Identify two consecutive R waves on the ECG
Measure the distance between them in large boxes
Use the 300 rule: 300 ÷ number of large boxes between R waves
Use the 1500 rule: 1500 ÷ number of small boxes between R waves
For irregular rhythms, count the number of QRS complexes in 6 seconds and multiply by 10
For a 10-second strip, count all QRS complexes and multiply by 6
Confirm the result by checking the ECG paper speed
Use 25 mm/s or 50 mm/s calibration as applicable