Heart & circulation
QTc calculator (corrected QT interval)
Enter the QT interval and heart rate printed on an ECG report to get the corrected QT interval (QTc) by four formulas: Fridericia, Bazett, Framingham and Hodges. The result is compared with the normal QTc limits for men and women. This tool works from a number someone has already measured; it cannot measure QT from a tracing.
Enter your values above to see your result and what it means.
Normal QTc for men and women
The QT interval is the time from the start of ventricular activation to the end of recovery on an ECG. Because it shortens as the heart speeds up, it is corrected for heart rate to give the QTc. A QTc above the 99th percentile counts as abnormally prolonged: about 470 ms for men and 480 ms for women. Above 500 ms is highly abnormal for both. This calculator grades the Fridericia QTc against those limits. It does not grade short QT intervals, so an unusually short value is something to ask a clinician about.
| Category | Men | Women |
|---|---|---|
| Normal | 470 or less | 480 or less |
| Prolonged | 471 to 500 | 481 to 500 |
| Very prolonged | over 500 | over 500 |
QT vs QTc and how QTc is calculated
Each formula uses the QT and the RR interval, the time between beats (RR in seconds = 60 divided by heart rate). Bazett divides QT by the square root of RR; Fridericia divides by the cube root, which corrects less aggressively. Take a QT of 400 ms at 90 bpm: Bazett gives 490 ms, above the normal limit for both sexes, while Fridericia gives 458 ms, within it. In a study of 6,609 ECGs, Fridericia and Framingham corrected for heart rate best and Bazett worst, which is why Fridericia is the headline here.
| Formula | Equation (QT and RR in seconds) | Example result |
|---|---|---|
| Bazett | QT / √RR | 490 ms |
| Fridericia | QT / ∛RR | 458 ms |
| Framingham | QT + 0.154 × (1 - RR) | 451 ms |
| Hodges | QT + 1.75 × (HR - 60), in ms | 452 ms |
What a prolonged QTc means
A long QTc matters because it raises the risk of torsade de pointes, a fast, irregular rhythm that can cause fainting or cardiac arrest. Risk rises with a QTc above 500 ms, with a rise of 60 ms or more after a medicine is started, and with factors such as older age, female sex, heart disease, low potassium or magnesium, and a slow heart rate. Many people check their QTc because they take a medicine that can lengthen it. That decision belongs to the prescriber, who weighs the benefit of the medicine against the ECG.
Limits of this calculator
The result is only as good as the QT you enter. QT is hard to measure by hand, automated readings depend on the lead and the algorithm, and the end of the T wave can be unclear. Use the QT and heart rate from the same ECG. At heart rates below 50 or above 120 bpm all formulas are less reliable. The calculator cannot diagnose long QT syndrome or replace a clinician reading the full tracing.
Frequently asked questions
What is a normal QTc?
For adults, a QTc up to about 470 ms in men and 480 ms in women is within normal limits, based on the 99th percentile of healthy people. Above 500 ms is highly abnormal for both sexes. Some ECG machines label values above 440 ms as borderline, which 10% to 20% of people exceed.
What is the difference between QT and QTc?
QT is the measured interval on the ECG. QTc is the same interval corrected to what it would be at a heart rate of 60 bpm, so readings taken at different heart rates can be compared. At exactly 60 bpm all four formulas return the QT unchanged.
Why does my ECG report show a different QTc?
Most ECG machines use Bazett, which reads high at faster heart rates, especially above 85 bpm. This calculator leads with Fridericia, which corrects for heart rate more accurately. At 90 bpm a QT of 400 ms gives 490 ms by Bazett and 458 ms by Fridericia.
When does a prolonged QTc need urgent attention?
A QTc above 500 ms, or a rise of 60 ms or more from your baseline after starting a medicine, needs prompt clinical review. Fainting, palpitations or a seizure need emergency care. Do not stop a prescribed medicine on your own; talk to the prescriber first.
Can this calculator measure my QT interval?
No. It takes a QT value that has already been measured on an ECG and corrects it for heart rate. Measuring QT requires the ECG tracing itself.
Sources
- Drew BJ, Ackerman MJ, Funk M, et al. Prevention of torsade de pointes in hospital settings: a scientific statement from the American Heart Association and the American College of Cardiology Foundation. Circulation. 2010;121(8):1047-1060.
- Vandenberk B, Vandael E, Robyns T, et al. Which QT correction formulae to use for QT monitoring? J Am Heart Assoc. 2016;5(6):e003264.
- Sagie A, Larson MG, Goldberg RJ, Bengtson JR, Levy D. An improved method for adjusting the QT interval for heart rate (the Framingham Heart Study). Am J Cardiol. 1992;70(7):797-801.