QTc Calculator

Background.

A QT interval of 360 milliseconds means one thing at a heart rate of 60 and something quite different at 100. The QT, measured on an electrocardiogram from the start of the QRS complex to the end of the T wave, spans the time the heart's ventricles take to activate electrically and then recover. Recovery speeds up as the heart speeds up, so the raw QT shortens at fast rates and lengthens at slow ones. To compare one tracing with another, or with a reference limit, clinicians convert it to a corrected QT, the QTc: an estimate of what the QT would be at 60 beats per minute.

There is no single agreed way to do that conversion. Henry Bazett in England and Louis Sigurd Fridericia in Denmark each proposed one in 1920. The Framingham Heart Study group under Sagie published a linear correction in 1992, and Hodges and colleagues a heart-rate-based one in 1983. All four agree exactly at 60 beats per minute and drift apart as the rate moves away from it. At 75 beats per minute a QT of 360 ms becomes 402.5 ms by Bazett but 387.8 ms by Fridericia; at 100 beats per minute the gap between those two widens to almost 38 ms.

That spread is why the calculator always works out all four and labels each one. Many ECG machines print a Bazett QTc by default, while drug-safety studies and many clinicians prefer Fridericia or one of the linear formulas, particularly at fast heart rates, where Bazett is known to overcorrect. You can choose which formula fills the 'Selected primary QTc' line, and the other three remain on screen for comparison.

The arithmetic is only as sound as the QT you enter. Measuring where the T wave ends is a judgement, and U waves, a wide QRS, pacing and irregular rhythms all make it harder. A QTc worked out here is a check on a reading, to be interpreted alongside the tracing, symptoms, medicines and electrolytes; it supports clinical judgement and does not replace it.

What is qtc calculator?

The QT interval is the time from the beginning of ventricular depolarisation, the start of the QRS complex, to the end of repolarisation, the end of the T wave, usually measured in milliseconds in a lead where the T wave ends clearly, often lead II or V5. The RR interval is the time between successive heartbeats; with a regular rhythm it equals 60 divided by the heart rate in beats per minute, so 75 beats per minute gives 0.8 seconds. A corrected QT expresses the measured QT as if the RR interval were 1 second, the RR at 60 beats per minute. Bazett and Fridericia do this by dividing by a power of RR, the square root and the cube root respectively; Framingham and Hodges add or subtract a fixed amount per unit of RR or heart rate. The QTc is a derived figure, not something read directly off the tracing, which is why the formula used should always be stated with it.

How to use this calculator.

1. 01Enter the measured QT in milliseconds, between 200 and 800. On standard ECG paper running at 25 mm/s, each small square is 40 ms and each large square 200 ms; take the QT from a lead where the end of the T wave is clear.
2. 02Enter the heart rate in beats per minute. The calculator converts it to an RR interval of 60 ÷ rate seconds.
3. 03If you measured the RR interval directly from the beats preceding the QT you measured, enter it in seconds in the optional RR field; four large squares at 25 mm/s is 0.8 seconds. Any value above 0 replaces the heart rate in all four formulas, including Hodges, which then uses 60 ÷ RR as its rate. Leave it at 0 to use the heart rate.
4. 04Choose the formula for the 'Selected primary QTc' line, or leave it on All formulas, which places Bazett there. The four individual results are shown whichever you choose.
5. 05Compare the results with the QTc printed on the ECG, and have any long or unexpected value reviewed by a clinician with the tracing in hand.

The formula.

First RR = 60 ÷ HR seconds, unless a measured RR is supplied. Then, with QT in milliseconds and RR in seconds: Bazett QTc = QT ÷ √RR. Fridericia QTc = QT ÷ ∛RR. Framingham QTc = QT + 154 × (1 − RR). Hodges QTc = QT + 1.75 × (HR − 60). At RR = 1 second, a heart rate of 60, every formula returns the QT unchanged. Away from 60 they diverge because they model the QT-to-rate relationship differently. Bazett's square root makes the correction steep: at fast rates RR is small, its square root is smaller still relative to the cube root, and dividing by it inflates the QTc, so Bazett tends to overcorrect in tachycardia and undercorrect in bradycardia. Fridericia's cube root is gentler. Framingham adds 154 ms for every second the RR falls short of 1 second, a straight line fitted to Framingham Heart Study participants. Hodges adds 1.75 ms for every beat per minute above 60. None of the formulas fully removes the effect of rate in every person: the QT adapts to a change in rate over a minute or more rather than instantly, and individuals differ in how steeply their QT follows their RR. In the example the RR is 0.8 s, so √0.8 = 0.8944 and ∛0.8 = 0.9283, which is why Bazett gives the highest value of the four.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Sagie, A., Larson, M.G., Goldberg, R.J., Bengtson, J.R., and Levy, D. (1992). "An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study)." American Journal of Cardiology 70(7):797-801.
2. [2]Luo, S., Michler, K., Johnston, P., and Macfarlane, P.W. (2004). "A comparison of commonly used QT correction formulae: the effect of heart rate on the QTc of normal ECGs." Journal of Electrocardiology 37 Suppl:81-90.
3. [3]Malik, M. et al. (2015). "Nomenclature, categorization and usage of formulae to adjust QT interval for heart rate." Journal of Electrocardiology.
4. [4]Cuthbertson, J. et al. (2020). "A Retrospective Analysis of Hospital Electrocardiogram Auto-Populated QT Interval Calculation." Cureus.
5. [5]American Psychiatric Association (2018). "Resource Document on QTc Prolongation and Psychotropic Medications." APA.
6. [6]Academy of Consultation-Liaison Psychiatry (2024). "How-To Guide: QTc Prolongation." ACLP.