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Corrected QT (Fridericia) Calculator

Enter the measured QT interval in ms and the heart rate to get the Fridericia-corrected QTc.

How it is calculated

The tool corrects a measured QT interval for heart rate with Fridericia's cube-root rule. First it converts the heart rate (beats per minute) into the beat-to-beat interval in seconds, RR = 60 ÷ HR. Then QTcF = QT ÷ RR^(1/3), where QT is the measured interval in milliseconds, so the corrected value is also in milliseconds. It is rounded to a whole number. Both QT and heart rate must be above zero, otherwise nothing is shown. At exactly 60 beats per minute RR is 1 second, so the corrected value equals the measured one. Above 60 the divisor is below 1 and the corrected QT is longer than the raw one; below 60 it is shorter. The tool only does this arithmetic: it does not interpret the number or say whether it is normal, and it is no substitute for a clinician's reading of the ECG.

Worked example

An ECG shows a QT of 400 ms at a heart rate of 75 beats per minute. Step 1: the RR interval is 60 ÷ 75 = 0.8 seconds. Step 2: the cube root of 0.8 is about 0.9283. Step 3: 400 ÷ 0.9283. The corrected interval is 431 ms. The heart is faster than 60 beats per minute, so the corrected value is longer than the raw 400 ms.

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Common mistakes

  • Entering the QT in seconds or the RR interval instead of the heart rate. QT is in milliseconds (400, not 0.4; 0.4 would give 0 ms), and the second field is beats per minute.
  • Using the heart rate from a different moment than the QT measurement. The correction is only meaningful when both come from the same beat or the same stretch of the tracing.
  • Comparing a Fridericia value with a limit that was set for Bazett's formula. The two formulas give different numbers, especially away from 60 beats per minute, so use the same method as the reference.
  • Expecting no change at a fast pulse. At 120 beats per minute a raw QT of 350 ms corrects to 441 ms, so large differences between raw and corrected values are normal at fast or slow rates.
  • Treating the result as a diagnosis. It is one number from a formula, and the ECG still has to be read as a whole.

Sources

About this calculator

The QT interval on an ECG shortens as heart rate rises and lengthens as it slows, so raw QT can't be compared across patients or heart rates without correction. Fridericia's formula corrects for this using a cube root: QTcF = QT ÷ ∛(RR), where RR is the interval between heartbeats in seconds — the same time base used to derive heart rate.

Fridericia's cube-root exponent is a milder correction than Bazett's square-root formula (QTcB = QT ÷ √RR), and clinical studies show it is more accurate at both fast (tachycardic) and slow (bradycardic) heart rates, where Bazett's formula overcorrects and undercorrects respectively. Cardiologists and QT-monitoring protocols, especially in drug-safety trials evaluating torsades de pointes risk, often prefer Fridericia's correction for this reason.

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  • Editorial team

    Different correction formulas give different numbers from the same ECG, so always note which one a value used. For a QT of 400 ms at 80 beats per minute, Fridericia gives 440 ms here, whereas the Bazett formula would give about 462 ms. Comparing a Fridericia value against a limit written for Bazett is a common source of confusion. This tool only does the arithmetic and cannot interpret the result. Which correction formula does your source use?

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