- Enter the measured QT interval in milliseconds — read it from lead II or V5–V6, take the longest clearly measurable interval, and exclude any U wave.
- Choose whether to enter the rate as heart rate (bpm) or RR interval (seconds), then type the value. The tool converts internally (RR = 60 ÷ HR).
- Select sex — the normal/borderline/prolonged thresholds differ for men and women.
- The result shows QTc by Bazett and Fridericia as primary cards plus a risk band; Framingham and Hodges appear in the verdict for cross-checking.
- Read the verdict and recommended action. Re-measure and recompute after any electrolyte correction or drug change.
All computation runs in your browser; no values are stored or transmitted.
When to Use
Use QTc whenever you start, up-titrate, or combine QT-prolonging drugs (ondansetron and other 5-HT3 antiemetics, metoclopramide, macrolides such as clarithromycin and azithromycin, fluoroquinolones, hydroxychloroquine, haloperidol and other antipsychotics, methadone, and many antiarrhythmics), or whenever you read an ECG in a patient at risk of torsades de pointes. It is especially important in CKD and dialysis patients, where electrolyte shifts (hypokalemia, hypomagnesemia, hypocalcemia), polypharmacy, and reduced drug clearance stack the risk — and where peri-dialysis potassium and magnesium swings can transiently prolong the QT.
Appropriate population
Adults with a measurable QT and a known heart rate (or RR interval) in sinus rhythm. The correction is most reliable at heart rates near 60 bpm; at heart-rate extremes, prefer Fridericia or Framingham over Bazett (see Pearls).
When NOT to rely on it
Rate-correction formulas are unreliable in atrial fibrillation or other markedly irregular rhythms (average several beats), and in a wide QRS (>120 ms) — bundle branch block, paced rhythm, pre-excitation — where a long QRS itself inflates the QT. In wide QRS, consider the JT interval or a QRS-adjusted QT (QT − (QRS − 100)) before correcting for rate. A single QTc is a snapshot — trend it and compare to the patient's own baseline.
Pearls & Pitfalls
Bazett is standard; Fridericia is better at the extremes
Bazett (QT ÷ √RR) is the most widely reported correction and the one most thresholds were defined against, but it over-corrects at fast heart rates and under-corrects at slow rates. At HR <60 or >100 bpm, prefer Fridericia (QT ÷ RR1/3) or Framingham, which are flatter across the rate range. This tool emphasizes Bazett and Fridericia side by side so the divergence is visible.
Measure the QT correctly
Use lead II or V5–V6, pick the longest clearly measurable QT, end at the return of the T wave to baseline by the tangent method, and do not include a U wave. Mismeasurement — particularly counting the U wave — is the commonest source of a falsely "prolonged" QTc.
Pitfalls
(1) Corrections diverge at heart-rate extremes — never act on Bazett alone at HR <60 or >100. (2) A wide QRS inflates QT; use JT or a QRS-adjusted QT instead. (3) QTc ≥ 500 ms, or an increase of >60 ms from baseline, markedly raises torsades risk regardless of the formula. (4) Correct electrolytes before blaming the drug — keep K+ > 4.0 mmol/L and Mg2+ > 2.0 mg/dL, which is especially relevant peri-dialysis.
Why Use It
The raw QT interval shortens at fast heart rates and lengthens at slow rates, so it cannot be interpreted without correcting for rate. QTc normalizes the QT to a heart rate of 60 bpm, letting you apply a single set of thresholds and track change over time. In CKD and dialysis patients — who frequently receive multiple QT-prolonging drugs against a background of electrolyte derangement and impaired drug clearance — a prolonged QTc is the actionable early-warning sign of torsades de pointes risk. Computing all four standard corrections, and comparing to baseline, prevents both false alarms (Bazett over-correcting a tachycardic patient) and missed risk (Bazett under-correcting a bradycardic one).
