What does a 5 x 5 mm square measure in seconds on an EKG?

• A. 0.20
• B. 0.04
• C. 0.50
• D. 1.00

Answer: (A)

Time on an ECG is read along the horizontal axis, and with standard paper speed of 25 mm per second, each small square (1 mm) represents 0.04 seconds. A large square measures 5 mm by 5 mm, which means it spans five small squares across horizontally. So its time width is 5 × 0.04 = 0.20 seconds. The vertical size of the square doesn’t affect the time measurement. Therefore, a 5x5 mm square corresponds to 0.20 seconds on the tracing. The other values would correspond to different counts of small squares across the paper (for example, 0.04 s is just one small square, 0.50 s is about 12.5 small squares, and 1.00 s is 25 small squares).