Corrected QT Interval (QTc) Calculator
The QT interval on an electrocardiogram changes with heart rate: it shortens when the heart beats faster. To compare QT across patients and over time, it is corrected to a standard rate of 60 beats per minute, giving the corrected QT (QTc). This calculator takes a measured QT interval and a heart rate (or RR interval) and returns the QTc using four published correction formulas so you can see how they differ. It is an arithmetic aid only and is not a substitute for clinical assessment.
QTc correction formulas
RR (seconds) = 60 / heart rate
Bazett: QTc = QT / sqrt(RR)
Fridericia: QTc = QT / RR^(1/3)
Framingham: QTc = QT + 154 * (1 - RR)
Hodges: QTc = QT + 1.75 * (heart rate - 60)
QT and QTc are in milliseconds, RR is in seconds, and heart rate is in beats per minute. Each formula corrects the measured QT to the value expected at 60 beats per minute (RR = 1 second). When RR equals 1 second, all four formulas return the original QT.
Worked example
For a measured QT of 400 ms at a heart rate of 75 bpm, RR = 60 / 75 = 0.80 seconds. Bazett gives 400 / sqrt(0.80) = 400 / 0.8944 = 447.21 ms. Fridericia gives 400 / 0.80^(1/3) = 400 / 0.9283 = 430.89 ms. Framingham gives 400 + 154 * (1 - 0.80) = 430.80 ms. Hodges gives 400 + 1.75 * (75 - 60) = 426.25 ms. The spread shows why the chosen formula matters near reference limits.
QTc interval: frequently asked questions
What is the QTc interval?
The QTc is the QT interval on an ECG corrected for heart rate. The raw QT interval shortens as heart rate rises, so correction formulas express it as the value it would have at a standardised rate of 60 beats per minute (an RR interval of 1 second). This makes QT comparable across different heart rates.
Which QTc formula should I use?
Bazett (QTc = QT / sqrt(RR)) is the historical standard and the most widely reported, but it over-corrects at fast rates and under-corrects at slow rates. Fridericia (QT / cube root of RR) performs better at extremes. This calculator shows Bazett, Fridericia, Framingham, and Hodges so you can compare. The number must be interpreted by a clinician alongside the full ECG.
What is a normal QTc value?
Reference values are not produced by this calculator and are not built in. Published guidance commonly cites upper limits around 450 ms for adult men and 460 ms for adult women, with values above roughly 500 ms considered high risk, but thresholds vary by source, sex, and clinical context. Always interpret against your own institution's reference ranges.
Is this tool a substitute for medical advice?
No. This is an arithmetic tool that applies published correction formulas to numbers you enter. It does not diagnose, does not read an ECG, and does not replace assessment by a qualified clinician. Do not use it for clinical decisions.
Sources
- Bazett HC, original heart-rate correction, summarised by the U.S. National Library of Medicine: QT Interval, StatPearls (NCBI Bookshelf).
- U.S. National Center for Biotechnology Information: PubMed Central for the Fridericia, Framingham, and Hodges correction methods.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. Educational tool, not medical advice. See our methodology.