QTc Prolongation Calculator
Correct the QT interval for heart rate with Bazett, Fridericia, Framingham, or Hodges. Classify against gender-specific thresholds, flag the 500 ms Torsades de Pointes risk band, and surface common QT-prolonging drug classes. Clinical decision support only.
QTc Prolongation Thresholds
The calculator applies sex-specific prolonged QTc cutoffs and a severe threshold.
| Category | Male QTc | Female QTc | Calculator rule |
|---|---|---|---|
| Short | Below 320 ms | Below 320 ms | Both sexes |
| Normal | 320-440 ms | 320-440 ms | At or below 440 ms |
| Borderline | Above 440 to below 450 ms | Above 440 to below 460 ms | Before sex-specific threshold |
| Prolonged | 450-500 ms | 460-500 ms | At or above threshold |
| Severely prolonged | Above 500 ms | Above 500 ms | Both sexes |
Frequently Asked Questions about the QTc Prolongation Calculator
Why does the QT interval have to be corrected for heart rate?
The raw QT interval on an ECG shortens as the heart rate climbs and lengthens as it slows, because ventricular repolarization (the T wave) is tied to the cardiac cycle length. A QT of 400 ms means very different things at HR 50 (long, RR = 1.2 s) versus HR 110 (borderline-short, RR = 0.55 s). Correcting for rate (QTc) lets you compare values across patients and across the same patient at different rates, and it is what every published QT prolongation threshold (450 ms male, 460 ms female, 500 ms TdP risk) refers to. Reporting an uncorrected QT and applying those thresholds is the most common error in QT interpretation.
Bazett (1920) vs Fridericia (1920): which is more accurate?
Both formulas date to 1920 and use the same RR interval, but with different exponents: Bazett uses a square root (QTc = QT / sqrt(RR)), while Fridericia uses a cube root (QTc = QT / cbrt(RR)). Bazett systematically overestimates QTc at heart rates above 100 bpm (it flags healthy tachycardia as prolonged) and underestimates at heart rates below 60 (it misses real prolongation in bradycardia). Fridericia and Framingham (a linear correction, also from the same era and refined by Sagie 1992) track the underlying rate adjustment more closely at the extremes and are preferred in drug-safety research and ICU work. The catch: Bazett is still the most widely reported formula on automated ECG carts, so most published thresholds were derived against Bazett. Use Fridericia outside 60 to 100 bpm and always check both.
Why are the QTc thresholds different for men and women?
Women have a baseline QTc that runs roughly 10 ms longer than men starting at puberty, driven by sex-hormone effects on the cardiac potassium channels (IKr) that control repolarization. The AHA/ACCF/HRS 2009 scientific statement codified this into separate cutoffs: upper limit of normal is 450 ms in males and 460 ms in females. This calculator classifies values at or above those cutoffs as prolonged, and above 500 ms as the high-risk band for either sex. Forgetting the sex-specific cut-off in a female patient leads to false-positive prolongation alerts at the 451 to 460 ms range, which can trigger unnecessary medication holds.
What is Torsades de Pointes and why does QTc above 500 ms matter?
Torsades de Pointes is a polymorphic ventricular tachycardia that can deteriorate into ventricular fibrillation. A QTc above 500 ms is associated with higher risk, but there is no precise risk increase that applies to every additional 10 ms or every patient. Formula choice, rhythm, medications, electrolytes, congenital risk, and symptoms all matter. Do not stop medication or replace electrolytes based on this calculator; obtain prompt clinical review, and seek emergency care for fainting, severe palpitations, or instability.
Which drugs commonly prolong the QT interval?
The class with the highest known TdP risk is class IA and class III antiarrhythmics: quinidine, procainamide, disopyramide, sotalol, dofetilide, ibutilide, and amiodarone (lower TdP risk than its prolongation suggests). Outside cardiology, common culprits include macrolide antibiotics (azithromycin, erythromycin, clarithromycin), fluoroquinolones (levofloxacin, moxifloxacin, ciprofloxacin), azole antifungals (fluconazole, ketoconazole, voriconazole), antipsychotics (haloperidol IV, ziprasidone, thioridazine), SSRIs (citalopram and escitalopram in particular, with dose-related risk), tricyclic antidepressants, methadone (cumulative, dose-dependent), and the 5HT3 antiemetic ondansetron. CredibleMeds.org maintains the authoritative public list (Known, Possible, Conditional Risk of TdP) and is the reference cited by US and European guidelines. Check it before starting any new drug in a patient with QTc above 470 ms or with prior TdP.
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