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QTc Calculator
The QT interval on an ECG lengthens as the heart rate slows and shortens as it speeds up, so it is "corrected" to what it would be at 60 beats per minute — the QTc. A prolonged QTc matters because it is associated with a higher risk of a specific ventricular arrhythmia (torsades de pointes) and is affected by many medicines and electrolyte problems.
Enter the values or tick the options above — the result updates automatically.
How to interpret the result
Frequently quoted upper limits are about 450 ms in men and 470 ms in women, and a QTc above 500 ms is generally treated as markedly prolonged. Treat any single number with care:
— QT measurement varies by lead and by observer, and a U wave can be mistaken for the end of the T wave.
— Bazett's formula over-corrects at high heart rates, so a "long" QTc in a tachycardic patient should be checked against Fridericia.
— Low potassium, low magnesium, and low calcium prolong the QT, as do many medicines — the Drug Guide cautions flag several.
Related calculators
Frequently asked questions
What is the difference between Bazett and Fridericia?
Both correct the QT for heart rate. Bazett divides by the square root of the RR interval and is the most widely used, but it over-corrects when the heart is fast and under-corrects when slow. Fridericia uses the cube root and behaves better at rate extremes.
What QTc is considered prolonged?
Commonly used upper limits are about 450 ms for men and 470 ms for women. Values above 500 ms are considered markedly prolonged and carry the greatest concern for torsades de pointes.
Why can two QTc values for the same ECG differ?
Because the QT interval itself is measured differently — different leads, manual versus machine measurement, and whether a U wave is included all change the input number, and each formula then corrects it differently.