Where can you find an explanation of how to measure QT interval?

To measure the QT interval, count the number of squares, and multiply the sum by the unit time per square, according to Kansas City University. For example, if there are 12 squares and the unit time per square is .04 seconds, the QT interval is 0.48 seconds.

The QT interval denotes the time between the start of the Q wave and the end of the T wave, which represents ventricular depolarization and repolarization, notes Practical Clinical Skills. The QT interval typically ranges from 0.36 to 0.44 seconds, over nine to 11 boxes. The QT interval varies from patient to patient, according to gender, age and heart rate.