QTc Calculator

The standard uncorrected QT interval varies significantly with heart rate—shortening as the heart beats faster and lengthening as it slows down. To standardize this measurement, mathematical corrections are applied using the preceding RR interval (measured in seconds, calculated as RR = 60 / Heart Rate). This calculator evaluates four widely accepted formulas: Bazett (QTc = QT / sqrt(RR)), Fridericia (QTc = QT / cube_root(RR)), Framingham (QTc = QT + 0.154 x (1 - RR)), and Hodges (QTc = QT + 0.00175 x (HR - 60)). Each model approaches heart rate dependency differently to provide a reliable clinical index.

Consider a patient with a measured QT interval of 400 milliseconds (0.400 seconds) and a heart rate of 75 beats per minute. First, calculate the RR interval: RR = 60 / 75 = 0.80 seconds. Applying Bazett's formula, we take the square root of 0.80, which is approximately 0.8944. Dividing the QT interval by this value gives 0.400 / 0.8944 = 0.447 seconds, or a QTc of 447 milliseconds. Using alternative formulas like Fridericia or Framingham would yield slightly different adjusted values, allowing clinicians to cross-reference results for maximum diagnostic precision.

When measuring QT intervals manually from an ECG strip, always use lead II or V5, as these typically provide the clearest representation of the T-wave terminus. Be aware that Bazett's formula tends to overcorrect at high heart rates and undercorrect at low heart rates, making Fridericia or Framingham preferable in cases of marked tachycardia or bradycardia. Always verify automated ECG machine readings with manual calculations when patient safety or critical medication dosing is at stake.