Good afternoon! See the ECG recording speed. Consider the distance in mm between two adjacent RR intervals. If the ECG recording speed is 50 mm/sec., then multiply this distance by 0.02. If the ECG recording speed is 25 mm/sec., then multiply this distance by 0.04. After that, divide 60 by the resulting number (duration RR in sec.). This will be the heart rate (Heart rate = 60/RR per second).
Thanks a lot for your reply and these explanations dear Svetlana Chuprova . I would like to know if this method is correct by counting the number of QRS complexes per minute and dividing them by the number of 5mm squares separating the QRS complex.