QTc Calculator
Correct the QT interval for heart rate. Enter the measured QT and the heart rate; results use both the Bazett and Fridericia formulas.
What is the QTc?
The QT interval on an ECG spans ventricular depolarisation and repolarisation — essentially, how long the ventricles take to fire and then reset. The catch is that the QT naturally shortens as the heart speeds up and lengthens as it slows. To compare one reading with another (or with a normal range), you correct the QT for heart rate, giving the corrected QT, or QTc.
Why bother? Because a long QTc is the warning sign for torsades de pointes, a dangerous polymorphic ventricular tachycardia. Getting the correction right can be the difference between reassurance and a red flag.
QTc formulas
All corrections use the R-R interval in seconds (RR = 60 ÷ heart rate). The two you will use most:
- Bazett: QTc = QT ÷ √RR. The classic, most widely reported formula — but it over-corrects at fast heart rates and under-corrects at slow ones.
- Fridericia: QTc = QT ÷ ∛RR (the cube root). More accurate at the extremes of heart rate, which is why many labs prefer it for tachycardia or bradycardia.
Two linear alternatives — the Framingham and Hodges corrections — are also used and tend to track Fridericia better than Bazett at rate extremes. The calculator above reports both Bazett and Fridericia so you can compare them side by side.
How to measure the QT interval
Measure from the start of the QRS complex to the end of the T wave, ideally in lead II or V5–V6. Use the tangent method — extend a line down the steepest part of the T wave's downslope to the baseline — and take the longest QT you can find. Avoid leads with prominent U waves, and don't include the U wave in the measurement.
Normal QTc values
| QTc | Interpretation |
|---|---|
| < 440 ms (men) / < 460 ms (women) | Normal |
| 440–470 ms (men) / 460–480 ms (women) | Borderline |
| > 470 ms (men) / > 480 ms (women) | Prolonged |
| > 500 ms | Markedly prolonged — high risk of torsades de pointes |
What prolongs the QT interval?
- Medications — many antiarrhythmics (sotalol, amiodarone), macrolide and fluoroquinolone antibiotics, several antipsychotics, methadone, and ondansetron, among others.
- Electrolyte disturbances — low potassium, low magnesium, and low calcium.
- Congenital long QT syndrome, bradycardia, myocardial ischaemia, and hypothermia.
Why it matters: torsades de pointes
A QTc over 500 ms markedly raises the risk of torsades de pointes. Management centres on intravenous magnesium, stopping any QT-prolonging drugs, and correcting electrolytes. For the full picture, read the QT interval guide and review rhythm recognition in the ECG interpretation guide.
References
- American Heart Association / American College of Cardiology scientific statements on QT measurement and drug-induced QT prolongation. AHA · ACC.
Educational reference only — not a substitute for clinical judgement.
Frequently asked questions
How do you calculate QTc?
Correct the measured QT for heart rate. Bazett's formula is QTc = QT ÷ √RR, where RR is the R-R interval in seconds (60 ÷ heart rate). Fridericia uses the cube root of RR.
What is a normal QTc?
Generally under 440 ms in men and under 460 ms in women. Values of 470 ms (men) or 480 ms (women) and above are prolonged, and over 500 ms markedly raises the risk of torsades de pointes.
What is the difference between Bazett and Fridericia?
Both correct QT for heart rate. Bazett (÷√RR) is the most reported but over-corrects at fast rates and under-corrects at slow ones; Fridericia (÷∛RR) is more accurate at the extremes.
How do you measure the QT interval?
Measure from the start of the QRS to the end of the T wave, preferably in lead II or V5–V6, using the tangent method, and take the longest QT. Don't include a U wave.
What QTc is dangerous?
A QTc over 500 ms is considered markedly prolonged and carries a high risk of torsades de pointes. The longer the QTc, the greater the risk.
What causes a prolonged QT interval?
Common causes include QT-prolonging drugs (some antiarrhythmics, macrolides, antipsychotics, methadone, ondansetron), low potassium, magnesium or calcium, congenital long QT syndrome, bradycardia, and ischaemia.
Which QTc formula should I use?
Bazett is the traditional default and fine at normal heart rates. At fast or slow rates, Fridericia (or the Framingham correction) is more reliable, so many clinicians report both.