Enter your patient's QT interval (in milliseconds) and heart rate (in bpm) to calculate the corrected QT interval (QTc) using Bazett's formula. The tool returns the QTc value and flags whether it falls within normal, borderline, or prolonged ranges — helping you quickly assess cardiac risk at the bedside.
QTc (Bazett)
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RR Interval
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Interpretation
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QTc Status
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Bazett's formula calculates the corrected QT interval as QTc = QT / √RR, where QT is the measured QT interval in seconds and RR is the RR interval in seconds (60 / heart rate). It is the most widely used formula in clinical practice, though it can overcorrect at very high or very low heart rates.
First, measure the QT interval from the ECG in milliseconds. Then calculate the RR interval by dividing 60 by the heart rate (in seconds). Finally, divide the QT interval by the square root of the RR interval. For example, a QT of 400 ms at 75 bpm gives an RR of 0.8 s and a QTc of 400 / √0.8 ≈ 447 ms.
A normal QTc is generally below 440 ms in men and below 460 ms in women. Values between 440–470 ms (men) or 460–480 ms (women) are considered borderline. A QTc above 500 ms is associated with a significantly increased risk of serious arrhythmias, including Torsades de Pointes.
QTc prolongation refers to an abnormally long corrected QT interval on the ECG, indicating delayed cardiac repolarization. It increases the risk of life-threatening arrhythmias such as Torsades de Pointes and sudden cardiac death. It can be caused by medications, electrolyte imbalances, congenital conditions, or underlying heart disease.
At a heart rate of 60 bpm, the RR interval is exactly 1.0 second. Using Bazett's formula, QTc = 360 / √1.0 = 360 ms. At a heart rate of exactly 60 bpm, the QTc equals the raw QT interval, since the RR interval normalizes to 1 second.
Yes. Fridericia's formula uses the cube root of RR (QTc = QT / ∛RR), which performs better at extreme heart rates. The Framingham formula (QTc = QT + 0.154 × (1 − RR)) and Hodges formula (QTc = QT + 1.75 × (HR − 60)) are also used. Bazett's formula remains the most common in clinical settings despite its limitations.
Many commonly prescribed medications can prolong the QT interval, including certain antibiotics (e.g., azithromycin, fluoroquinolones), antiarrhythmics (e.g., amiodarone, sotalol), antipsychotics (e.g., haloperidol, quetiapine), and antidepressants. Checking crediblemeds.org or similar resources before prescribing in patients with already prolonged QTc is strongly recommended.
Yes. Hypokalemia, hypomagnesemia, and hypocalcemia are all associated with QT prolongation. Correcting these electrolyte abnormalities is an important step in managing patients with prolonged QTc, especially before or alongside other treatments.