The next step in the orderly approach for 12-lead ECG interpretation is determination of the heart rate.
Heart rate calculation
Now, let us calculate the heart rate. At a normal paper speed of 25 mm/second, one large box is 0.2 seconds. If one ECG complex occurred every large box, there would be one heart beat every 0.2 seconds and the heart rate would be 300 beats/minute. Hence, the heart rate can be determined by dividing 300 by the number of large boxes between each R-wave.
Example
As an example for heart rate calculation, here is a patient who has a complex every 4 large boxes. 300 divided by 4 large boxes equals 75, which is the heart rate.