What is a normal QTc interval?

A normal QTc interval is generally less than 440 ms in males and less than 460 ms in females using Bazett's formula. A QTc above 500 ms is associated with significantly increased risk of torsades de pointes (TdP) and sudden cardiac death.

Which QTc formula should I use?

Bazett is most widely used in clinical practice and appears in most drug labels. Fridericia is more accurate at extreme heart rates and is recommended by the EMA for drug safety studies. For routine clinical use at normal heart rates (60–100 bpm), all four formulas give similar results.

QTc Calculator – Bazett, Fridericia, Framingham, Hodges

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.

The Four QTc Formulas Compared

Formula	Year	Equation	Best For	Limitation
Bazett ✦	1920	QT ÷ √(RR in sec)	Standard clinical practice, drug labelling	Overestimates at HR >100; underestimates at HR <60
Fridericia	1920	QT ÷ ∛(RR in sec)	Drug safety studies (EMA preferred), extreme HR	Less familiar in routine practice
Framingham	1992	QT + 154×(1−RR in sec)	Wide HR range, linear correction	Less used in drug safety
Hodges	1983	QT + 1.75×(HR−60)	Simple mental calculation	Less accurate at extreme rates

Clinical Risk Thresholds

QTc (Bazett)	Classification	Action
<440 ms (M) / <460 ms (F)	Normal	Routine monitoring with QT-prolonging drugs
440–500 ms	Borderline / Prolonged	Review QT-prolonging medications, correct electrolytes, monitor closely
>500 ms	Markedly prolonged	High TdP risk — consider drug cessation, specialist review, continuous monitoring
Drug-induced ΔQTc >60 ms	Significant prolongation	FDA/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.

QTc Interval Calculator