What is the duration of a standard small box on ECG paper?

• A. 0.04 seconds
• B. 0.2 seconds
• C. 0.1 seconds
• D. 0.02 seconds

Answer: (A)

Understanding how time is encoded on ECG paper helps you read these traces quickly. At the standard paper speed of 25 millimeters per second, each small box is 1 millimeter wide. Time per small box is 1 mm divided by 25 mm/s, which equals 0.04 seconds. A large box, made up of five small boxes, therefore represents 5 × 0.04 = 0.20 seconds. This scaling lets you estimate intervals and heart rate by counting boxes.

So, the duration of a standard small box is 0.04 seconds. The other durations correspond to different box sizes (like a large box) or do not align with the standard small-box timing.