QTc calculator: QT interval corrected by Bazett, Fridericia and Framingham
The QT interval shortens as heart rate rises. The heart-rate corrected QT interval (QTc) converts it to a heart rate of 60/min (RR interval 1 s). The calculator shows the three most common correction formulas side by side.
⚠︎ Learning tool for medical education – not for use in patients or for treatment decisions.
Formulas
RR interval in seconds = 60 ÷ heart rate. Bazett (1920): QTc = QT ÷ √RR. Fridericia (1920): QTc = QT ÷ ∛RR. Framingham (Sagie 1992): QTc = QT + 154 ms × (1 − RR). At a heart rate of 60/min (RR = 1 s) all three formulas return the measured QT interval.
Sagie et al. derived the linear Framingham formula from the ECGs of 5,018 participants of the Framingham study. According to their data, Bazett’s formula overcorrects at fast heart rates (QTc too long) and undercorrects at slow heart rates.
Thresholds: two sources
There is no single threshold for a “prolonged” QTc. The calculator classifies each value according to both sources:
| Source | Men | Women |
|---|---|---|
| Goldenberg 2006: normal | < 430 ms | < 450 ms |
| Goldenberg 2006: borderline | 430–450 ms | 450–470 ms |
| Goldenberg 2006: prolonged | > 450 ms | > 470 ms |
| Drew 2010 (AHA/ACCF): prolonged, above the 99th percentile | > 470 ms | > 480 ms |
The values of Goldenberg et al. (2006) apply to adults. Drew et al. (2010) consider a QTc above the 99th percentile of otherwise healthy postpubertal individuals to be prolonged (about 470 ms in men and 480 ms in women) and a QTc above 500 ms in either sex to be highly abnormal. They also point out that some ECG algorithms already label values above 440 ms as “borderline”, although 10–20% of the population exceed this value.
Limitations
The QT interval varies from beat to beat, particularly with the changing RR interval in atrial fibrillation. In bundle branch block the QT interval is lengthened by the wide QRS complex and cannot be compared with the usual thresholds. The formulas differ most at very fast or very slow heart rates.
FAQ
Which QTc formula is correct?▾
No formula works equally well at all heart rates. Bazett is the most widely used but overcorrects at fast heart rates. The linear Framingham formula corrects more reliably (Sagie 1992); Fridericia also corrects more appropriately at high heart rates (Drew 2010).
When is the QTc prolonged?▾
That depends on the source: according to Goldenberg 2006 above 450 ms in men and above 470 ms in women, according to Drew 2010 above about 470 ms and 480 ms respectively (99th percentile). The calculator shows both sources side by side.
References
- Bazett HC. An analysis of the time-relations of electrocardiograms. Heart 1920;7:353–70. Nachdruck: Ann Noninvasive Electrocardiol 1997;2:177–94 (nicht in PubMed gelistet). · DOI
- Fridericia LS. Die Systolendauer im Elektrokardiogramm bei normalen Menschen und bei Herzkranken. Acta Med Scand 1920;53:469–86. Englische Übersetzung: Ann Noninvasive Electrocardiol 2003;8:343–51. · PubMed
- Sagie A et al. An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study). Am J Cardiol 1992;70:797–801. · PubMed
- Goldenberg I, Moss AJ, Zareba W. QT interval: how to measure it and what is “normal”. J Cardiovasc Electrophysiol 2006;17:333–6. · PubMed
- Drew BJ et al. Prevention of torsade de pointes in hospital settings: a scientific statement from the American Heart Association and the American College of Cardiology Foundation. Circulation 2010;121:1047–60. · PubMed
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Open in browser Download on the App StoreIntended purpose: learning tool for medical education only. Not intended for use in patients or for diagnostic or treatment decisions; not a medical device. Calculations follow the cited original publications; no guarantee of completeness or correctness. Last updated: 10/2026 · Dr. Pascal Bafteh