From the start of the QRS complex to the end of the T wave.
The RR interval in seconds is 60 divided by this.
QT / √RR. What most ECG machines print.
Prolonged above 450 ms.
The equation, and where it comes from
Bazett: QTc = QT / √RR, where RR in seconds = 60 / heart rate
Bazett HC. An analysis of the time-relations of electrocardiograms. Heart. 1920;7:353-370. Fridericia LS. Acta Med Scand. 1920;53:469-486. Thresholds per the AHA/ACCF/HRS 2009 recommendations.
How to read the result
A QTc above 450 ms in men or 460 ms in women is generally called prolonged, and above 500 ms is the threshold most often quoted for materially raised torsades risk. These are population thresholds, not a judgement about an individual tracing.
Your patient's numbers never leave this device
The qtc interval arithmetic runs in JavaScript inside your own browser. There is no server call when you type, nothing is written to a database, nothing is logged, and no account is involved. Once the page has loaded you can disconnect from the network and it still works.
That matters more here than on an ordinary calculator. Values entered on a hospital workstation come off a real chart, and this tool deliberately does not remember them: close the tab and they are gone, including for the next person who uses that machine.
Frequently asked questions
- Why does the QT interval need correcting for heart rate at all?
- The QT interval shortens as the heart speeds up, so a raw QT of 400 ms means something quite different at 50 bpm than at 110 bpm. Rate correction expresses the interval as what it would be at 60 bpm, which is what makes two tracings comparable.
- Bazett or Fridericia, which should I use?
- Bazett is the one nearly every ECG machine prints, so it is the one your colleagues will expect. Its weakness is well described: it over-corrects at fast rates and under-corrects at slow ones, because the square root is too aggressive. Fridericia's cube root behaves better outside roughly 60 to 100 bpm, and this calculator warns you when Bazett is being used there.
- What counts as a prolonged QTc?
- Conventionally above 450 ms in men and 460 ms in women, using the AHA/ACCF/HRS 2009 thresholds. The sex difference is real rather than arbitrary: adult women have a slightly longer repolarisation time. This tool applies whichever threshold matches the sex you select.
- How do I measure the QT interval in the first place?
- From the beginning of the QRS complex to the end of the T wave, conventionally in the lead where the T wave ends latest and most clearly, often lead II or V5. Where a U wave follows, the tangent method on the T wave downslope is the usual way to decide where T ends. This calculator takes the interval you measured; it does not read a tracing.
- Can I use this with atrial fibrillation?
- With caution. Rate correction assumes a regular rhythm and a stable RR interval, neither of which holds in AF. The usual workaround is to average the QT and the RR over several beats, and to treat the resulting number as a rough guide rather than a precise figure.