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Calculate corrected QT (QTc) from QT interval and heart rate or R-R interval. Outputs all four major formulas simultaneously with sex-specific Bazett interpretation. Use with our CHA₂DS₂-VASc calculator for AF stroke risk context. Educational—not a substitute for cardiology review.
Last updated: June 5, 2026
Bazett over-corrects in tachycardia—QT 360 ms at HR 120 can yield Bazett ~509 ms while Fridericia ~453 ms. Compare formulas before labeling “prolonged.”
Bazett QTc
400
ms
Fridericia
400 ms
Framingham
400 ms
Hodges
400 ms
Normal HR 60 (default)
Bazett 400 ms
QT 400 ms · all formulas ≈400
Tachycardia QT 360 / HR 120
Bazett 509 · Frid 454
Bazett over-corrects
Bradycardia QT 480 / HR 48
Bazett 429 · Hodges 459
Normal
Long QT 520 / HR 55
Bazett 498 ms
Prolonged
Ward monitor: QT 410 ms, HR 72 → Bazett 449 ms (Normal, female thresholds)
QT in milliseconds; RR in seconds (RR = 60 / HR).
| Formula | Equation | Worked example |
|---|---|---|
| Bazett | QTc = QT / √RR | QT 400 ms, RR 1.0 s → 400 ms |
| Fridericia | QTc = QT / ∛RR | QT 360 ms, RR 0.5 s (HR 120) → ~453 ms |
| Framingham | QTc = QT + 154 × (1 − RR) | QT 360 ms, RR 0.5 → 437 ms |
| Hodges | QTc = QT + 1.75 × (HR − 60) | QT 360 ms, HR 120 → 465 ms |
| Category | Male | Female | Action |
|---|---|---|---|
| Normal | ≤429 ms | ≤449 ms | Routine monitoring per indication |
| Borderline | 430–450 ms | 450–470 ms | Repeat ECG, meds, K⁺/Mg²⁺ |
| Prolonged | >450 ms | >470 ms | Hold QT-prolonging drugs if possible; correct electrolytes |
| High risk | >500 ms | >500 ms | Urgent cardiology/telemetry evaluation |
| HR context | Preferred | Note |
|---|---|---|
| 50–90 bpm | Bazett or Framingham | Formulas often agree near HR 60 |
| >100 bpm | Fridericia, Framingham, Hodges | Bazett over-corrects → false prolonged QTc |
| <50 bpm | Fridericia, Framingham | Bazett under-corrects at slow rates |
| Wide QRS | Adjusted QT (JT) or specialist formulas | Subtract excess QRS before standard correction |
| Abnormality | ECG clue | Action |
|---|---|---|
| Hypokalemia (K⁺ low) | Flattened T, U waves, apparent QT prolongation | Replete K⁺; recheck QTc |
| Hypomagnesemia | May potentiate QT prolongation | Mg²⁺ repletion especially if on QT drugs |
| Hypocalcemia | QT prolongation | Correct Ca²⁺; distinguish from true long QT |
| Hypercalcemia | Shortened QT | Treat cause; QTc may shorten |
| Drug | Class | Risk note |
|---|---|---|
| Macrolides (azithromycin) | Antibiotic | Moderate–high QT risk |
| Fluoroquinolones | Antibiotic | QT prolongation—avoid combo with other QT drugs |
| Ondansetron (high IV dose) | Antiemetic | FDA QT warning at higher doses |
| Haloperidol / ziprasidone | Antipsychotic | Common inpatient QT concern |
| Amiodarone | Class III antiarrhythmic | Paradoxical—prolongs QT but ↓ TdP risk at steady state |
| SSRIs (citalopram > others) | Antidepressant | Dose-dependent QT; psychiatry monitoring |
Clinical takeaway: Bazett 509 ms may look dangerously prolonged while Fridericia 454 ms and Framingham 437 ms are lower—do not stop medications based on Bazett alone at HR 120 without context.
| Condition | Adjustment | Note |
|---|---|---|
| LBBB / RBBB | Consider JT = QT − (QRS − 120) ms before correction | Raw QT overestimates repolarization time |
| Paced rhythm | Use institution-specific paced-QT protocols | Standard Bazett may mislead |
| Ventricular rhythm | Cardiology interpretation required | Different thresholds apply |
This QTc calculator applies published formulas for learning and bedside estimates. It does not measure QT from ECG images, account for wide QRS automatically, or replace electrophysiology consultation. QTc >500 ms or syncope with QT prolongation warrants urgent evaluation.