Corrected QT Interval (QTc) Calculator

The QT interval on an ECG lengthens or shortens with heart rate on its own, independent of any underlying cardiac problem, so it must be "corrected" to a standardized rate of 60 bpm before it can be compared across patients or over time. This calculator computes that correction four ways from the same two inputs, measured QT Interval and Heart Rate. Bazett's formula (1920), QTc = QT / sqrt(RR), is the most widely used clinically despite being known to over-correct at heart rates outside the roughly 60-100 bpm range. Fridericia's formula, QTc = QT / RR^(1/3), uses a gentler cube-root exponent and is generally preferred at heart rate extremes.

Framingham's linear regression formula (Sagie et al., 1992), QTc = QT + 0.154 x (1 - RR), and Hodges' formula (1983), QTc = QT + 1.75 x (HR - 60), take a different, additive approach rather than dividing by a power of RR. All four ultimately move in the same direction as the two inputs: raising QT Interval raises every QTc value one-for-one in the underlying units, and -- because none of the four correction formulas is perfectly rate-independent -- raising Heart Rate also raises all four QTc values across this calculator's full 30-200 bpm range, with Bazett's correction climbing the most steeply of the four at high heart rates, the well-documented over-correction that is exactly why Fridericia or Hodges are often favored when heart rate is far from normal. This calculator's single Prolonged flag uses a general 460 ms Bazett cutoff rather than the sex-specific thresholds (over 450 ms in men, over 460 ms in women in the 2009 AHA/ACCF/HRS ECG recommendations; some drug-safety guidance uses 470 ms for women) since it takes no sex input -- treat it as a rough screen, not a diagnostic cutoff. A QTc over 500 ms is read as markedly prolonged: the 2010 AHA/ACCF statement on torsades de pointes reports a 2- to 3-fold higher risk above that line, with each further 10 ms adding roughly 5-7%.