How long is a large box on ECG paper?

• A. 0.04 seconds
• B. 0.2 seconds
• C. 0.5 seconds
• D. 1.0 seconds

Answer: (B)

Time intervals on ECG paper are read along the horizontal axis. A small box represents 0.04 seconds, and five adjacent small boxes make one large box. Multiplying 0.04 by five gives 0.2 seconds, so a large box lasts 0.2 seconds. This aligns with the standard paper speed of 25 mm per second, where one second spans five large boxes. This quick reference helps you estimate intervals and heart rate directly from the tracing.