Corrected QT Interval (QTc) Calculator — Bazett · Fridericia · Framingham · Hodges
Enter the measured QT and the heart rate (or RR interval) and select sex to compute QTc by all four standard formulas, with sex-specific thresholds and torsades-risk flagging. Built for safe prescribing of QT-prolonging drugs in CKD and dialysis patients.
⚕ With RR in seconds (RR = 60 ÷ HR): Bazett QTc = QT ÷ √RR; Fridericia QTc = QT ÷ RR1/3; Framingham QTc = QT + 154 × (1 − RR); Hodges QTc = QT + 1.75 × (HR − 60). Thresholds: prolonged >470 ms (men) / >480 ms (women); ≥500 ms = high torsades risk. A single QTc is a snapshot — confirm on the ECG, correct electrolytes, and compare to baseline. Not a substitute for clinical judgment. Source: Postema PG, Wilde AAM. Curr Cardiol Rev. 2014;10(3):287–294.
Next Steps
Use the result to support — not replace — clinical judgment.
- Review every QT-prolonging drug on the chart (check a CredibleMeds-style list); stop, switch, or dose-reduce where the QTc is borderline or prolonged, and avoid combining two or more QT-prolongers.
- Correct electrolytes: keep potassium > 4.0 mmol/L and magnesium > 2.0 mg/dL, and address hypocalcemia — especially relevant peri-dialysis, where K+ and Mg2+ shift rapidly.
- If QTc ≥ 500 ms (or it has risen >60 ms from baseline): place on continuous telemetry, treat reversible causes, give IV magnesium for torsades, and obtain cardiology input.
- Recompute after each intervention and trend serial QTc values against the patient's own baseline rather than acting on a single reading.
Evidence & References
Formulas & Equations
| Correction | Equation (RR in seconds, HR in bpm) |
|---|---|
| RR interval | RR = 60 ÷ HR |
| Bazett | QTc = QT ÷ √RR (standard; over-corrects fast, under-corrects slow) |
| Fridericia | QTc = QT ÷ RR1/3 (preferred at HR <60 or >100) |
| Framingham | QTc = QT + 154 × (1 − RR) |
| Hodges | QTc = QT + 1.75 × (HR − 60) |
Sanity check: QT 400 ms at HR 60 → RR 1.0 s → all four corrections return ≈400 ms.
Interpretation thresholds (QTc, ms)
| Band | Men | Women |
|---|---|---|
| Normal | ≤ 440 | ≤ 450 |
| Borderline | 441–470 | 451–480 |
| Prolonged | > 470 | > 480 |
| High torsades risk | ≥ 500 (either sex), or an increase > 60 ms from baseline | |
In wide QRS (>120 ms), the QRS itself inflates the QT — consider the JT interval or a QRS-adjusted QT, e.g. QT − (QRS − 100), before applying these cut-offs.
Evidence & References
Bazett's and Fridericia's corrections both date to 1920; Framingham and Hodges are later rate-linear alternatives that perform better across the heart-rate range. Modern consensus statements emphasize that QTc ≥ 500 ms, or a > 60 ms rise from baseline, identifies patients at materially increased risk of torsades de pointes, and that electrolyte repletion and review of QT-prolonging drugs are first-line.
- Bazett HC. An analysis of the time-relations of electrocardiograms. Heart. 1920;7:353–370.
- Fridericia LS. Die Systolendauer im Elektrokardiogramm bei normalen Menschen und bei Herzkranken. Acta Med Scand. 1920;53:469–486.
- Postema PG, Wilde AAM. The measurement of the QT interval. Curr Cardiol Rev. 2014;10(3):287–294.
- Drew BJ, Ackerman MJ, Funk M, et al. Prevention of Torsade de Pointes in Hospital Settings: AHA/ACCF Scientific Statement. Circulation. 2010;121(8):1047–1060.
- CredibleMeds (Arizona CERT). QTdrugs Lists — drugs that prolong the QT and/or cause torsades de pointes. crediblemeds.org.
