Count the number of large squares between two consecutive R waves
Divide 300 by the number of large squares
Count the number of small squares between two consecutive R waves
Divide 1500 by the number of small squares
Count the number of QRS complexes in a 6-second strip
Multiply that number by 10
Use the formula Heart rate = 60 / R-R interval in seconds
Measure the R-R interval in seconds
Convert the interval to beats per minute using 60 divided by the interval
For irregular rhythms, use the 6-second method
For regular rhythms, use the large-box or small-box method