The QT interval on an ECG shortens and lengthens with heart rate even in perfectly healthy hearts, which makes the raw QT interval hard to compare across patients or across readings taken at different heart rates. QTc — the corrected QT interval — strips out that heart-rate dependence so a single number can be tracked over time and compared against a normal range. This guide explains why four different correction formulas exist, how to read the sex-specific cutoffs, and where QTc fits into the bigger clinical picture.

Why four formulas exist

Bazett's 1920 square-root formula was the first QT correction and remains the most widely cited in clinical practice and in most ECG machine software. Its weakness is well documented: because RR appears under a square root, the correction grows too aggressively as heart rate rises, systematically over-correcting QTc at tachycardic heart rates and under-correcting it at bradycardic ones.

Fridericia's cube-root formula, published the same year, corrects less aggressively at the extremes and is generally considered more accurate across a wider heart-rate range — many cardiology references now prefer it for exactly this reason. Framingham and Hodges take a different approach entirely, applying a linear adjustment rather than a root-based one; both tend to sit between Bazett and Fridericia at abnormal heart rates. No single formula is universally 'correct' — comparing more than one, especially outside a normal resting heart rate, gives a fuller picture than trusting any single number in isolation.

Reading the sex-specific cutoff

Women have a longer baseline QTc than men on average, so the standard cutoffs are sex-specific: a QTc is generally considered prolonged above 450 ms in men and above 460 ms in women, with borderline zones just below each threshold. Regardless of sex, a QTc of 500 ms or higher is treated as a high-risk value associated with a substantially increased risk of torsades de pointes and warrants prompt clinical attention.

These cutoffs are population-level reference points, not individualized diagnostic thresholds — a single reading should always be interpreted alongside the patient's baseline, trend over time, and clinical context.

QTc in clinical context

A prolonged QTc is a screening flag, not a diagnosis. Common reversible causes include QT-prolonging medications (a long list spanning antiarrhythmics, certain antipsychotics, some antibiotics, and antiemetics), and electrolyte disturbances such as hypokalemia, hypomagnesemia, and hypocalcemia. Congenital long QT syndrome — an inherited ion-channel disorder — can also present with a prolonged QTc, often first noticed incidentally on a routine ECG.

This calculator performs the arithmetic only. It does not check a patient's medication list, review electrolyte results, or diagnose long QT syndrome — a prolonged or high-risk QTc reading should always be correlated with the full clinical picture by a qualified clinician, not acted on in isolation.