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QT Interval Calculator

Calculate the corrected QT interval (QTc) to assess cardiac repolarization and identify potential arrhythmia risk based on heart rate and QT interval measurements.

Enter the QT interval in milliseconds (ms)
Enter the heart rate in beats per minute (bpm)

How to Use This Calculator

  1. Enter the QT interval in milliseconds (ms) as measured on ECG
  2. Enter the heart rate in beats per minute (bpm)
  3. Select the correction formula you prefer (Bazett's is most commonly used)
  4. Select the patient's sex (QTc values differ slightly between males and females)
  5. Click Calculate to see the corrected QT interval and its clinical interpretation

Formula Used

QTc = QT / √(RR interval)

Where:

  • QTc = Corrected QT interval in milliseconds
  • QT = Measured QT interval in milliseconds
  • RR interval = 60 / Heart Rate (in seconds)

Alternative Formulas:

  • Bazett's: QTc = QT / √(RR)
  • Fridericia's: QTc = QT / ∛(RR)
  • Framingham: QTc = QT + 0.154 × (1 - RR)
  • Hodges': QTc = QT + 1.75 × (HR - 60)

Example Calculation

Real-World Scenario:

A 45-year-old female patient with a heart rate of 80 bpm and a measured QT interval of 380 ms.

Given:

  • QT Interval = 380 ms
  • Heart Rate = 80 bpm
  • Formula = Bazett's
  • Sex = Female

Calculation:

RR interval = 60 / 80 = 0.75 seconds

QTc = 380 / √(0.75) = 380 / 0.866 = 438.6 ms

Result: QTc of 439 ms, which is borderline prolonged for a female patient (normal range is typically ≤ 450 ms)

Why This Calculation Matters

Practical Applications

  • Assessing risk of ventricular arrhythmias
  • Evaluating medication effects on cardiac repolarization
  • Screening for congenital long QT syndrome

Key Benefits

  • Standardizes QT interval assessment across different heart rates
  • Helps identify patients at risk for torsades de pointes
  • Guides medication selection and dosing decisions

Common Mistakes & Tips

Bazett's formula is most commonly used but can overcorrect at high heart rates and undercorrect at low heart rates. For patients with very high (>100 bpm) or very low (<50 bpm) heart rates, consider using Fridericia's or Framingham's formula instead.

Females naturally have slightly longer QT intervals than males. The normal upper limit for QTc is typically 450 ms in males and 470 ms in females. Always consider the patient's sex when interpreting QTc values.

The QT interval should be measured from the beginning of the QRS complex to the end of the T wave. For best accuracy, measure multiple beats and use the average. Lead II or V5 are typically used for measurement as they often have the clearest T waves.

Frequently Asked Questions

A normal QTc is typically ≤ 440 ms in males and ≤ 470 ms in females. Values between 440-470 ms in males and 470-500 ms in females are considered borderline. Values above these thresholds are considered prolonged and may indicate increased risk of arrhythmias.

Prolonged QT can be congenital (genetic long QT syndrome) or acquired. Common acquired causes include certain medications (antiarrhythmics, antibiotics, antipsychotics), electrolyte imbalances (hypokalemia, hypomagnesemia, hypocalcemia), and medical conditions like hypothyroidism or myocardial ischemia.

No single formula is universally most accurate. Bazett's formula is most widely used despite its limitations. Fridericia's formula tends to be more accurate at high heart rates, while Framingham's formula performs better at low heart rates. The choice often depends on institutional preference and the specific clinical scenario.

A prolonged QT interval increases the risk of a specific type of ventricular tachycardia called torsades de pointes, which can cause syncope or sudden cardiac death. It's particularly important to monitor QTc when patients are taking medications known to prolong QT or have electrolyte abnormalities.

References & Disclaimer

Medical Disclaimer

This QT Interval Calculator is for informational purposes only and should not be used as a substitute for professional medical advice, diagnosis, or treatment. The calculations are estimates only and should not be used to make clinical decisions without consulting with a qualified healthcare provider. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.

References

Accuracy Notice

This calculator provides corrected QT interval estimates based on established formulas. However, manual measurement of the QT interval can have variability depending on the ECG lead used and the interpreter's technique. For clinical decision-making, especially in borderline cases, consultation with a cardiologist or electrophysiologist is recommended. The calculator does not account for all factors that may affect QT interval interpretation, such as bundle branch blocks or ventricular pacing.

About the Author

Kumaravel Madhavan

Web developer and data researcher creating accurate, easy-to-use calculators across health, finance, education, and construction and more. Works with subject-matter experts to ensure formulas meet trusted standards like WHO, NIH, and ISO.

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