Last updated: 2026-06-28

QTc (Corrected QT Interval)

The corrected QT interval (QTc) is an estimate of how long ventricular repolarization would last if the heart were beating at a standard rate of 60 beats per minute. It takes the QT interval measured on an electrocardiogram and adjusts it for the patient's heart rate, so that a single set of thresholds can be used to judge whether repolarization is prolonged. Clinicians use the QTc to screen for drug effects, electrolyte disturbances, and inherited long-QT syndromes, all of which can raise the risk of a dangerous ventricular arrhythmia.

Why the QT interval is corrected for heart rate

The QT interval shortens as the heart speeds up and lengthens as it slows down. Because of this, a raw QT value cannot be compared against a fixed limit: a QT of 400 ms is unremarkable at 60 bpm but distinctly long at 100 bpm. Rate correction removes the heart-rate dependence by projecting the measured QT onto a reference rate of 60 bpm, where the RR interval (the time between two heartbeats) equals one second.

The correction always works from the RR interval, which is simply

with $RR$ in seconds and $HR$ in beats per minute.

The four correction formulas

No single formula is correct at every heart rate, so this calculator reports all four of the most widely used corrections. Writing $QT$ and $RR$ in seconds:

Bazett, published in 1920, is the oldest and most frequently reported. Its square-root term over-corrects when the heart is fast and under-corrects when it is slow, so it tends to exaggerate the QTc during tachycardia. Fridericia replaces the square root with a cube root, which tracks the true interval far more closely away from 60 bpm, and is the formula of choice in drug-safety studies. Framingham and Hodges are linear corrections derived from large population samples; they behave similarly to Fridericia across the usual range of rates.

Thresholds by sex

The upper limit of normal QTc is slightly higher for women than for men. The categories below follow the AHA/ACCF/HRS 2009 recommendations and are applied to whichever formula you select:

A QTc above 500 ms is the value most often cited as carrying a substantially increased risk of torsades de pointes, regardless of sex. Risk rises gradually rather than at a sharp line, and a rise of 60 ms or more above a person's own baseline is also considered significant.

Which formula to trust

Near a heart rate of 60 bpm all four formulas agree, because the RR interval is close to one second and the corrections collapse to the raw QT. The formulas diverge as the rate moves away from 60. At fast rates, Bazett reads noticeably higher than the others, which is why a Bazett-based reading can flag a falsely prolonged QTc in a tachycardic patient. When the heart rate is far from 60 bpm — or when the question is whether a medication is lengthening the QT — Fridericia or Framingham is generally the more reliable choice.

Worked example

Consider a QT of 360 ms measured at a heart rate of 120 bpm. The RR interval is $60 / 120 = 0.5$ s, so:

Bazett labels this reading markedly prolonged at 509 ms, while Fridericia places it at a normal 454 ms. The gap of more than 50 ms comes entirely from the choice of correction, and it illustrates why the formula should be matched to the heart rate before acting on the number.

Measuring the QT interval

The QT interval is measured from the start of the QRS complex to the end of the T wave, usually in lead II or V5. The end of the T wave is taken where the steepest part of its downslope meets the baseline. Because the interval varies slightly from beat to beat, several beats are averaged, and very fast or irregular rhythms make the measurement less dependable. The corrected value is only as good as the QT it starts from, so a careful measurement matters as much as the choice of formula.