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QTc Calculator (Bazett, Fridericia and more)

Correct a measured QT interval for heart rate with the Bazett, Fridericia, Framingham and Hodges formulas, side by side, from a heart rate or an R-R interval.

Formula

Bazett:QTc=QTRR\text{Bazett:}\quad \mathrm{QTc} = \dfrac{\mathrm{QT}}{\sqrt{\mathrm{RR}}}
Fridericia:QTc=QTRR1/3\text{Fridericia:}\quad \mathrm{QTc} = \dfrac{\mathrm{QT}}{\mathrm{RR}^{1/3}}
Framingham:QTc=QT+154 (1−RR)\text{Framingham:}\quad \mathrm{QTc} = \mathrm{QT} + 154\,(1 - \mathrm{RR})
Hodges:QTc=QT+1.75 (HR−60)\text{Hodges:}\quad \mathrm{QTc} = \mathrm{QT} + 1.75\,(\mathrm{HR} - 60)
QT\mathrm{QT}
the measured QT interval, in ms
RR\mathrm{RR}
the R-R interval in seconds (60 / heart rate)
HR\mathrm{HR}
heart rate, in beats per minute

How it works

The QT interval shortens as the heart rate rises, so a QT measured at 100 beats/min cannot be compared directly with one at 60. A correction formula converts it to the value it would have at a standard rate. At 60 beats/min the R-R interval is 1 s and every formula here returns the measured QT unchanged.

The formulas differ most at fast and slow rates. Bazett's square-root correction is the oldest and most widely used, but it over-corrects at high rates and under-corrects at low ones; Fridericia's cube-root and the linear Framingham and Hodges formulas are generally preferred when the rate is far from 60. Corrected values are comparable only if calculated the same way.

Worked example

QT = 400 ms at 75 beats/min (RR = 0.8 s).

  1. Bazett: 400 / √0.8 = 447 ms.
  2. Fridericia: 400 / 0.8^(1/3) = 431 ms.
  3. Framingham: 400 + 154 × 0.2 = 431 ms.
  4. Hodges: 400 + 1.75 × 15 = 426 ms.

The QTc is 426 to 447 ms depending on the formula, which shows why the method must be stated with the value.

These are the values the calculator opens with, so you can check its output against this example.

Assumptions

  • The QT was measured accurately, in the same lead and by the same method as any value it is compared with.
  • A regular rhythm; the formulas are not validated for atrial fibrillation without averaging.

Common mistakes

  • Comparing a Bazett value with a Fridericia value, or with a threshold defined for another formula.
  • Entering the R-R interval in milliseconds where seconds are expected. The tool accepts either, with the basis selected.
  • Applying a formula at extreme heart rates without checking it is valid there.

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Service documentation, failure modes and parts for the instruments this calculation is used with.