ECG PR Interval Calculator

How to calculate PR interval?

The PR interval is measured from the onset of the P-wave to the beginning of the QRS complex. This duration represents the time taken for electrical depolarization to travel from the sinoatrial (SA) node through the atria and the atrioventricular (AV) node until ventricular depolarization begins.

PR (ms) = (Boxes / Speed) × 1000

Using the PR Interval calculator: an example

For an ECG tracing with a paper speed of 25 mm/s where you measure a distance of 4 small boxes for the PR interval, the calculation is performed as follows:

Step-by-step calculation:

1. Count the small squares between the P-wave start and QRS start (e.g., 4 boxes).
2. Determine the ECG paper speed (e.g., 25 mm/s).
3. Calculate: (4 boxes / 25 mm/s) × 1000 = 160 ms.

What is the normal PR interval range?

The normal PR interval is between 120 ms and 200 ms, which is equivalent to 3 to 5 small boxes on standard ECG paper.

What indicates a first-degree block?

A PR interval longer than 200 ms (more than 5 small boxes) is indicative of a first-degree atrioventricular (AV) block, suggesting a delay in impulse conduction.

What do short PR intervals mean?

A short PR interval (less than 120 ms) may suggest an accessory pathway that bypasses the AV node, commonly seen in pre-excitation syndromes like Wolff-Parkinson-White.