How to Measure Intervals
From Ask Dr Wiki
What are the Intervals including PR interval, QRS interval, and QT interval
To measure the PR interval measure from the beginning of the p wave to the beginning of the QRS. A normal range is between 120ms (3 small boxes) and 200 ms (5 small boxes).
To measure the QRS complex measure from the beginning of the Q wave to the end of the S wave. A normal range is from 40 to 100 msec (1 small box to 2.5 small boxes)
The next step in reading an EKG is How to Look for Hypertrophy.