A heart rate can be calculated from a six-second segment of an EKG strip by counting the number of R waves in the section and multiplying by 10, according to RnCeus.com. Each small square on an EKG strip's horizontal axis represents 0.04 seconds, so a six-second segment is 150 squares long.
The six-second method is best used for regular heart rates, according to Nurses Learning Network. For more irregular hearts, a longer period is used and the full QRS wave observed, which is represented by the highest peak in the trace. One way to do this is to take two separate six-second traces, count the number of waves in both and use a multiplier of five to find the number of beats in a minute. These calculations only work if the EKG is run at the standard speed of 25 millimeters per second; any amendments in the speed need to be accounted for when calculating the heart rate.
An alternate method of calculating heart rate from the standard six-second printout is to count the number of squares between consecutive R waves. For standard rhythms, this is 300 divided by the number of large squares between the waves. For fast rhythms, the calculation is 1,500 divided by the number of small squares in the gap, as stated by Life in the Fastlane.