QTc Calculator

The QT interval on an ECG represents the time for ventricular depolarization and repolarization. Because QT naturally shortens at faster heart rates and lengthens at slower rates, it must be corrected for heart rate to allow meaningful comparison between patients. The corrected value is called QTc.

Bazett's formula is the most widely used in clinical practice and automated ECG machines, but it is known to over-correct at high heart rates and under-correct at low heart rates. Fridericia's formula (cube-root correction) is generally considered more accurate across a wider range of heart rates. For heart rates between 60–100 bpm, both formulas give similar results.

A normal QTc is generally considered to be below 440 ms in men and below 460 ms in women. Values between 440–470 ms (men) or 460–480 ms (women) are considered borderline. A QTc above 500 ms is associated with a significantly increased risk of torsades de pointes and ventricular arrhythmias.

Women naturally have slightly longer QT intervals than men, a difference that becomes more pronounced after puberty. As a result, clinical guidelines use a higher upper limit of normal for women (460 ms) compared to men (440 ms). This difference is thought to be related to hormonal influences on cardiac repolarization.

The RR interval (in milliseconds) is calculated as 60,000 divided by the heart rate in bpm. For example, a heart rate of 70 bpm gives an RR interval of approximately 857 ms. You can enter the RR interval directly from your ECG strip if you prefer, and this calculator will use that value instead of deriving it from heart rate.

A prolonged QTc indicates delayed ventricular repolarization, which can predispose a patient to dangerous arrhythmias such as torsades de pointes and ventricular fibrillation. Causes include congenital long QT syndrome, electrolyte abnormalities (low potassium, magnesium, or calcium), and many medications. Clinical context is always required for interpretation.

The QT interval is traditionally measured in lead II or in the precordial lead with the longest visible QT interval (often V5 or V6). Lead II is preferred because the P-QRS-T vector axis is predominantly directed along this lead, giving a clear baseline and T-wave end. Avoid leads with biphasic or inverted T-waves as the endpoint can be difficult to identify.

Yes. Many common medications are known to prolong the QTc interval, including certain antibiotics (e.g. azithromycin, fluoroquinolones), antipsychotics, antiarrhythmics, antihistamines, and antidepressants. Always check a drug interaction resource such as CredibleMeds when prescribing QT-prolonging drugs, especially in combination.