QTc Calculator
The QT interval shortens as the heart beats faster, so it is corrected to a standard heart rate before it is judged. Enter the measured QT and the heart rate from the same ECG.
| Result | Interpretation |
|---|---|
| Below 390 ms | Short (390 ms or less) |
| 390 ms to 450 ms | Normal |
| 450 ms to 460 ms | Prolonged in men, normal in women |
| 460 ms to 500 ms | Prolonged |
| 500 ms and above | Markedly prolonged (above 500 ms) |
Source: 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. (doi:10.1016/0002-9149(92)90562-d); Rautaharju PM, Surawicz B, Gettes LS, et al. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part IV: the ST segment, T and U waves, and the QT interval. Circulation 2009;119(10):e241-e250. (doi:10.1161/CIRCULATIONAHA.108.191096); 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. (doi:10.1161/CIRCULATIONAHA.109.192704)
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QTc formulas
Each formula uses the RR interval in seconds, which is 60 divided by the heart rate.
- Framingham (linear): QTc = QT + 154 × (1 − RR)
- Bazett: QTc = QT ÷ √RR
- Fridericia: QTc = QT ÷ ∛RR
The American Heart Association recommends linear correction formulas, such as Framingham, rather than Bazett, which is why Framingham is the main result here. Bazett is the traditional formula, so it is shown too.
Worked example
QT 380 ms at a heart rate of 90 (RR = 0.667 s):
- Framingham: 380 + 154 × (1 − 0.667) = 431 ms
- Bazett: 380 ÷ √0.667 = 465 ms
- Fridericia: 380 ÷ ∛0.667 = 435 ms
Why the formula matters at fast heart rates
The same measured QT of 400 ms, corrected at different heart rates:
| Heart rate | Framingham (ms) | Bazett (ms) | Fridericia (ms) |
|---|---|---|---|
| 50 | 369 | 365 | 376 |
| 60 | 400 | 400 | 400 |
| 75 | 431 | 447 | 431 |
| 90 | 451 | 490 | 458 |
| 100 | 462 | 516 | 474 |
| 120 | 477 | 566 | 504 |
At 60 beats per minute all three agree. As the rate rises, Bazett climbs fastest and can label a normal QT as prolonged. The AHA notes that Bazett “may produce false QT prolongations”, and a later AHA/ACCF statement says it gives overlong values at fast rates, particularly above 85 beats per minute.
What counts as prolonged
| QTc | Meaning (AHA/ACCF/HRS 2009) |
|---|---|
| 390 ms or less | Short QT |
| 391 to 449 ms | Normal |
| 450 to 459 ms | Prolonged in men, normal in women |
| 460 to 500 ms | Prolonged |
| Above 500 ms | Highly abnormal; 2 to 3 times the risk of torsade de pointes |
There is no QTc at which a dangerous rhythm is certain to occur. Clinicians also act on a rise of 60 ms or more from a person’s own baseline, for example after starting a new medicine.
When not to correct
The AHA advises against correcting the QT when the heart rhythm is very irregular, as in atrial fibrillation, or when the end of the T wave cannot be identified reliably. In those cases the numbers from any formula can mislead.
FAQs
What is a normal QTc?
Above 390 ms and below 450 ms in men, or below 460 ms in women, under the AHA/ACCF/HRS cut-offs.
Which QTc formula is best?
The AHA recommends linear formulas such as Framingham over Bazett. Bazett is accurate near 60 beats per minute but overcorrects at faster rates.
What QTc is dangerous?
A QTc above 500 ms is considered highly abnormal and is linked to a 2 to 3 times higher risk of torsade de pointes, a dangerous heart rhythm. That needs prompt medical attention, especially alongside medicines that prolong the QT interval.