Health & Medicine · Cardiology & Hemodynamics
Corrected QT Interval Calculator (Bazett)
Calculates the heart-rate-corrected QT interval (QTc) using Bazett's formula to identify prolongation risk from a measured QT interval and RR interval.
Calculator
Formula
QTc is the corrected QT interval in milliseconds (ms). QT is the measured QT interval in milliseconds from the ECG. RR is the RR interval in seconds (the time between consecutive R-wave peaks), calculated as 60 divided by the heart rate in beats per minute. Dividing by the square root of RR normalises the QT interval to a heart rate of 60 bpm.
Source: Bazett HC. An analysis of the time-relations of electrocardiograms. Heart. 1920;7:353–370.
How it works
The QT interval on an ECG represents the total duration of ventricular depolarisation and repolarisation. Because the QT interval naturally shortens at higher heart rates and lengthens at lower heart rates, a raw QT measurement cannot be reliably compared between patients or across different physiological states without rate correction. A QT interval that appears normal at a slow resting heart rate may in fact indicate an abnormally prolonged repolarisation phase once corrected.
Bazett's formula corrects the QT interval by dividing it by the square root of the RR interval (in seconds): QTc = QT ÷ √RR. The RR interval is derived from the heart rate as 60 ÷ HR (bpm). This simple relationship was empirically derived from a small dataset and assumes that QT varies as the square root of cycle length — an approximation that works well at normal resting heart rates (50–100 bpm) but can over-correct at high heart rates and under-correct at very low heart rates. Normal QTc values are generally accepted as below 430 ms in males and below 450 ms in females, with values above 500 ms considered markedly prolonged and associated with substantially elevated arrhythmic risk.
QTc calculation is critically important in drug safety monitoring. Over 50 commonly prescribed medications — including certain antibiotics (fluoroquinolones, macrolides), antipsychotics, antiarrhythmics, and antiemetics — are known to prolong the QT interval. Before prescribing such drugs, clinicians routinely obtain a baseline ECG and calculate QTc. The FDA and ICH E14 guideline require thorough QT studies for all new non-antiarrhythmic drugs, making automated QTc calculation an essential tool in both clinical and pharmaceutical research settings.
Worked example
A 58-year-old male patient presents for a pre-operative assessment. His ECG shows a measured QT interval of 420 ms and a heart rate of 65 bpm.
Step 1 — Calculate the RR interval:
RR = 60 ÷ HR = 60 ÷ 65 = 0.923 seconds
Step 2 — Apply Bazett's formula:
QTc = QT ÷ √RR = 420 ÷ √0.923 = 420 ÷ 0.9607 = 437.2 ms
Interpretation: In a male patient, a QTc of 437 ms falls in the borderline range (430–450 ms). The clinical team would note this result and exercise caution when prescribing any QT-prolonging medications. A repeat ECG after any new drug introduction would be advisable, and cardiology review should be considered if additional risk factors are present.
Limitations & notes
Bazett's formula has well-documented limitations that clinicians should keep in mind. It over-corrects at high heart rates (above 100 bpm), producing falsely elevated QTc values, and under-corrects at low heart rates (below 60 bpm), potentially masking true prolongation in bradycardic patients. For heart rates outside the 60–100 bpm range, alternative correction formulas such as Fridericia (cube-root correction), Framingham (linear regression), or Hodges corrections are considered more accurate and are preferred in research settings and drug trials. Additionally, QT interval measurement itself carries significant variability depending on lead selection (lead II and V5 are standard), measurement technique (manual versus automated), and inter-observer differences. This calculator should never replace a full clinical ECG interpretation by a qualified healthcare professional. Always verify automated measurements and consider the patient's full clinical context, medications, and electrolyte status (particularly potassium, magnesium, and calcium) before acting on QTc values.
Frequently asked questions
What is a normal QTc value according to Bazett's formula?
Normal QTc values are generally below 430 ms in males and below 450 ms in females. Values between 430–450 ms (male) or 450–470 ms (female) are considered borderline. QTc above 500 ms in either sex is regarded as markedly prolonged and associated with significantly elevated risk of Torsades de Pointes arrhythmia.
Why does sex matter for QTc interpretation?
Biological sex influences QT interval duration due to hormonal differences. After puberty, males typically have shorter QTc intervals than females, likely due to the effects of testosterone shortening ventricular repolarisation. As a result, the thresholds for borderline and prolonged QTc are approximately 20 ms higher in females than in males, and sex-specific cutoffs should always be used for clinical interpretation.
When should I use Fridericia's formula instead of Bazett's?
Fridericia's formula (QTc = QT ÷ RR^(1/3)) is preferred when the patient's heart rate is significantly above 100 bpm or below 60 bpm, as Bazett's square-root correction becomes unreliable at these extremes. The ICH E14 regulatory guideline for pharmaceutical QT studies recommends evaluating multiple correction formulas. In clinical practice, Bazett remains the standard, but for bradycardic or tachycardic patients, Fridericia or Framingham corrections provide more accurate rate-independent QTc estimates.
Which drugs are most commonly associated with QT prolongation?
Many drug classes carry QT prolongation risk. High-risk examples include antiarrhythmics (amiodarone, sotalol, quinidine), certain antipsychotics (haloperidol, quetiapine), macrolide antibiotics (azithromycin, clarithromycin), fluoroquinolones (ciprofloxacin, moxifloxacin), antiemetics (ondansetron, domperidone), and some antifungals (fluconazole). The CredibleMeds/AzCERT database (crediblemeds.org) maintains a continuously updated risk classification of QT-prolonging drugs and is widely used by clinicians and pharmacists.
What electrolyte abnormalities affect the QT interval?
Hypokalaemia (low potassium) is the most clinically significant electrolyte cause of QT prolongation, as potassium channels are integral to ventricular repolarisation. Hypomagnesaemia and hypocalcaemia also prolong the QT interval and are frequently co-present with hypokalaemia. Before attributing QT prolongation to a drug or intrinsic cardiac disease, clinicians should always check serum electrolytes and correct any deficiencies, as normalising electrolytes can substantially shorten the QTc without any other intervention.
Last updated: 2025-01-15 · Formula verified against primary sources.