Corrected QT Interval (Bazett) Calculator
The corrected QT interval (QTc) adjusts the measured QT interval for the patient's heart rate, enabling identification of pathological QT prolongation independent of rate. The Bazett formula, published in 1920, is the most widely used correction: QTc = QT / sqrt(RR), where both QT and RR are in the same units (seconds or milliseconds). QTc prolongation is associated with risk of torsades de pointes, a form of polymorphic ventricular tachycardia. Many medications, electrolyte disorders, and congenital conditions can prolong QTc. Enter the measured QT and RR intervals in milliseconds; the RR interval can be derived from heart rate as 60,000 / HR in bpm. This tool is for educational reference only.
Bazett formula
QTc = QT / sqrt(RR / 1000)
(QT and RR in ms; divide RR by 1000 to get seconds for sqrt)
QT is measured from onset of Q wave to end of T wave. RR is measured from one R peak to the next. Bazett's formula normalizes QT to a heart rate of 60 bpm (RR = 1 second). It tends to over-correct at tachycardia and under-correct at bradycardia.
QTc interpretation (adults)
- Normal male: QTc below 440 ms.
- Normal female: QTc below 450 ms.
- Borderline: 440-470 ms (male), 450-480 ms (female).
- Prolonged: above 470 ms (male), above 480 ms (female).
- High risk of torsades de pointes: above 500 ms in either sex.
Frequently asked questions
What is the QTc interval?
QTc is the QT interval corrected for heart rate, because QT naturally shortens at faster heart rates. Correcting for rate allows comparison across different heart rates and identification of pathological prolongation.
What is the Bazett formula?
QTc = QT / sqrt(RR), where QT is the measured QT interval in seconds (or milliseconds) and RR is the R-R interval in seconds (or milliseconds). Both must be in the same units. Bazett's formula tends to over-correct at fast heart rates.
What QTc values are considered prolonged?
Generally, QTc above 450 ms in males and above 460 ms in females is considered borderline prolonged. QTc above 500 ms is associated with increased risk of torsades de pointes, a potentially life-threatening ventricular arrhythmia.
What causes QTc prolongation?
Causes include congenital long QT syndrome, medications (many antiarrhythmics, antipsychotics, and antibiotics), electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia), hypothyroidism, and structural heart disease.
How do I calculate RR interval from heart rate?
RR (in ms) = 60,000 / heart rate (bpm). For example, at 60 bpm, RR = 1,000 ms. At 75 bpm, RR = 800 ms. At 100 bpm, RR = 600 ms. The QT interval is measured from the start of the Q wave to the end of the T wave on ECG.
Official sources
- American Heart Association: Long QT Syndrome.
- NIH National Heart, Lung, and Blood Institute: Long QT Syndrome Overview.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.