Bazett QTc Calculator
The QT interval on an electrocardiogram measures the time from the start of ventricular depolarisation to the end of repolarisation, but it varies with heart rate. To compare measurements taken at different rates, the QT is corrected to a standard rate using a correction formula. Bazett's formula, the most widely used, divides the measured QT by the square root of the RR interval to produce the corrected QT, or QTc. This calculator takes a measured QT in milliseconds and a heart rate in beats per minute, derives the RR interval, and returns the Bazett QTc along with the RR interval used. It is an educational arithmetic tool and does not provide a diagnosis.
Bazett QTc formula
RR interval (s) = 60 / heart rate (bpm)
QTc (s) = QT (s) / square root of RR (s)
In milliseconds: QTc (ms) = QT (ms) / square root of RR (s)
Worked example: QT = 400 ms and heart rate = 60 bpm gives RR = 60 / 60 = 1.00 s, so QTc = 400 / square root of 1.00 = 400 ms. At 75 bpm the same QT gives RR = 0.80 s and QTc = 400 / 0.894 = 447.21 ms.
QT correction context
- Bazett's correction normalises the QT to a heart rate of 60 bpm, where the RR interval is exactly 1 second.
- At a rate of 60 bpm the corrected QTc equals the measured QT, because the square root of 1 is 1.
- Bazett tends to overcorrect at fast heart rates and undercorrect at slow heart rates compared with newer formulas.
- QT and RR must be measured in the same units before correction; this tool converts the heart rate to RR for you.
- This is an educational arithmetic tool only and is not a substitute for clinical interpretation by a qualified clinician.
Bazett QTc: frequently asked questions
What is Bazett's formula for QTc?
Bazett's formula is QTc = QT / square root of RR, where QT is the measured QT interval and RR is the R-R interval, both in seconds. When the inputs are in milliseconds, the corrected interval is QTc(ms) = QT(ms) / square root of RR(seconds). The RR interval in seconds equals 60 divided by the heart rate in beats per minute.
What is a normal QTc value?
Reference ranges are not produced by this calculator and depend on the patient and clinical context. As a general guide often cited in cardiology references, a QTc up to roughly 450 ms in men and 460 ms in women is commonly regarded as within normal limits, with prolongation defined above those thresholds. Always interpret QTc against the values used by your own institution and clinician.
Why does QT need to be corrected for heart rate?
The raw QT interval shortens as heart rate rises and lengthens as it falls. Correcting for heart rate produces QTc, an estimate of what the QT would be at a standard rate of 60 beats per minute (an RR interval of 1 second), so intervals measured at different rates can be compared.
What are the limits of Bazett's formula?
Bazett's correction is the most widely used but is known to overcorrect at high heart rates and undercorrect at low heart rates. Alternative corrections (Fridericia, Framingham, Hodges) behave differently across the rate range. This tool implements Bazett's formula exactly; use the correction your clinical setting specifies.
Can I enter the RR interval instead of heart rate?
Yes. Enter the heart rate in beats per minute and the calculator derives the RR interval as 60 divided by heart rate. The RR interval used in the correction is shown in the outputs so you can confirm it against your ECG measurement.
Official sources
- National Center for Biotechnology Information, NCBI Bookshelf (StatPearls): QT interval reference.
- American Heart Association journals: Circulation and related ECG standards literature.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.