About the QTc calculator
The QT interval measured on an ECG is not fixed - it naturally shortens when the heart beats faster and lengthens when it beats slower. Because of that, a raw QT reading cannot be compared meaningfully between two people, or even between two readings on the same person, unless their heart rates match. QTc (the rate-corrected QT interval) applies a standard formula to remove that heart-rate dependence, producing a number that better reflects the heart's underlying repolarization time.
The four standard correction formulas
- Bazett: QTc = QT / sqrt(RR), where RR is the RR interval in seconds (RR = 60 / heart rate). The oldest and most widely used formula; it tends to over-correct at fast heart rates and under-correct at slow ones.
- Fridericia: QTc = QT / RR^(1/3). Uses a cube root instead of a square root and is generally more accurate than Bazett across a wide range of heart rates.
- Framingham: QTc = QT + 154 x (1 - RR). A linear correction derived from the Framingham Heart Study.
- Hodges: QTc = QT + 1.75 x (heart rate - 60). A linear correction expressed directly in terms of heart rate rather than RR interval.
All four formulas take a QT interval in milliseconds and a heart rate in beats per minute, and all four agree exactly when heart rate is 60 bpm (RR = 1 second), since no correction is needed at that reference rate.
Putting the result in context
Commonly cited reference bands classify QTc as normal up to roughly 430 ms in men and 450 ms in women, borderline up to about 450 ms in men and 470 ms in women, and prolonged above those levels. A QTc at or above 500 ms is generally flagged as high risk for dangerous arrhythmias such as torsades de pointes, regardless of sex. These are population-based bands, not a diagnosis, and a single calculated value does not capture everything a clinician looks at on an ECG.
When to consult a professional
This tool performs the standard QTc arithmetic for education and reference. It is not a diagnosis and should not be used to start, stop, or adjust any medication. QT-prolonging drugs, electrolyte abnormalities, and inherited long-QT syndromes can all affect QTc, so any borderline or prolonged result - or any result paired with symptoms such as fainting or palpitations - should be reviewed with a licensed healthcare provider.