When taking your pulse, you count 18 beats in 15 seconds. Calculate the time period of your heart rate in seconds.
You will have to multiply 18 which is the number of beats in 15 seconds by 4 to get 72 beats per minute. To measure the heart rate, you need to count the number of pulses in a 10 second time period using your stopwatch or clock.
18 divided by 15 gives you 1.2 beats per second.