QTc Calculator
Enter QT interval and either heart rate or RR interval. The calculator returns QTc using Bazett, Fridericia, Framingham, and Hodges formulas.
Educational use only. Always confirm ECG interpretation with a qualified clinician.
What is QTc and why calculate it?
The QT interval on an ECG reflects the time from ventricular depolarization to repolarization. Because QT shortens at higher heart rates and lengthens at lower rates, clinicians often use a corrected QT (QTc) to compare values across different heart rates.
Calculating QTc helps with risk assessment for arrhythmias, especially torsades de pointes. It is commonly reviewed when:
- Starting medications known to prolong QT
- Monitoring electrolyte disturbances
- Evaluating syncope, palpitations, or inherited channelopathies
- Reviewing ICU, telemetry, or perioperative ECGs
Common QTc formulas
No single correction method is perfect at all heart rates. Different formulas behave differently in tachycardia and bradycardia.
| Formula | Expression | Typical use notes |
|---|---|---|
| Bazett | QTc = QT / √RR | Most familiar and widely reported; may overcorrect at high HR and undercorrect at low HR. |
| Fridericia | QTc = QT / RR^(1/3) | Often more stable than Bazett, especially when HR is not near 60 bpm. |
| Framingham | QTc(ms) = QT(ms) + 154 × (1 − RR) | Linear correction; often used in epidemiologic and clinical research contexts. |
| Hodges | QTc(ms) = QT(ms) + 1.75 × (HR − 60) | Uses heart rate directly; useful for quick checks and comparative review. |
How to measure QT accurately before you calculate
1) Choose the best lead
Pick a lead where the T-wave end is clear and not distorted by baseline artifact. Leads II and V5 are common choices.
2) Identify the end of the T wave
Measure from the start of the QRS complex to the end of the T wave. Avoid including U waves unless local guidance specifically advises it.
3) Use representative beats
In sinus rhythm, averaging 3 to 5 similar beats can improve reliability. In irregular rhythms, values may vary beat-to-beat, so interpretation needs extra caution.
4) Match QT and RR from the same beat context
QT adaptation to heart rate can lag, especially after abrupt rate changes. A stable rhythm segment is best.
Interpreting the result
Reference cutoffs vary slightly by guideline, method, and patient population. A practical approach:
- Marked concern: QTc ≥ 500 ms (higher proarrhythmic risk)
- Male: prolonged usually considered > 450 ms
- Female: prolonged usually considered > 470 ms
Interpretation should always include symptoms, ECG context, medication profile, and electrolytes (especially potassium, magnesium, and calcium).
Frequent causes of prolonged QTc
- QT-prolonging drugs (antiarrhythmics, some antibiotics, antipsychotics, and others)
- Hypokalemia, hypomagnesemia, hypocalcemia
- Bradycardia
- Structural heart disease and ischemia
- Congenital long QT syndromes
- Drug interactions that increase medication levels
Practical workflow for clinicians and learners
- Measure QT and RR on a clean ECG segment.
- Calculate QTc using at least one method (ideally compare more than one).
- If QTc is near a threshold, recheck manually and repeat ECG if needed.
- Review medications and lab abnormalities promptly.
- Escalate urgent findings (especially QTc around or above 500 ms) according to local protocol.
Final takeaway
To calculate QTc well, accuracy starts with careful QT measurement, then thoughtful formula choice, and finally clinical interpretation. The calculator above gives multiple correction methods so you can compare outputs quickly and avoid over-reliance on a single number.