QT Interval Calculator

The QT Interval Calculator estimates the corrected QT interval (QTc) based on heart rate and QT duration. This tool helps medical professionals quickly assess cardiac repolarization and potential arrhythmia risks. Whether you are evaluating medication side effects or screening for long QT syndrome, this calculator provides accurate, standardized results for clinical decision-making.

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

How Corrected QT Interval Is Calculated

The corrected QT interval represents the time it takes for the heart's electrical system to recharge after a beat, adjusted for how fast the heart is beating. Because the raw QT interval naturally shortens when the heart beats faster, a correction formula is necessary to get a standard value.

QTc = QT / √(RR)

Where:

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

To calculate, first convert the heart rate into the RR interval, which is the time between beats in seconds. Then, divide the measured QT by the square root of the RR interval using Bazett's formula. This mathematical adjustment ensures that the result is reliable regardless of whether the patient has a slow or fast pulse.

While Bazett's formula is the most widely used method, it can overcorrect at very high heart rates, so other formulas like Fridericia's are available for specific cases.

What Your Corrected QT Interval Means

Your result indicates the adjusted duration of the heart's electrical activity, helping you spot patients who may be at risk for dangerous heart rhythms like torsades de pointes. A higher number suggests that the heart is taking longer than normal to recharge, which can lead to instability.

Normal QTc

If the result is 440 ms or lower for males, or 470 ms or lower for females, the repolarization time is within healthy limits. No immediate action regarding the QT interval is typically required for these patients.

Borderline QTc

Values between 441-470 ms in males and 471-500 ms in females are considered borderline. You should review the patient's medications and electrolyte levels, as these factors often contribute to slight prolongations.

Prolonged QTc

A result above 470 ms for males or 500 ms for females indicates a high risk for arrhythmias. Immediate clinical assessment is necessary to determine if causes like drug interactions or genetic conditions are present.

Important: This calculator is a screening tool for educational purposes and is not a substitute for professional medical diagnosis. Always consult with a qualified healthcare provider before making clinical decisions based on these results.

To ensure accuracy, consider measuring the QT interval over several beats and averaging the result, as single-beat variations can occur.