QTc Calculator (Corrected QT Interval)
Enter a measured QT interval and heart rate to estimate QTc using Bazett, Fridericia, Framingham, and Hodges formulas.
What is QTc?
The QT interval on an ECG represents ventricular depolarization plus repolarization. Because QT duration naturally changes with heart rate, clinicians use a corrected QT (QTc) to compare values more consistently across patients and situations.
How this calculator qtc works
This tool calculates QTc from your measured QT and heart rate. Internally, it first estimates the RR interval using:
RR (seconds) = 60 / heart rate
Then it applies four common correction methods:
- Bazett: QTc = QT / √RR
- Fridericia: QTc = QT / RR1/3
- Framingham: QTc = QT + 154 × (1 − RR)
- Hodges: QTc = QT + 1.75 × (HR − 60)
All results are shown in milliseconds (ms).
How to interpret the output
General thresholds used in this page
- Male: normal < 430 ms, borderline 430–450 ms, prolonged > 450 ms
- Female: normal < 450 ms, borderline 450–470 ms, prolonged > 470 ms
- Both: QTc ≥ 500 ms is usually considered high risk for malignant arrhythmias such as torsades de pointes
Why multiple formulas matter
No single formula is perfect in every heart rate range. Bazett is very common, but it may over-correct at high heart rates and under-correct at low rates. Seeing multiple methods together gives a better clinical sense than relying on one number.
Practical tips for accurate QTc estimation
- Measure QT in leads with a clearly defined T-wave end.
- Avoid single-beat measurements when rhythm is irregular.
- Re-check values when heart rate is very high or very low.
- Review medications that may prolong QT (e.g., antiarrhythmics, certain antibiotics, antipsychotics).
- Consider electrolyte abnormalities (K, Mg, Ca) if QTc is prolonged.
Important clinical note
This calculator qtc is educational and supports screening, not diagnosis. Final decisions should be made by qualified clinicians using full ECG context, symptoms, medication list, and laboratory data.