In the small-box method, how is heart rate estimated?

• A. Count the number of small boxes between two consecutive R waves and divide into 1500
• B. Count the number of large boxes between two consecutive R waves and divide into 300
• C. Count the number of RR intervals in 1 minute and multiply by 1
• D. Count the number of P waves between two consecutive R waves and divide into 1000

Answer: (A)

Estimating heart rate from an ECG strip relies on how long the RR interval lasts, measured in small boxes on standard paper. On 25 mm/s paper, each small box represents 0.04 seconds, and there are 1500 small boxes in one minute. If the RR interval spans N small boxes, the heart rate is 1500 divided by N (beats per minute). So counting the small boxes between consecutive R waves and dividing into 1500 gives the heart rate.

Counting large boxes between R waves is a related but separate method (using 300 divided by the number of large boxes). The other option involves counting RR intervals in a minute, which isn’t the small-box approach.