QTc Calculator (Corrected QT Interval)
Correct a measured QT interval for heart rate with the Bazett, Fridericia, Framingham and Hodges formulas, and flag a prolonged QTc.
How it works
RR = 60 ÷ heart rate (s) · Bazett: QTc = QT ÷ √RR · Fridericia: QT ÷ ∛RR · Framingham: QT + 154 × (1 − RR) · Hodges: QT + 1.75 × (HR − 60)
The QT interval on an ECG shortens as heart rate rises, so it is corrected to a standard 60 bpm — the QTc — to judge whether ventricular repolarisation is truly prolonged. Bazett’s square-root correction is the historical default but over-corrects at fast rates and under-corrects at slow ones; Fridericia’s cube-root and the linear Framingham and Hodges formulas behave better across the range. A QTc above roughly 450 ms in men or 460–470 ms in women raises concern for a long-QT state and torsades risk, which is why several corrections are shown side by side.
Worked example
With a QT of 400 ms at 75 bpm, RR = 60 ÷ 75 = 0.80 s. Bazett gives 400 ÷ √0.80 = 447 ms, Fridericia 400 ÷ ∛0.80 = 431 ms, and Framingham 400 + 154 × (1 − 0.80) = 431 ms — all just under the 450 ms male threshold.
Sources: QT interval corrections
Frequently asked questions
Which QTc formula is best?
Bazett is the most widely reported but is inaccurate at heart rates far from 60 bpm. Fridericia and Framingham are generally preferred at fast or slow rates. Showing several lets you see whether the conclusion is robust.
What QTc is considered prolonged?
Commonly, a QTc above about 450 ms in men and 460–470 ms in women is called prolonged, with values over 500 ms carrying a markedly higher risk of dangerous arrhythmias. Thresholds vary between guidelines.
Why does heart rate matter for QT?
The QT interval naturally shortens as the heart beats faster and lengthens when it slows. Correcting to a reference rate removes that dependence so QT can be compared between readings and people.
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Educational estimate using standard published formulas — not medical advice, a diagnosis, or a substitute for clinical judgement. Dosing, diagnosis and treatment must be decided by a qualified clinician using validated tools and the full clinical picture.