What Is a QTc Interval (Bazett) Calculator?
The QT interval on an ECG represents the time it takes the heart's ventricles to depolarize and repolarize. Because that time naturally changes with heart rate, clinicians correct it to a standardized value called QTc, so it can be compared against normal ranges regardless of how fast or slow the heart is beating. Bazett's formula is the most widely used method for that correction.
This calculator takes your measured QT interval and heart rate and returns QTc along with a normal/borderline/prolonged classification.
How to Read Your Results
QTc (Bazett)
The corrected QT interval in milliseconds — the value typically compared against reference ranges and tracked over time or on medication.
RR Interval
The time between consecutive heartbeats, derived from heart rate (RR = 60 ÷ HR), used in the correction formula.
Category & Normal Upper Limit
General reference ranges: normal QTc is under about 440 ms for men and 460 ms for women; QTc over 500 ms is widely considered high risk for dangerous ventricular arrhythmias such as torsades de pointes.
Real-World Example
A patient has a measured QT interval of 400 ms and a heart rate of 100 bpm (faster than typical resting rate).
| Value | Result |
|---|---|
| RR interval (60 ÷ 100) | 0.60 s |
| QT interval | 0.400 s |
| QTc (0.400 ÷ √0.60) | ≈ 516 ms |
Even though the raw QT of 400 ms looks unremarkable, correcting for the fast heart rate reveals a markedly prolonged QTc — this is exactly the over-correction at high heart rates that clinicians must keep in mind with Bazett's formula.
Tips for Using QTc
- Bazett's formula over-corrects at fast heart rates and under-corrects at slow rates — consider Fridericia's formula as a cross-check at extreme heart rates.
- Recheck QTc after starting antiarrhythmics, certain antibiotics, antipsychotics, or antiemetics known to prolong QT.
- Correct for bundle branch block or paced rhythms separately — Bazett's formula is designed for normal QRS morphology.
- Trend QTc over time rather than relying on a single measurement.
Related Calculators
Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team