QTc Calculator – Corrected QT Interval
Calculate the corrected QT interval (QTc) using Bazett's formula. Enter the measured QT interval and either the RR interval or heart rate to get an instant QTc value in seconds.
Correct the QT Interval for Heart Rate
Bazett’s Formula for QT Correction
Where:
- \(QT\) — measured QT interval (seconds), the time from the start of the Q wave to the end of the T wave
- \(RR\) — RR interval (seconds), the time between two consecutive R waves
- \(HR\) — heart rate (beats per minute)
Bazett's formula corrects the measured QT interval to a heart rate of 60 bpm, so that QT values measured at different heart rates can be compared against common reference thresholds. It is the most widely used QT correction in clinical practice.
QTc – Calculation Example
Example: an ECG shows a measured QT interval of 400 ms with an RR interval of 1000 ms.
\[ QT = 400\ \text{ms} = 0.400\ \text{s}, \quad RR = 1000\ \text{ms} = 1.000\ \text{s} \]
\[ QTc = \frac{0.400}{\sqrt{1.000}} = \frac{0.400}{1.000} = 0.400\ \text{s} \]
Result: \(QTc = 0.400\) s (400 ms) — within the normal range.
How to Use the QTc Calculator
- Enter the measured QT value in the “QT measured” field and select its unit: ms, sec, or mm (1 mm of standard ECG paper at 25 mm/s = 0.04 s).
- Choose the Parameter: either “RR interval” or “Heart rate”, depending on what you measured.
- Enter the Value — the RR interval (with its unit) or the heart rate in bpm — in the “Value” field.
- Read the Corrected QT result, displayed in seconds.
What Is the Corrected QT Interval and Why It Matters
The QT interval on an ECG represents the total time of ventricular depolarization and repolarization — from the beginning of the Q wave to the end of the T wave. Because the QT interval shortens as the heart beats faster and lengthens as it slows, a raw QT value cannot be interpreted on its own. Correction formulas, the most common being Bazett's formula published in 1920, adjust the measured QT to a standardized heart rate of 60 bpm, producing the corrected QT interval (QTc).
The QTc is a key screening measure for the risk of ventricular arrhythmias, particularly Torsades de Pointes, a polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation and sudden cardiac death. QTc prolongation may be congenital (long QT syndrome) or acquired — most often caused by medications such as certain antiarrhythmics, antibiotics, antipsychotics, and antiemetics, or by electrolyte disturbances like hypokalemia and hypomagnesemia.
General interpretation thresholds for Bazett-corrected QTc in adults: values below 440 ms in men and 460 ms in women are considered normal; values of 500 ms or more in either sex indicate pronounced prolongation with a substantially elevated arrhythmia risk and usually warrant urgent clinical attention. Very short QTc values (below about 340 ms) are also abnormal and may suggest short QT syndrome. Interpretation should always account for the patient's sex, age, medications, and the clinical context.
For context in broader cardiovascular assessment, you may also find our CHA₂DS₂-VASc score calculator, mean arterial pressure calculator, and target heart rate calculator useful.
Assumptions, Limitations and Practical Tips
- Bazett's limitations: the formula overcorrects (gives falsely long QTc) at heart rates above about 90–100 bpm and undercorrects at rates below about 60 bpm. At extreme heart rates, alternative corrections such as Fridericia (QT divided by the cube root of RR) or Framingham are more accurate.
- Measurement quality matters: the end of the T wave can be difficult to identify, especially with biphasic T waves, U waves, or artifact. If the T wave merges with a U wave, measure to the nadir between them and note this in the record.
- Which beats to use: in atrial fibrillation the RR intervals vary beat to beat; average the QT and RR over several (usually 3–5) consecutive beats.
- Units: keep units consistent. This calculator converts milliseconds, seconds, and millimeters (at standard 25 mm/s paper speed) to seconds automatically; if you enter heart rate, the RR interval is derived as \(60/HR\) in seconds.
- Clinical judgment: a single QTc value is a screening tool, not a diagnosis. Borderline or ambiguous results, especially in patients on QT-prolonging drugs, should be reviewed by a clinician and confirmed with a formal 12-lead ECG.
Frequently Asked Questions
How do I calculate QTc by hand without a calculator?
Take the measured QT interval in seconds and divide it by the square root of the RR interval in seconds. To find the RR interval yourself, divide 60 by the heart rate in bpm. For example, with QT = 0.38 s at 100 bpm: RR = 60/100 = 0.6 s, and \(QTc = 0.38/\sqrt{0.6} \approx 0.49\) s (about 490 ms). A quick bedside trick: at 60 bpm the square root is 1, so the QT equals the QTc.
What is a normal QTc value for men and women?
Using Bazett's formula, a QTc below 440 ms is normal in men and below 460 ms in women. Values of 440–460 ms in men or 460–480 ms in women are borderline. A QTc of 500 ms or more in either sex is markedly prolonged and carries a higher risk of dangerous arrhythmias. Women normally have slightly longer QTc values than men, partly due to hormonal effects on repolarization.
How do I measure the RR interval on ECG paper?
On standard ECG paper running at 25 mm/s, each small square is 0.04 s (40 ms) and each large square is 0.20 s. Count the small squares between two consecutive R waves and multiply by 0.04. For example, 25 small squares = 1.0 s, which corresponds to 60 bpm. If the rate is irregular, measure several RR intervals and use their average.
Is Bazett's formula accurate at high or low heart rates?
No — this is its main weakness. Bazett overcorrects at heart rates above ~90–100 bpm, producing falsely long QTc values, and undercorrects below ~60 bpm. At tachycardia or bradycardia, Fridericia's formula \((QT/\sqrt[3]{RR})\) or the Framingham correction are more reliable. If your patient's heart rate is far from 60–90 bpm, consider recalculating with one of these alternatives before making clinical decisions.
Why does the calculator ask for heart rate when the formula needs RR?
Bazett's formula requires the RR interval, but many ECG machines report heart rate instead. The calculator converts automatically using \(RR = 60/HR\) in seconds. For instance, 75 bpm gives RR = 60/75 = 0.8 s. This works because the heart rate in bpm and the RR interval in seconds are exact reciprocals of each other (up to the 60 seconds-per-minute factor).
Can I use this QTc calculator for children or during pregnancy?
Use with caution in both groups. Children have age-specific normal ranges, and Bazett's formula performs poorly at the high heart rates typical of infants and young children — pediatric cardiology typically uses other corrections. In pregnancy, heart rate rises and QTc normally increases slightly, but a prolonged QTc still needs evaluation. Always confirm borderline results in children or pregnant patients with a cardiologist.
How do I measure QT if the T wave merges with a U wave?
This is a common source of error. If a U wave is separate, measure the QT to the end of the T wave only, ignoring the U wave. If the T and U waves are fused, measure to the nadir (trough) between them and document this, because the true QT is likely somewhere between the two readings. Also avoid measuring in leads with biphasic or flattened T waves — choose the lead with the clearest T-wave termination, often lead II or V5.