how to find qt interval ecg
How to Calculate a QT Interval. 1  Count the number of small boxes on the standard ECG printout (with 5-mm boxes) between the beginning of the QRS complex and the end of the T wave. 2  Multiply the number of boxes you counted by the speed that the ECG paper was run (in mm/sec). 3  Multiply this number by 8 to get the QT interval in milliseconds.