Look at the rhythm strip and determine the RATE of the EKG.  To do this, find a complex whose apex falls on the line of a large box on the EKG paper.  Then count the number of large boxes you cross until you reach the apex of the next complex.  Take that number and call it N.  Then, plug it into the formula:

RATE = 300 / N

If the complexes are regularly spaced across the EKG, this formula will give you the approximate RATE of the EKG.   If the complexes are irregularly spaced (as in atrial fibrillation) then you will have to calculate your rate in a different manner.  However, this simple method works for a large percentage of EKG’s.

If you use this formula enough times, eventually you will start to learn a pattern for the different rates as the complexes fall on 1 large box, 2 large boxes, 3 large boxes and so on.  Three hundred divided by 1 is 300.  Three hundred divided by 2 is 150.  Three hundred divided by 3 is 100 down to 300 divided by 6 is 50.  The set of numbers you will end up with is:

300, 150, 100, 75, 60, 50

If you mention these 6 numbers to any physician that reads EKG’s, they will recognize the pattern immediately.

If you want to get a more exact estimate of the rate, you can count the number of small boxes instead of large boxes.  Call that “n” and plug it into the formula:

RATE = 1500 / n

This is intuitive since there are 5 small boxes for each large box on EKG paper and 5 times 300 is 1500

Source: https://kchemekg.wordpress.com/ekg-test-home/