation requires risk-benefit assessment. Always correct electrolytes first.

What is congenital long QT syndrome?

Congenital long QT syndrome (LQTS) is a genetic disorder of cardiac ion channels causing prolonged QT and risk of sudden death. There are 17 subtypes, with LQT1, LQT2, and LQT3 being most common. Beta-blockers are first-line treatment; some patients need ICDs.

How does hypokalemia affect QT?

Hypokalemia prolongs the QT interval by delaying cardiac repolarization. The lower the potassium, the longer the QT. Severe hypokalemia (<2.5 mEq/L) can cause dangerous QT prolongation. Correct potassium to 4.0-4.5 mEq/L in patients at risk.

Can exercise affect QTc?

Yes. Heart rate increases during exercise, and QT shortens physiologically. In some patients with congenital LQTS (especially LQT1), the QT fails to shorten appropriately with exercise, which can trigger arrhythmias. Exercise stress testing can unmask LQTS.

What is the Schwartz score for LQTS?

The Schwartz score is a clinical scoring system for diagnosing congenital long QT syndrome, incorporating QTc duration, T wave morphology, symptoms, and family history. A score ≥3.5 indicates high probability of LQTS. It's used alongside genetic testing.

Is QTc different in athletes?

Athletes often have lower resting heart rates (40-60 bpm), which can cause Bazett's formula to undercorrect QTc. Some athletes have slightly longer QT intervals without pathology. Use Fridericia in bradycardic athletes and interpret cautiously.

How does the FDA evaluate QT prolongation?

The FDA requires 'thorough QT studies' for most new drugs. A mean QTc prolongation >20 ms or upper confidence bound >10 ms raises concern. The FDA recommends Fridericia correction and requires warnings for drugs causing >5 ms mean QTc prolongation.

Can COVID-19 treatments affect QT?

Hydroxychloroquine and azithromycin, used early in the pandemic, both prolong QT. Their combination further increases risk. Current guidelines recommend ECG monitoring when these drugs are used together. Remdesivir and antivirals have minimal QT effects.

What is short QT syndrome?

Short QT syndrome is a rare inherited condition where QTc is abnormally short (<340 ms), increasing risk of atrial and ventricular arrhythmias and sudden death. Unlike long QT, it's less well-characterized and treatment options are limited.

Online Medical Tools — QTc Calculator

Printed on 8/14/2026

For informational purposes only. This is not medical advice.

Clinical

QTc Calculator

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)

Sex

QT Interval

Heart Rate

bpm

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How It Works

Enter your ECG measurements

Input your sex, the measured QT interval in milliseconds, and the heart rate in beats per minute from the ECG strip or monitor.

Calculate corrected QT values

The calculator applies Bazett, Fridericia, and Framingham correction formulas, adjusting the raw QT for heart rate to produce QTc values.

Interpret your results

Compare your QTc against sex-specific normal ranges. Values above 450ms (men) or 460ms (women) warrant attention; values above 500ms indicate significant risk.

Who Uses the QTc Calculator

Pre-Medication Screening

Prescribing physicians, pharmacists

Check baseline QTc before starting medications known to prolong QT — antiarrhythmics, fluoroquinolones, antipsychotics, certain antidepressants.

Drug Monitoring

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) or [SOFA Score](/tools/sofa-score) for comprehensive risk assessment.

Syncope Workup

Emergency physicians, electrophysiologists

Evaluate patients with unexplained syncope or seizures for congenital or acquired long QT syndrome by calculating QTc from their ECG.

Electrolyte Imbalance Assessment

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.

Family Screening for Long QT

Genetic counselors, cardiologists

Screen family members of patients with congenital long QT syndrome. First-degree relatives have a 50% chance of inheriting the condition.

Pre-Operative Evaluation

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.

Pro Tips

Use Fridericia for extreme heart rates

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.

Measure QT from lead II or V5

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.

Include the U wave if it merges with T

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.

Check multiple beats and average

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.

QTc >500 ms is the danger zone

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.

The motor component has two points in Bazett

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.

Potassium and magnesium first

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.

Check CredibleMeds for drug interactions

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.

Congenital long QT has different thresholds

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.

Serial ECGs beat single measurements

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.

Common Questions About Your Results

Evidence-Based Methodology

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.

Clinical Content Trust

Last reviewed:: April 21, 2026
Guideline version:: General evidence framework v2026.04
Source set version:: Primary-source set v1

How to Interpret Your Result

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.

When to Use This Tool

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.

Limitations

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.

Changelog

April 21, 2026 · trust-baseline
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