How many seconds are represented by 1 small box (1mm) on an ECG?

In standard electrocardiogram (ECG) interpretation, each small box on the ECG paper represents a specific duration of time. The ECG paper is typically printed at a speed of 25 millimeters per second. Since each small box is 1 millimeter in width, you can determine the time it represents by dividing the time taken for the paper to move by the number of boxes that fit into that time frame.

Since the paper moves at a speed of 25 mm per second, and there are 25 small boxes in this 1 second interval, it follows that each small box corresponds to 1/25 of a second. Mathematically, this translates to 0.04 seconds for 1 small box (1 mm).

Understanding this is crucial for accurately interpreting the rhythm and timing of heartbeats on an ECG, as it allows for precise measurement of intervals between different electrical events in the cardiac cycle.