Cardiology Tool

QTc Interval Calculator

Calculate corrected QT interval (QTc) using all four validated formulas simultaneously, Bazett, Fridericia, Framingham, and Hodges. Instant drug-induced QT prolongation risk classification, sex-specific normal ranges, and TdP risk assessment.

4 formulas simultaneously Sex-specific thresholds PDF export
QTc Calculator
ms
Measure from start of QRS to end of T-wave. Use the longest QT in any lead (usually V5–V6). Please enter a valid QT interval (200–700 ms).
bpm
Use the average ventricular rate from the ECG rhythm strip. Please enter a valid heart rate (20–300 bpm).
QTc Results
-
ms, QTc (Bazett, primary clinical reference)

All four formula results:

Bazett Most Used
-
ms
QTc = QT ÷ √RR
Fridericia EMA Preferred
-
ms
QTc = QT ÷ ∛RR
Framingham
-
ms
QTc = QT + 154×(1−RR)
Hodges
-
ms
QTc = QT + 1.75×(HR−60)

Normal QTc ranges (Bazett):

Male, Normal
< 440 ms
Female, Normal
< 460 ms
Borderline (Both)
440–500 ms
High TdP Risk
> 500 ms

About QTc Interval Correction

The QT interval, measured from the start of the QRS complex to the end of the T-wave, varies inversely with heart rate. The corrected QT interval (QTc) normalises for heart rate, allowing comparison across different rates. Drug-induced QT prolongation and the resulting risk of torsades de pointes (TdP), a potentially fatal polymorphic ventricular tachycardia, is a major cardiac safety concern and has led to the withdrawal of multiple drugs from the market. See the original derivation of the Bazett formula for background.

The Four QTc Formulas Compared

FormulaYearEquationBest ForLimitation
Bazett ✦1920QT ÷ √(RR in sec)Standard clinical practice, drug labellingOverestimates at HR >100; underestimates at HR <60
Fridericia1920QT ÷ ∛(RR in sec)Drug safety studies (EMA preferred), extreme HRLess familiar in routine practice
Framingham1992QT + 154×(1−RR in sec)Wide HR range, linear correctionLess used in drug safety
Hodges1983QT + 1.75×(HR−60)Simple mental calculationLess accurate at extreme rates

Clinical Risk Thresholds

QTc (Bazett)ClassificationAction
<440 ms (M) / <460 ms (F)NormalRoutine monitoring with QT-prolonging drugs
440–500 msBorderline / ProlongedReview QT-prolonging medications, correct electrolytes, monitor closely
>500 msMarkedly prolongedHigh TdP risk, consider drug cessation, specialist review, continuous monitoring
Drug-induced ΔQTc >60 msSignificant prolongationFDA/EMA safety signal, consider drug discontinuation

Common QT-Prolonging Drugs

Many commonly used drugs prolong the QT interval. High-risk categories include: antipsychotics (haloperidol, quetiapine, risperidone), antidepressants (tricyclics, citalopram), antibiotics (azithromycin, ciprofloxacin, moxifloxacin), antifungals (fluconazole), antiemetics (ondansetron, domperidone), antiarrhythmics (amiodarone, sotalol), and antimalarials (hydroxychloroquine). Check CredibleMeds (crediblemeds.org) for drug-specific QT risk classification.

Frequently Asked Questions

Why does Bazett formula overestimate QTc at high heart rates?+
Bazett's formula uses a square root correction (QTc = QT ÷ √RR), which over-corrects at high heart rates because the relationship between QT and RR is not truly square-root linear, it flattens at higher rates. At heart rates above 100 bpm, Bazett can overestimate QTc by 20–40 ms compared to Fridericia. This can lead to false-positive QTc prolongation diagnoses. In tachycardic patients, Fridericia is recommended.
How do I measure the QT interval on an ECG?+
Measure from the earliest onset of the QRS complex to the latest point of the T-wave return to the isoelectric baseline in the lead with the largest T-wave amplitude (usually V5 or V6). Avoid leads with flat T-waves. If a U-wave is present, measure to the nadir between the T and U waves if the U is incorporated into the T-wave morphology. Use digital calipers for accuracy. The QT should be measured over 3–5 consecutive beats and averaged. In atrial fibrillation, average the QT over ≥10 beats due to beat-to-beat variability.
What electrolyte abnormalities prolong the QT interval?+
Hypokalaemia is the most common electrolyte cause of QT prolongation, it reduces cardiac repolarisation reserve and significantly increases TdP risk, especially in the context of QT-prolonging drugs. Hypomagnesaemia also prolongs QT and is important to correct. Hypocalcaemia prolongs the ST segment component of the QT. Correction of hypokalaemia (target K⁺ >4.0 mmol/L) and hypomagnesaemia (Mg²⁺ >0.8 mmol/L) is a priority before initiating or continuing QT-prolonging medications. Check Lab Values Converter to convert between mmol/L and mEq/L for electrolytes.

Related Tools on MediCalc Pro

For AF stroke risk and anticoagulation with DOAC monitoring, see CHA₂DS₂-VASc Score. For 10-year cardiovascular risk, see Framingham Risk Score. For renal function impacting QT-prolonging drug dosing, see Creatinine Clearance. For ACS risk, see TIMI Score and HEART Score. For electrolyte lab value conversion, see Lab Values Converter.

References

  • Bazett HC. "An analysis of the time-relations of electrocardiograms." Heart. 1920;7:353-370.
  • Fridericia LS. "Die Systolendauer im Elektrokardiogramm bei normalen Menschen und bei Herzkranken." Acta Med Scand. 1920;53:469-486.
  • Sagie A, et al. "An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study)." Am J Cardiol. 1992;70(7):797-801.
  • European Medicines Agency. ICH E14 Clinical Evaluation of QT/QTc Interval Prolongation and Proarrhythmic Potential for Non-Antiarrhythmic Drugs. EMA, 2005.
⚠️ Medical Disclaimer: QTc calculations are clinical reference tools. Interpretation must account for the full clinical context including drugs, electrolytes, and patient history. A prolonged QTc always warrants clinical assessment by a qualified healthcare professional.