QTc Calculator — Corrected QT Interval (Bazett, Fridericia, Framingham Formulas)
Calculate the corrected QT interval (QTc) from ECG measurements using Bazett, Fridericia, or Framingham formulas to assess risk of Torsades de Pointes and sudden cardiac death.
QTc Calculator
Enter values below to calculate
Measure from QRS onset to T-wave end in lead II or V5. Avoid U-wave.
Ventricular rate at time of ECG measurement.
This calculator is for informational and educational purposes. Results should be interpreted with clinical judgment and local guidelines.
What is the Corrected QT Interval (QTc)?
The corrected QT interval (QTc) is the QT interval from an electrocardiogram (ECG) normalized for heart rate. Because the QT interval shortens at faster heart rates and lengthens at slower rates, raw QT measurements cannot be safely compared across patients with different heart rates. QTc correction standardizes the QT interval to a theoretical heart rate of 60 bpm (RR interval = 1 second). The most widely used formula is Bazett's formula (QTc = QT / √RR), though Fridericia's formula (QTc = QT / ∛RR) is preferred at extreme heart rates. Prolonged QTc (> 450 ms in males, > 470 ms in females) is a risk factor for polymorphic ventricular tachycardia (Torsades de Pointes, TdP), potentially leading to ventricular fibrillation and sudden cardiac death. QTc monitoring is mandatory when initiating QT-prolonging medications.
QTc (Bazett) = QT (ms) / √RR (seconds). QTc (Fridericia) = QT (ms) / ∛RR (seconds). Normal QTc: ≤ 440 ms (males), ≤ 460 ms (females). Concerning: > 500 ms in either sex — high TdP risk.
When to Use the QTc Calculator
- Pre-treatment baseline ECG and QTc measurement before initiating QT-prolonging medications including antipsychotics (haloperidol, quetiapine, ziprasidone), antiarrhythmics (amiodarone, sotalol, dofetilide), antibiotics (azithromycin, moxifloxacin, chloroquine), and antidepressants (TCAs, citalopram).
- ICU and inpatient cardiac monitoring: Serial QTc monitoring during treatment with multiple QT-prolonging drugs, electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia), or sepsis-related cardiac dysfunction.
- Evaluation of suspected congenital long QT syndrome (LQTS types 1–3) in patients with unexplained syncope, near-drowning episodes, seizures, or family history of sudden cardiac death in young individuals.
- Drug safety review in clinical pharmacy: Identifying high-risk drug combinations with additive QT-prolonging potential before dispensing concurrent medications.
- Acute poisoning and overdose: Assessing QTc in patients with tricyclic antidepressant, organophosphate, or antipsychotic overdose where QT prolongation predicts arrhythmia risk.
QTc Correction Formulas — Bazett, Fridericia, Framingham
Bazett Formula (Most Widely Used)
QTc = QT (ms) / √(RR interval in seconds) = QT / √(60/HR)
HR = 75 bpm, QT = 380 ms → RR = 60/75 = 0.8s → QTc = 380/√0.8 = 425 ms (Normal)
Fridericia Formula (Better at Extremes of HR)
QTc = QT (ms) / ∛(RR interval in seconds) = QT / (RR)^(1/3)
HR = 100 bpm, QT = 360 ms → RR = 0.6s → QTc = 360/(0.6)^0.333 = 427 ms
Framingham Formula
QTc = QT + 0.154 × (1 − RR)
HR = 60 bpm, QT = 420 ms → RR = 1.0 → QTc = 420 + 0.154 × (1-1) = 420 ms
Heart Rate to RR Interval Conversion
RR (seconds) = 60 / Heart Rate (bpm)
HR = 72 bpm → RR = 60/72 = 0.833 seconds
Variables
QT interval (milliseconds (ms))
Measured from the beginning of the QRS complex to the end of the T wave. Measure in lead II or the lead with the largest T wave. Average 3 consecutive beats in sinus rhythm.
RR interval (seconds)
Time between consecutive R wave peaks. RR = 60/heart rate (bpm). At regular heart rates, average 3 consecutive RR intervals.
Heart Rate (beats per minute (bpm))
Ventricular rate at time of QT measurement. QTc correction requires accurate heart rate at time of ECG.
