QTc calculator (Bazett & Fridericia)
The QT interval on an ECG shortens as the heart beats faster and lengthens as it slows, so the raw number can't be read on its own. The corrected QT (QTc) adjusts the measured QT to a standard heart rate of 60 bpm so you can compare it against thresholds and track it over time. This calculator gives both the Bazett QTc (the value most ECG machines print) and the Fridericia QTc, which is usually the more reliable of the two.
How it works
You supply two numbers: the measured QT interval in milliseconds and the heart rate in beats per minute (from which the RR interval in seconds is just 60 divided by the heart rate). Bazett's formula divides the QT by the square root of the RR interval; Fridericia's divides it by the cube root. At a heart rate of exactly 60 bpm the RR interval is 1 second, so both formulas leave the QT unchanged and QTc equals QT. Away from 60 bpm they diverge: Bazett over-corrects when the heart is fast (it inflates the QTc and can flag a normal tracing as prolonged) and under-corrects when the heart is slow. Because of that, Fridericia — and the linear formulas the AHA/ACC/HRS favor, such as Framingham and Hodges — tracks the true rate-independent value better outside the roughly 60-100 bpm range, which is why we show it alongside Bazett. For interpretation, a QTc under about 450 ms in men and 460 ms in women is normal; 450/460 up to roughly 470 (men) or 480 (women) is borderline; higher than that is prolonged. A QTc above 500 ms signals a markedly increased risk of the dangerous arrhythmia torsades de pointes regardless of sex.
Worked example
Say the ECG measures a QT of 420 ms at a heart rate of 100 bpm. The RR interval is 60 / 100 = 0.6 seconds. Bazett: 420 / sqrt(0.6) = 420 / 0.7746 = 542 ms, which would land in the "markedly prolonged, high torsades risk" zone. Fridericia: 420 / cbrt(0.6) = 420 / 0.8434 = 498 ms, just under the 500 ms alarm line. The two differ by about 44 ms at this fast rate — a real-world example of Bazett over-correcting when the heart is fast, and the reason the raw Bazett number alone should not drive a decision at rates far from 60 bpm.
Important
Educational estimate only — not medical advice, a diagnosis, or a substitute for a clinician interpreting the actual ECG tracing. The result is only as reliable as its inputs: a mismeasured QT, a wrong heart rate, or QT read from a single lead all yield a misleading QTc. Every correction formula loses accuracy at very high or very low heart rates, and Bazett in particular over-corrects when the heart is fast (it can flag a normal tracing as prolonged) and under-corrects when it is slow — so a high Bazett value alone, especially far from 60 bpm, should not drive any decision; compare it against Fridericia or a linear formula. Correction is unreliable in atrial fibrillation, bundle branch block, or a wide QRS, where a manual QT/QTc adjustment is required. The thresholds shown are general adult reference values; they are NOT validated for children, and normal ranges shift with sex and with low potassium, magnesium, or calcium and QT-prolonging medications. A QTc in the borderline or prolonged range — and especially any QTc above 500 ms, or a rise of 60 ms or more from a prior baseline — needs prompt evaluation by a qualified clinician; do not start, stop, or change any medication on your own based on this number. If you have fainting, palpitations, seizures, or chest symptoms, seek emergency care now.