QTc calculator (Bazett & Fridericia)
Use this QTc calculator to correct the QT interval for heart rate using the Bazett or Fridericia formulas from the same ECG tracing.
Last reviewed: February 26, 2026
Clinical disclaimer
Use the QT interval from the same ECG as the heart rate or RR interval.
Bazett is commonly used but can over‑correct at high heart rates.
QTc formula comparison
| Formula | Expression | Best fit | Main caution |
|---|---|---|---|
| Bazett | QTc = QT / sqrt(RR) | Fast bedside use and the most familiar default formula. | Can over-correct in tachycardia and under-correct in bradycardia. |
| Fridericia | QTc = QT / RR^(1/3) | Heart rates that are clearly high or low compared with baseline. | Often preferred when Bazett looks exaggerated at rate extremes. |
| Framingham | QTc = QT + 0.154(1 − RR) | Cross-checking rate-related distortion when you want a linear correction option. | More common in comparative ECG review than bedside pediatric habit. |
| Hodges | QTc = QT + 1.75(HR − 60) | Another rate-adjustment cross-check when formula disagreement matters. | Useful for comparison, but local ECG interpretation practice still governs which value is operational. |
Interpretation checkpoints
Clearly normal context
QTc is comfortably below the concerning threshold for the child, the tracing is measured cleanly, and the clinical story does not suggest an acquired or congenital long-QT problem.
Document the formula used and continue routine interpretation in clinical context.
Borderline or formula-sensitive result
Bazett and Fridericia disagree, the heart rate is extreme, or the value sits close to a threshold where age, sex, and measurement technique matter.
Re-measure the tracing, compare formulas, review electrolytes and medications, and set a repeat-ECG plan.
Clearly prolonged or rising trend
The corrected QT is clearly above the threshold your local pediatric guidance uses or is worsening compared with prior ECGs.
Correct reversible contributors, document escalation thresholds, and involve the appropriate senior or specialty team promptly.
Exact normal and abnormal cutoffs vary by age, sex, ECG system, and local protocol. Use the interpretation standard your pediatric team follows.
When to use
- When reviewing QT interval on an ECG and you need a heart-rate corrected value.
- Before starting or adjusting QT-prolonging medications.
- When evaluating syncope, palpitations, or electrolyte abnormalities.
How it works
QTc adjusts the measured QT interval for heart rate. The two most common correction formulas are:
- Bazett: QTc = QT / sqrt(RR).
- Fridericia: QTc = QT / RR^(1/3).
Enter QT in milliseconds and either heart rate (bpm) or RR interval (ms). The calculator converts to seconds internally.
Rapid bedside workflow
- Confirm QT and heart-rate/RR values come from the same ECG tracing.
- Calculate with both Bazett and Fridericia when heart rate is clearly high or low.
- Interpret results with electrolytes, medication list, and symptom context.
- Set repeat-ECG timing and escalation threshold if QTc is prolonged or trend worsens.
Interpretation and caveats
QTc values vary by age, sex, and measurement technique. Bazett tends to over‑correct at high heart rates, which are common in infants and toddlers.
- Use multiple beats if the rhythm is irregular.
- Consider Fridericia in tachycardia or bradycardia when Bazett may be less reliable.
- Review medications, electrolytes, and clinical context when QTc is prolonged.
This tool does not replace formal ECG interpretation or cardiology review.
Reassessment cadence
- After electrolyte correction or QT-prolonging medication change: repeat ECG and recalculation in the expected response window.
- If stable and asymptomatic with borderline values: scheduled interval reassessment per local protocol.
- If syncope, palpitations, or worsening prolongation occurs: immediate reassessment and escalation.
Shift handoff checklist
- Measured QT, heart rate/RR source, and formula(s) used.
- Current QTc trend versus prior ECG(s).
- Medication/electrolyte contributors reviewed and actions taken.
- Next ECG timing and escalation trigger.
Related calculators
References
- LITFL ECG Library: QT Interval and QTc formulas.
- Bazett HC. An analysis of time‑relations of electrocardiograms. Heart. 1920.
Frequently asked questions
Use a QTc calculator when you need to correct the QT interval for heart rate, especially when reviewing medications, electrolytes, or symptoms that may prolong QT.
Bazett is the most common formula but can over‑correct at high heart rates and under‑correct at low heart rates. Fridericia is often preferred when the heart rate is very fast or slow. Follow your local ECG interpretation guidance.
Yes. This calculator converts heart rate to RR internally using RR = 60 / HR. Use the QT and heart rate from the same ECG tracing.
Normal QTc ranges vary by age, sex, and lab/ECG system. Use pediatric reference ranges and interpret in the clinical context rather than relying on a single cutoff.
Measure the QT interval from the start of the QRS to the end of the T wave in a lead where the T wave is well‑defined, then use an average of several beats if the rhythm is irregular.
Document the formula used, ECG context, trend direction, and management actions (electrolyte correction or medication review). This supports safer serial interpretation across teams.
That often happens when the heart rate is clearly fast or slow. Re-check the QT measurement, compare both formulas in context, and follow the local pediatric interpretation standard rather than relying on a single automatic value.