ECG interval analysis is the foundation of cardiac rhythm interpretation. Our EKG calculator handles the four most common interval calculations: heart rate from RR interval (or vice versa), QTc using both Bazett (QT/√RR) and Fridericia (QT/∛RR) corrections, PR interval categorization, and QRS duration assessment. Bazett is the historical standard but tends to overcorrect at high heart rates and undercorrect at low heart rates — Fridericia is generally more accurate. Each interval is classified against standard clinical cutoffs.
Provide either HR or RR interval — the other is calculated.
Input the measured QT interval in ms.
Add PR and QRS for complete interval interpretation.
Review Bazett and Fridericia QTc plus interval categories.
The QT interval lengthens at slow heart rates and shortens at fast ones, so it must be corrected before comparing values across patients. Bazett's formula divides by the square root of RR; Fridericia uses the cube root, which is more accurate at extreme heart rates. PR > 200 ms = first-degree AV block; QRS > 120 ms = bundle branch block.
QTc(Bazett) = QT/√RR • QTc(Fridericia) = QT/∛RR • HR = 60000/RR
QTc normal: <440 ms men, <460 ms women
QTc >500 ms = high risk for torsades de pointes
PR interval normal: 120-200 ms; >200 = first-degree AV block
QRS normal: <120 ms; >120 = bundle branch block
Bazett overcorrects at high HR; Fridericia is more reliable at extreme rates
Many medications prolong QTc — always check before prescribing in patients with borderline QTc
Electrocardiogram (ECG/EKG) interpretation is a fundamental clinical skill required of every physician, nurse, and paramedic. Among the most critical measurements on a 12-lead ECG are the QT interval and its rate-corrected value, QTc. The QT interval represents the total time for ventricular depolarization and repolarization, and when prolonged beyond 500 milliseconds (corrected), it significantly increases the risk of torsades de pointes — a life-threatening ventricular arrhythmia. Over 100 commonly prescribed medications can prolong the QT interval, including certain antibiotics (azithromycin, fluoroquinolones), antipsychotics (haloperidol, quetiapine), and antiarrhythmics (amiodarone, sotalol). The Bazett formula (QTc = QT divided by the square root of RR) has been the clinical standard since 1920, though the Fridericia correction (QTc = QT divided by the cube root of RR) is increasingly preferred for its accuracy at extreme heart rates. This EKG calculator computes QTc using both Bazett and Fridericia formulas simultaneously, derives heart rate from the RR interval, classifies the QTc as normal or prolonged based on sex-specific thresholds, and flags clinically significant prolongation requiring immediate attention.
The QT interval physiologically lengthens at slow heart rates and shortens at fast ones. Without correction, comparing QT values across different heart rates would be meaningless. Bazett's 1920 formula is the most commonly cited but tends to overcorrect at high HR. Modern guidelines often prefer Fridericia or Framingham corrections for more reliable results.
Data sourced from trusted institutions
All formulas verified against official standards.