QTc Calculator

By Kacper Pawlik
Last updated: Mar 12, 2021

With our QTc calculator, you will be able to perform a correction of QT interval for your patient’s heart rate. In the further text, you will learn how to calculate QTc using Bazett’s formula and other equations (e.g. Fridericia, Framingham). We also explain what is a normal QT interval, what are the risks of QT prolongation and which QT prolonging drugs you should evade in the treatment of patients with a prolonged QT interval.

Remember that using this calculator is by no means equivalent to a consultation with a specialist. If the result you obtained is troubling you, make sure to visit your physician!

Are you interested in cardiology? We have many other calculators that you may want to see. Please, try stroke volume calculator and MAP calculator.

QT interval

QT interval is a part of an electrocardiographic representation of an electrical function of a heart. It starts at the beginning of QRS complex (caused by depolarization of ventricles) and ends at the end of T wave (the repolarization of ventricles).

Schematic representation of normal ECG trace, with waves, segments, and intervals labeled

QT interval is often measured by cardiologists since its prolongation correlates with a prevalence of ventricular arrhythmias such as torsades de pointes and is a risk factor of sudden cardiac death. Duration of QT interval is highly influenced by heart rate: the faster the rhythm, the shorter QT interval is. Because of that, a correction of QT interval for heart rate should be made. It can be performed with, e.g. a Bazett’s formula.

Bazett’s formula

The most common equation used for QT interval correction all around the world is a formula presented by physiologist Henry Cuthbert Bazett in the article “An Analysis of the Time-Relations of Electrocardiograms” published in 1920 in the Heart Journal. Bazett’s formula looks like this:

QTc = QT / √RR,


QTc = corrected QT,

QT = duration of QT interval,

RR = duration of RR interval.

Although very popular, Bazett’s formula has its limitations – it is known for overcorrecting at high heart rates and undercorrecting at low heart rates.

Several other equations have been proposed:

  • A formula by Louis Sigurd Fridericia:

    QTc = QT / ∛RR

    This method is also thought to give more consistent results at faster heart rates.

  • The Framingham correction based on the Framingham Heart Study:

    QTc = QT + 0.154(1-RR)

  • An equation by M Hodges:

    QTc = QT + 1.75(Heart_Rate - 60)

  • And by Pentti M Rautaharju:

    QTc = QT - 185 * ((60 / Heart_Rate) - 1) + sex_coefficient

    Sex coefficient equals 6 ms for man and 0 ms for woman.

To see the QTc calculated with these formulas, click on the advanced mode button.

In a recent study published in the Journal of the American Heart Association authors compared previously listed methods of QT correction and concluded that Framingham and Fridericia equations provided the best rate correction and mortality prediction.

How to calculate QTc?

To get your patient’s QTc with our QTc calculator you need to:

  1. Determine the length of the QT interval:
  • Find the beginning of Q wave (and QRS complex at the same time).
  • Localize the end of T wave.
  • Measure the distance between these two points on X-axis. You can use a ruler or a caliper.
  • Transform the length of QT interval from millimeters or boxes to milliseconds. With a paper speed of 25 mm/s one small box (1 mm) lasts 0.04 s, one big box (5 mm) – 0.2 s.
  • Type the result in the QT interval field of our QTc calculator.
  1. In the heart rate section, type the number of beats per minute of your patient’s heart rate. Alternatively, you can measure the RR interval proceeding already evaluated QT interval – use the same technique as in determination of QT interval length but start at the peak of one R wave and assess the distance to the peak of the next R wave (it should be a part of the QRS complex mentioned in the first point of this list).
  2. Read the result from QTc (Bazett) field. This is the corrected QT calculated with Bazett’s formula by our calculator! Now, you only need to verify whether it is normal QTc for your patient's sex. You can also click on the advanced mode button to discover the results of other correcting QT interval formulas.

It is often assumed, that a normal QTc lasts (after correction) under 430 ms for an adult male and 450 ms for an adult female. We can diagnose QT prolongation with QTc of over 450 ms for the adult male and 470 ms for the adult female.

Do you want to find an easy way to get your patient’s heart rate from his ECG? Check our ECG heart rate calculator.

QT prolongation

QT prolongation increases the risk of a premature action potential developing during the late phase of depolarization – before the repolarization is fully completed. This can lead to severe arrhythmias:

  • torsades de pointes - a polymorphic ventricular tachyarrhythmia, with characteristic morphology of QRS complexes changing their amplitude and twisting around the isoelectric line on ECG,
  • ventricular fibrillation- a state of complete disorganization of the electrical activity of the heart with a loss of its pumping function.

Usually, cardiomyocytes manage to repolarize entirely before the next wave of depolarization reaches them through the electrical conduction system of the heart and other cardiomyocytes. However, when repolarization is happening longer than physiologically (as in QT prolongation), or a premature depolarization develops, the depolarization wave may reach cardiomyocytes before they can fully repolarize. Such an event may cause serious electrical disturbances resulting in previously mentioned tachyarrhythmias.

Many factors may create QT prolongation, including:

  • genetic disorders – an inherited Long QT Syndrome, for example in Romano-Ward syndrome or Jervell and Lange-Nielsen syndrome,
  • QT prolonging drugs
  • low blood potassium – hypokalemia,
  • low blood calcium– hypocalcemia,
  • low blood magnesium - hypomagnesemia,
  • heart diseases,
  • low heart rate – bradycardia,
  • central nervous system pathologies,
  • low body temperature - hypothermia,
  • insufficient function of the thyroid gland – hypothyroidism.

You may want to check a longer list of clinical factors associated with prolonged QTc.

Did you measure your patient’s blood calcium or magnesium? In some situations, the results have to be corrected – see our corrected calcium calculator and corrected magnesium calculator.

QT prolonging drugs

There are many drugs with prolongation of QT as one of their side effects. It can be a result of treatment with anti-arrhythmic drugs, such as amiodarone and sotalol, antibiotics like macrolides (erythromycin) and quinolones (levofloxacin), antihistamines, a number of pharmaceuticals used in psychiatry and many others. A list of QT prolonging drugs can be found under this link.

Before making a decision of including a QT prolonging drug in a patient’s therapy, a risk-benefit analysis should be performed, especially, when there are other risk factors present.

You already know how to calculate QTc. See how many other medical equations you can solve with us! For example, check out our pediatric GFR calculator!

Kacper Pawlik
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