By Zencalcy Editorial TeamLast reviewed About us
The QT interval shortens as heart rate rises, so raw QT values are hard to compare between people and between moments. Correcting for heart rate produces QTc, an estimate of what the QT would be at a standard rate. This calculator reports both the Bazett and Fridericia corrections because they behave differently across the heart-rate range.
Rate-corrected QT interval by Bazett and Fridericia.
Bazett: QTc = QT ÷ √(60 ÷ HR) Fridericia: QTc = QT ÷ ∛(60 ÷ HR)
Both formulas rescale QT to a heart rate of 60 bpm. Bazett over-corrects at high rates and under-corrects at low rates; Fridericia is less rate-dependent. A QTc above 450 ms in men or above 460 ms in women is commonly flagged as prolonged, but ECG interpretation depends on the lead, the rhythm and the patient's history.
| Symbol | Meaning |
|---|---|
| QT | Measured QT interval in milliseconds. |
| HR | Heart rate in beats per minute. |
| QTc | Rate-corrected QT interval in milliseconds. |
Bazett: 60 ÷ 80 = 0.75, √0.75 = 0.8660, so QTc = 400 ÷ 0.8660 = 461.9 ms. Fridericia: ∛0.75 = 0.9086, so QTc = 400 ÷ 0.9086 = 440.3 ms. For a man, the Bazett value is above the 450 ms threshold while the Fridericia value is not, which illustrates why both are shown.
The QT interval includes both depolarisation and repolarisation, and repolarisation speeds up as heart rate increases. A correction formula tries to remove that rate effect so values from different moments can be compared. Bazett divides by the square root of the RR interval ratio, which works reasonably near 60 bpm but exaggerates the correction when the rate is high. Fridericia uses a cube root and stays closer to measured values across a wider range. Neither formula accounts for individual repolarisation physiology, and both inherit measurement error from identifying the end of the T wave. That is why a borderline result is usually read with the whole ECG, the medication list and electrolyte results rather than as an isolated number.
Continue with the Mean Arterial Pressure Calculator, Pulse Pressure Calculator, or the CHA2DS2-VASc Calculator — or browse more cardiology tools and all health calculators.
Neither is perfect. Bazett is the most widely used but distorts at fast and slow rates. Fridericia is more stable across the rate range. Many clinicians look at both and at the clinical context rather than treating one formula as definitive.
Blood pressure, ECG and risk scores are screening aids, not diagnoses. A single reading can be misleading; trends and proper technique matter. Scores estimate population risk and do not replace clinical assessment, anticoagulation decisions or emergency care.
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Last reviewed: September 2026. See how we build and check every tool on the about page.
Content updated: September 2026