QTc Calculator — Corrected QT Interval (Bazett, Fridericia, Framingham, Hodges)

The QT interval on an ECG shortens as the heart rate rises, so a raw QT cannot be compared against a fixed threshold. QTc is the QT “corrected” to an equivalent value at a heart rate of 60 beats per minute, which lets you judge whether repolarisation is truly prolonged. Every correction takes the measured QT (in milliseconds) and the RR interval, where RR (s) = 60 ÷ heart rate. This tool computes the four most cited formulas:

• Bazett: QTc = QT ÷ √RR
• Fridericia: QTc = QT ÷ ∛RR
• Framingham: QTc = QT + 154 × (1 − RR)
• Hodges: QTc = QT + 1.75 × (HR − 60)

At exactly 60 bpm the RR interval is 1 second, so every formula returns the QT unchanged — a built-in self-check. Away from 60 bpm they diverge. Bazett's square-root correction, derived from a small 1920 cohort, over-corrects at fast rates (inflating QTc) and under-corrects at slow rates. Fridericia's cube-root correction and the linear Framingham and Hodges corrections are less rate-dependent, which is why showing all four side by side matters: the spread between them can exceed 40 ms and push a value across a clinical threshold.

Interpretation uses the sex-specific thresholds from the AHA/ACCF/HRS 2009 (Rautaharju et al.) recommendations for standardising the ECG. In men, a QTc below 450 ms is normal, 450–470 ms is borderline, and above 470 ms is prolonged; in women the bands are 460, 460–480 and above 480 ms. A QTc under 350ms is flagged as abnormally short. The calculator also runs an independent cross-check of Bazett — computing it from heart rate as QT × √(HR ÷ 60) rather than from RR — and shows a “cross-checked” badge when the two paths agree.

These thresholds assume a correctly measured QT: the longest interval, typically in lead II or V5–V6, measured with the tangent method and with U waves excluded. No QTc formula is reliable in atrial fibrillation, in marked brady- or tachycardia, or in wide-QRS rhythms such as bundle branch block or paced beats, where a JT-based assessment is preferred. This is decision-support education — a clinician interprets the ECG.

• Rautaharju PM, et al. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the ECG, Part IV: the QT interval. Circulation 2009;119:e241–e250 (sex-specific QTc thresholds)
• Sagie A, Larson MG, Goldberg RJ, et al. An improved method for adjusting the QT interval for heart rate (Framingham Heart Study). Am J Cardiol 1992;70:797–801
• Postema PG, Wilde AAM. The measurement of the QT interval. Curr Cardiol Rev 2014;10:287–294 (measurement method, lead choice, U-wave handling)

The formulas (Bazett 1920, Fridericia 1920, Sagie/Framingham 1992, Hodges 1983) and the AHA/ACCF/HRS 2009 (Rautaharju et al.) interpretation bands on this page were last cross-checked against these sources on 2026-07-12. This tool is for education and reference only; it does not provide medical advice and does not replace a physician-interpreted ECG.