QTc calculator
Enter QT interval and heart rate – the calculator shows the rate-corrected QTc with Fridericia, Bazett, Framingham and Hodges and classifies it using the usual limits.
Corrected QTc
For illustration only, not a diagnosis. Limits as in the app: men 450 / 470 / 500 ms, women 460 / 480 / 500 ms.
Why and how QT is corrected
The QT interval depends strongly on heart rate: the faster the heart beats, the shorter it gets. To make measurements at different rates comparable, QT is converted to a rate of 60 beats per minute (RR = 1 s) – this is the QTc. RR is the time between two heartbeats in seconds (60 / heart rate).
QTc = QT / RR1/3The standard – robust across a wide heart-rate range.QTc = QT / RR1/2The classic formula, the most widely used in clinics.QTc = QT + 0.154 · (1 − RR)Linear correction from the Framingham Heart Study.QTc = QT + 1.75 · (HR − 60)Linear correction based on heart rate.Fridericia is considered the standard today because it stays stable over a wide range of heart rates. Bazett is the most widely used but over-corrects at high heart rates and under-corrects at low ones.
Worked example
Measured: QT = 380 ms at a heart rate of 75 bpm. The RR interval is 60 / 75 = 0.8 s.
Fridericia: 380 / 0.81/3 = 409 msBazett: 380 / 0.81/2 = 425 msFramingham: 380 + 154 · (1 − 0.8) = 411 msHodges: 380 + 1.75 · (75 − 60) = 406 ms
QTc limits
| Classification | Men | Women |
|---|---|---|
| Normal | < 450 ms | < 460 ms |
| Borderline | 450–469 ms | 460–479 ms |
| Prolonged | 470–499 ms | 480–499 ms |
| Markedly prolonged | ≥ 500 ms | ≥ 500 ms |
In addition, a rise of more than 60 ms over baseline is considered a warning sign. Limits according to AHA/ACCF/HRS (2009) and Drew et al. (2010).
Frequently asked questions about QTc
What is the difference between QT and QTc?
QT is the measured time from the start of the QRS complex to the end of the T wave. QTc is the same interval, mathematically corrected to a heart rate of 60 bpm – only this makes measurements at different heart rates comparable.
Which QTc formula is best?
For everyday use Fridericia is a good standard because it is stable over a wide range of heart rates; the ICH E14 guideline also prefers it. Bazett is the best known but gives values that are too high at high heart rates and too low at low ones.
When is QTc prolonged?
Commonly, values from 450 ms in men and 460 ms in women are borderline, and from 470 or 480 ms prolonged. At 500 ms or more, or with a rise of more than 60 ms, the risk of torsades de pointes increases markedly.
How accurate is the calculator?
The calculator applies the formulas exactly. Accuracy depends on the measured QT interval, though – finding the end of the T wave is the hardest step. The result does not replace a physician's interpretation.
Can I measure my QTc with an Apple Watch?
The Apple Watch records a single-lead ECG but does not show the QT interval. QTc Tracker measures QT and QTc automatically from these ECGs in Apple Health and shows the trend.
QTc automatically from your ECGs
QTc Tracker measures QT itself and calculates all four formulas – for every ECG in Apple Health.