EKG Interval Calculator

This calculator takes four raw EKG measurements and derives the values clinicians actually use for interpretation. The QT interval on its own is not clinically meaningful because it shortens and lengthens with heart rate, so it needs a rate-correction formula. Bazett's formula (QTc = QT / sqrt(RR in seconds), Bazett 1920) is the oldest and most widely used correction and is what most automated EKG machines report, but it is known to over-correct at fast heart rates and under-correct at slow ones. Fridericia's formula (QTc = QT / RR^(1/3), Fridericia 1920) uses a cube root instead of a square root and stays more accurate across a wider range of heart rates, which is why it's often preferred in research and drug-safety studies. Hodges' formula (QTc = QT + 1.75 x (HR - 60), Hodges et al.

1983) takes a different, purely additive approach rather than dividing by a function of the RR interval. Because the RR interval itself is derived directly from heart rate (RR = 60,000 / HR in milliseconds), a faster heart rate always shortens the raw RR interval, which is exactly why Bazett's square-root correction can overstate QTc at high heart rates -- it's dividing by a smaller number. This calculator also flags PR interval (normal 120-200 ms; below is a short-PR/pre-excitation pattern, above is first-degree AV block) and QRS duration (normal under 100 ms; 100-119 ms suggests incomplete bundle branch block, 120 ms or more suggests a complete bundle branch block or another cause of wide-complex conduction) using standard duration cutoffs, and grades the Bazett-corrected QTc against commonly cited thresholds: under 440 ms normal, 440-469 borderline, 470-499 prolonged, and 500 ms or more critically prolonged with meaningfully increased torsades de pointes risk. This calculator does not differentiate the QTc cutoffs by sex (clinical practice commonly uses a slightly higher normal ceiling for women) and does not replace a clinician's full 12-lead interpretation.