QTc Calculator (Bazett)

How it is calculated

You enter the measured QT interval in milliseconds and the heart rate in beats per minute. The tool first derives the RR interval in seconds as RR = 60 ÷ HR, then applies Bazett's correction QTc = QT ÷ √RR. The result is rounded to whole milliseconds. The heart rate field is the only way to give the rate, so an RR interval must be converted to bpm first. Heart rate must be greater than zero. At exactly 60 bpm, RR is 1 second and QTc equals QT. Bazett's formula over-corrects at fast heart rates and under-corrects at slow ones, so other formulas exist for those cases. The tool does not say whether the value is normal; thresholds differ by sex, age and clinical setting and should be judged by a clinician.

Common mistakes

• Entering the QT in seconds. The field is in milliseconds, so 0.4 would be read as 0.4 ms; enter 400.
• Typing the RR interval in the heart-rate field. A 0.8 second RR is 75 bpm, and entering 0.8 gives a meaningless answer.
• Using a heart rate taken at a different time from the QT measurement. Both should come from the same beats on the tracing.
• Trusting Bazett at very fast or very slow rates. At 110 bpm, a QT of 360 ms gives 487 ms, which can overstate the problem.
• Reading the QTc as a diagnosis. The number needs to be interpreted with the clinical picture, medications and electrolytes.