QTc Formula Calculator
Enter QT interval and heart rate to calculate corrected QT (QTc) using the most common formulas.
Educational tool only. Clinical interpretation should be done by a qualified medical professional.
What is QTc and why does it matter?
The QT interval on an electrocardiogram (ECG) represents the total time it takes for the ventricles of the heart to depolarize and repolarize. Because QT changes with heart rate, clinicians often use a corrected value called QTc, which adjusts QT so values can be compared more consistently across different heart rates.
A prolonged QTc can be associated with increased risk of ventricular arrhythmias, including torsades de pointes. A very short QTc can also be clinically relevant in specific contexts. That is why a reliable QTc formula calculator can be useful for quick checks in educational, research, and bedside-support settings.
QTc formulas used in this calculator
This page computes four widely used correction formulas:
- Bazett: QTc = QT / √RR
- Fridericia: QTc = QT / RR^(1/3)
- Framingham: QTc = QT + 154 × (1 − RR)
- Hodges: QTc = QT + 1.75 × (HR − 60)
In this tool, QT is entered in milliseconds, HR in beats per minute, and RR is derived in seconds from RR = 60 / HR.
Which formula should you trust most?
No single correction formula is perfect at all heart rates. Bazett is historically common but can overcorrect at high heart rates and undercorrect at low heart rates. Fridericia often performs better across broader ranges. Framingham and Hodges are also frequently used in clinical studies and can provide useful comparison values.
Practical takeaway: compare multiple formulas and always interpret results in full clinical context (symptoms, medications, electrolytes, congenital syndromes, and ECG quality).
How to use this formula qtc calculator
- Enter the measured QT interval in milliseconds.
- Enter heart rate in beats per minute.
- Optionally choose sex for a simple reference interpretation.
- Click Calculate QTc.
- Review all formula outputs and the interpretation note.
Example calculation
Suppose QT = 420 ms and HR = 80 bpm. RR = 60/80 = 0.75 seconds. You will generally see:
- Bazett ≈ 485 ms
- Fridericia ≈ 463 ms
- Framingham ≈ 459 ms
- Hodges ≈ 455 ms
Notice how the output differs by formula. This is normal and exactly why side-by-side comparison is helpful.
Typical adult reference ranges (general guide)
Reference limits vary by source, age, and clinical setting. A common quick framework:
- Men: QTc > 450 ms may be prolonged
- Women: QTc > 460 ms may be prolonged
- Any adult: QTc ≥ 500 ms is often considered high risk
These are screening-level cutoffs, not a diagnosis. Borderline values need careful interpretation.
Common causes of prolonged QTc
Medication-related
Many drugs can lengthen ventricular repolarization, including certain antiarrhythmics, antibiotics, antipsychotics, antidepressants, and antiemetics. Drug interactions can amplify risk.
Electrolyte and metabolic factors
Low potassium, low magnesium, low calcium, thyroid disease, and severe systemic illness may affect QTc.
Inherited conditions
Congenital long QT syndromes involve ion channel abnormalities and often require specialist management.
Limitations of any online QTc calculator
- Input quality matters: QT measurement can vary with ECG lead selection and baseline noise.
- Formula assumptions break down at very high or very low heart rates.
- Clinical risk is never based on QTc alone.
- This tool does not replace physician judgment, ECG over-read, or emergency care.
Final note
Use this formula qtc calculator as a fast educational and comparison tool. If you are evaluating a real patient, especially with syncope, palpitations, medication changes, or QTc near/above 500 ms, seek formal clinical evaluation right away.