QTc Calculator (Bazett)

How it is calculated

The tool first converts the heart rate into the RR interval, the time between two heartbeats in seconds: RR = 60 ÷ HR. It then applies Bazett's correction: QTc = QT ÷ √RR. QT is the measured QT interval in milliseconds, HR is the heart rate in beats per minute, and the corrected value is returned in milliseconds with two decimals. At exactly 60 beats per minute RR is 1 second, so QTc equals the measured QT. Both values must be above zero. The result is only as good as the measurements: QT depends on where the end of the T wave is read, and the heart rate should be the one at the moment of that ECG beat. Bazett's formula tends to overcorrect at fast heart rates, which is why other formulas exist. The tool gives a number, not a diagnosis; interpretation belongs to a clinician.

Common mistakes

• Entering the QT in seconds (0.4) instead of milliseconds (400). The result is then given in ms but is 1000 times too small.
• Entering the RR interval, in ms or seconds, in the heart rate field. The tool needs beats per minute and calculates RR itself.
• Using the heart rate from the monitor at another time rather than the rate during the beats where QT was measured.
• Applying Bazett to a very fast or slow heart rate and treating it as exact. The formula overcorrects as the rate rises, so a borderline value at high HR may look longer than it is.
• Mixing up QT and QTc when comparing with reference limits. Limits are given for the corrected interval.