The Bazett formula over-corrects QTc at high heart rates (> 100 bpm) and under-corrects at slow rates (< 60 bpm). Use Fridericia formula for patients with tachycardia or bradycardia. The QT interval should be measured manually in lead II or V5 from the onset of the QRS to the end of the T wave. Avoid U-wave inclusion.
QTc Interval Classification & Torsades de Pointes Risk
QTc thresholds differ by sex because females have physiologically longer QTc intervals due to hormonal effects. All values > 500 ms carry significant TdP risk regardless of sex.
| Range | Classification | Health Risk |
|---|---|---|
| < 400 ms | Short QTc | Short QT — evaluate for Short QT Syndrome if < 360 ms with symptoms. |
| 400–440 ms (Males) / 400–460 ms (Females) | Normal QTc | Normal corrected QT interval. No intervention required. |
| 441–460 ms (Males) / 461–480 ms (Females) | Borderline Prolonged | Borderline — monitor; avoid additional QT-prolonging drugs; correct electrolytes. |
| 461–500 ms (Males) / 481–500 ms (Females) | Prolonged QTc | Prolonged — review QT-prolonging medications; cardiology review if new finding. |
| > 500 ms (Either Sex) | Critically Prolonged — High TdP Risk | High Torsades de Pointes risk. Discontinue offending drugs; IV magnesium sulphate 2g; cardiac monitoring; consider isoproterenol or pacing. |
| > 600 ms (Either Sex) | Extreme Prolongation — Emergency | Extreme risk. Immediate cardiology input. Consider IV magnesium, overdrive pacing. ICU level monitoring. |
The absolute QT interval should also be reviewed alongside QTc. A QT > 500 ms or ΔQTc increase of > 60 ms from baseline after drug initiation are clinically significant thresholds triggering drug review.
QTc Calculator Limitations & Clinical Warnings
Bazett formula accuracy limitations: Over-corrects at heart rates > 100 bpm (falsely prolonged QTc) and under-corrects at heart rates < 60 bpm (falsely shortened QTc). Use Fridericia formula at extreme heart rates.
Accurate QT measurement is critical: Errors in manual QT measurement directly produce inaccurate QTc. QT should be measured from the beginning of the earliest QRS deflection to the end of the T-wave return to baseline in the lead with the widest T-wave.
Atrial fibrillation: QT measurement in AF is unreliable due to irregular RR intervals. Measure QT in the longest RR interval or use the Hodges formula. Consider cardiology review.
Bundle branch blocks: Left bundle branch block (LBBB) and right bundle branch block (RBBB) artificially prolong the QRS, making QTc calculation inaccurate. Modified Bazett corrections exist for LBBB.
U-wave confusion: Large U waves (common in hypokalemia, bradycardia) may be mistakenly included in QT measurement, falsely prolonging QTc. Measure to T-wave nadir, not U-wave end.
Does not replace cardiology ECG interpretation: QTc calculators are decision support tools. All clinically significant QTc findings require review by a physician or cardiologist familiar with the full clinical context.
Medical Disclaimer
QTc prolongation (> 500 ms) is a medical emergency requiring immediate drug review, electrolyte correction, and cardiac monitoring. Any suspected Torsades de Pointes (TdP) presenting as presyncope, syncope, or polymorphic VT requires emergency resuscitation and immediate cardiology involvement.
References & Citations
- Bazett HC. “An analysis of the time-relations of electrocardiograms”. Heart. 1920. 7: 353–370. Original QT correction formula paper (1920)
- Fridericia LS. “The duration of systole in the electrocardiogram of normal subjects and of patients with heart disease”. Acta Medica Scandinavica. 1920. 53: 469–486. Fridericia QTc correction — cube root formula (1920)
- Priori SG, Blomström-Lundqvist C, Mazzanti A, et al.. “2015 ESC Guidelines for the Management of Patients with Ventricular Arrhythmias and the Prevention of Sudden Cardiac Death”. European Heart Journal. 2015. 36(41): 2793–2867. PMID: 26320108 | DOI: 10.1093/eurheartj/ehv316
- Roden DM. “Drug-Induced Prolongation of the QT Interval”. New England Journal of Medicine. 2004. 350(10): 1013–1022. PMID: 14999113 | DOI: 10.1056/NEJMra032426
Medically Reviewed
Reviewed by CliniqWise Clinical Advisory Board — Cardiology & Electrophysiology Panel · Last updated: 2026-08-27 · Next review: 2027-02-27
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