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Corrected QT (QTc) Calculator

Correct the QT interval for heart rate with Bazett

Result

424.9 ms
0.80 s

How it is calculated

Enter the measured QT interval in milliseconds and the heart rate in beats per minute. The tool first finds the RR interval, RR = 60 ÷ HR (seconds between beats), then applies Bazett's correction, QTc = QT ÷ √RR. The corrected value is displayed in ms with one decimal, and the RR interval in seconds with two.

Both inputs must be above zero. At exactly 60 bpm RR equals 1 s, so QTc equals the measured QT. Bazett's formula tends to over-correct at fast heart rates and under-correct at slow ones, which is why other formulas exist; this tool only uses Bazett. It is a calculation aid and does not interpret the number or replace a clinician's reading of the ECG.

Worked example

A patient has a measured QT of 380 ms at 75 beats per minute. RR = 60 ÷ 75 = 0.80 s. The square root of 0.80 is about 0.894. Then QTc = 380 ÷ 0.894 = 424.9 ms. The corrected value is higher than the raw QT because the heart is beating faster than 60 bpm, which shortens the QT.

Try this example →

Common mistakes

  • Entering QT in seconds (0.38). The field expects milliseconds, so the result would be meaningless.
  • Using the RR interval in the heart rate box. The field wants beats per minute.
  • Applying Bazett at very fast or slow heart rates and trusting it fully; it distorts the value at those extremes.
  • Reading the QT from a single beat when the rhythm is irregular, as in atrial fibrillation, where an average over several beats is normally used.
  • Comparing the corrected figure with limits that were set for a different formula.

Sources

About this calculator

This calculator applies Bazett's formula, QTc = QT / √RR, where QT is the measured QT interval (in milliseconds) and RR is the interval between heartbeats in seconds (60 divided by heart rate in bpm), so the corrected value also comes out in milliseconds. Because the QT interval naturally shortens as the heart beats faster, Bazett's square-root correction normalizes it to what it would be at a heart rate of 60 bpm, making QT values comparable across different heart rates.

Cardiologists and ECG technicians use QTc to screen for long QT syndrome and to monitor patients on QT-prolonging medications (certain antibiotics, antipsychotics and antiarrhythmics), since a QTc above roughly 450–470 ms raises the risk of the dangerous arrhythmia torsades de pointes.

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  • Editorial team

    The same measured QT gives a higher corrected value as the heart rate rises: 360 ms at 60 beats per minute stays 360.0 ms, but 360 ms at 100 beats per minute becomes 464.8 ms, and the tool also shows the RR interval it used, 0.60 s. So always note the rate together with the QT. Bazett is only one of several correction formulas; tell us if you would like another one added. Reading a QTc belongs with a clinician.

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