Cardiology

QTc Calculator (Bazett, Fridericia, Framingham, Hodges)

Corrected QT interval by 4 formulas: Bazett, Fridericia, Framingham, Hodges

Primary source: Bazett HC. Heart 1920; Fridericia LS. Acta Med Scand 1920; Sagie A. Am J Cardiol 1992; Hodges M. J Electrocardiol 1983

Aleksandr A. Aulov, MD, internal medicine Last clinical review: April 28, 2026

Online calculation

QT (ms)

Measured from the onset of the QRS complex to the end of the T wave (usually in lead II or V5)

RR (ms)

Interval between two adjacent R waves. RR = 60000 / heart rate. For a heart rate of 70 → RR = 857 ms

About this score

QTc (corrected QT) is the QT interval corrected for heart rate, a key ECG parameter for assessing the risk of arrhythmic events, primarily polymorphic ventricular tachycardia (torsade de pointes, TdP) and sudden cardiac death.

Standard correction formulas: Bazett (QT/√RR) — the most widely used, but overestimates QTc at tachycardia and underestimates it at bradycardia; Fridericia (QT/∛RR) — more accurate at extreme heart rates; Framingham and Hodges — used less often. The 2022 ESC guidelines recommend Fridericia or Framingham when heart rate falls outside the 60–100/min range.

Normal ranges: QTc up to 440 ms in men and up to 460 ms in women is normal; 440/460–500 ms is borderline prolongation; >500 ms or an increase ≥60 ms from baseline indicates high risk of TdP. QTc prolongation can be congenital (LQTS) or acquired (drugs, electrolyte disturbances).

When to use

Before starting QT-prolonging drugs (amiodarone, sotalol, macrolides, fluoroquinolones, antipsychotics).
Monitoring when congenital long QT syndrome is suspected (syncope, family history).
Monitoring electrolyte disturbances in the ICU.
Standard ECG analysis as part of preoperative assessment.

Clinical example

Case

67-year-old woman after IV azithromycin for pneumonia. ECG: heart rate 78/min, measured QT 480 ms, RR ≈ 770 ms. Serum potassium 3.2 mmol/L.

Calculation

QTc by Bazett = 480 / √0.77 = 480 / 0.877 ≈ 547 ms.

Interpretation and management

Markedly prolonged QTc, high risk of TdP. Management: immediate discontinuation of azithromycin, switch to a safer antibiotic, correct hypokalemia (KCl to target >4.0 mmol/L), replete magnesium (MgSO₄ 2 g IV), ECG monitoring in a monitored unit. If TdP occurs — defibrillation and bolus MgSO₄.

Limitations and cautions

Bazett distorts QTc at heart rates <60 or >100 — Fridericia is preferable.
Manual QT measurement accuracy suffers with wide QRS, bundle branch block, or artifacts.
Does not account for the patient's baseline QTc — an increase ≥60 ms is more informative than the absolute value.
In patients with LBBB, the JT interval is assessed instead or correction adjustments are applied.

Frequently asked questions

Which formula should I use?

At heart rates of 60–100, use Bazett (the conventional choice). At tachycardia or bradycardia, use Fridericia. In research settings, Framingham is often used.

At what QTc should a drug be discontinued?

An absolute value >500 ms or an increase ≥60 ms from baseline warrants reviewing therapy. If symptoms are present (presyncope, syncope), discontinue immediately.

Does sex affect QTc?

Yes. QTc is physiologically longer in women (normal up to 460 vs 440 ms). Risk thresholds are the same.

What defines congenital LQTS?

QTc >480 ms on repeated ECGs plus symptoms (syncope, family history of sudden cardiac death at a young age), or a confirmed genetic mutation.

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Aleksandr A. Aulov Physician, internal medicine, Sechenov School — independent medical education platform. Calculators are compiled from the original publications and current clinical guidelines. Interpretation thresholds follow the source study unless stated otherwise.

This calculator is intended for healthcare professionals. Do not use it for self-diagnosis or self-treatment. Management decisions are made by the treating clinician based on the full clinical picture of the individual patient.