🫀 Corrected QT Interval (QTc) Calculator – Bazett, Fridericia, Framingham & Hodges
The QT interval on an electrocardiogram (ECG) reflects the time required for the ventricles to depolarize and fully repolarize. Because the QT interval naturally shortens at faster heart rates and lengthens at slower ones, clinicians use a heart-rate correction formula to produce the corrected QT interval (QTc) — a standardized value that removes the influence of heart rate and allows meaningful comparison across patients and time points.
Why QTc Matters Clinically
A prolonged QTc is associated with an elevated risk of life-threatening ventricular arrhythmias, particularly torsades de pointes (TdP), a polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation. Causes of QT prolongation include congenital long QT syndrome (LQTS), electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia), certain medications (antiarrhythmics, antihistamines, antipsychotics, antibiotics), and cardiac or systemic disease. Monitoring QTc is essential when prescribing QT-prolonging drugs or evaluating syncope, palpitations, or unexplained sudden cardiac arrest risk.
The Four Supported Correction Formulas
| Formula | Equation | Best Used When |
|---|---|---|
| Bazett | QTc = QT ÷ √RR | Standard clinical practice; embedded in most ECG machines. Heart rate 60–100 bpm. |
| Fridericia | QTc = QT ÷ ∛RR | Better accuracy at heart rates outside the normal range; preferred in research. |
| Framingham | QTc = QT + 0.154 × (1 − RR) | Linear correction; derived from Framingham Heart Study population data. |
| Hodges | QTc = QT + 1.75 × (HR − 60) | Linear rate-based correction using heart rate directly in bpm. |
All formulas use QT and RR in seconds internally. If QT is measured in milliseconds, divide by 1,000 before applying the formula, then multiply the result by 1,000 to display in ms. The RR interval in seconds can be derived from heart rate as RR = 60 ÷ HR.
Interpreting QTc Results
Reference thresholds vary slightly by sex and age. The following ranges reflect widely used clinical guidelines (AHA/ESC) for adult patients:
A QTc exceeding 500 ms substantially increases torsades de pointes risk and warrants immediate clinical attention. Values in the borderline range require contextual evaluation — medication review, electrolyte assessment, and family history of sudden cardiac death are all relevant.
Bazett vs. Fridericia: When to Use Each
Bazett's formula is the global standard embedded in automated ECG systems and is used in most clinical trials that define QT prolongation thresholds. However, it is known to overcorrect at heart rates above 100 bpm (producing an artificially high QTc) and undercorrect at heart rates below 50 bpm (producing an artificially low QTc).
Fridericia's formula uses a cube-root correction that tracks QT-RR relationships more accurately across a wide heart rate range, making it the preferred method in pharmacological safety studies evaluating drug-induced QT changes (per ICH E14 guidance). For routine clinical screening in patients with normal sinus rhythm, both formulas are generally acceptable. For patients with tachycardia or bradycardia, Fridericia, Framingham, or Hodges provide more reliable estimates.
Wide QRS and Other Pitfalls
The QT interval encompasses both depolarization (QRS) and repolarization (ST-T wave). When the QRS is widened (≥ 120 ms) — due to left or right bundle branch block, ventricular pacing, or pre-excitation — the inflated depolarization component falsely lengthens the raw QT. Some guidelines recommend using the JT interval (QT minus QRS) or subtracting half the QRS duration from QT before applying the correction formula. The wide-QRS flag in this calculator provides a caution reminder, but specific adjustment methodology should follow institutional protocols.
ECG Lead Selection and Measurement Tips
- Lead II and V5 or V6 are most commonly used for QT measurement because they typically show a well-defined T-wave end.
- Measure from the start of the QRS complex to the point where the T wave returns to the isoelectric baseline (tangent method is preferred over visual return).
- If T waves are low-amplitude, biphasic, or followed by prominent U waves, consistent measurement is especially important for reproducibility.
- Automated ECG machine QT measurements can differ from manual measurements; both values may be useful to record.
Multi-Formula Comparison Mode
Because each formula may produce meaningfully different QTc values at the same heart rate, enabling the multi-formula comparison mode displays Bazett, Fridericia, Framingham, and Hodges results side by side. This is particularly valuable:
- When QTc is near the borderline threshold and formula choice could alter classification
- When evaluating drug-induced QT changes for pharmacovigilance purposes
- For educational review of how formulas diverge at fast or slow heart rates
- In academic settings comparing published studies that used different correction methods
Pediatric Considerations
QTc thresholds are different in children. Neonates and infants naturally have longer QTc values as part of normal cardiac development. The calculator's pediatric mode uses a threshold of 440 ms (normal), 460 ms (borderline), and 480 ms (prolonged) as general reference points, but institutional pediatric cardiology protocols and age-specific normative data should always be applied in clinical contexts.
Limitations and Disclaimer
This calculator is intended for educational and informational purposes only. Results depend entirely on the accuracy of the QT interval measurement entered. ECG interpretation, arrhythmia risk assessment, and drug safety decisions require evaluation by a qualified clinician. Electrolyte levels, genetic history, concurrent medications, and clinical context are all essential components of a complete QT assessment that this tool cannot replace.