What does each small box on an ECG strip represent in seconds?

Each small box on an ECG strip represents 0.04 seconds. This is based on the standard grid used on ECG paper, where the paper moves at a speed of 25 millimeters per second. Since the ECG graph is divided into small boxes that measure 1 millimeter in height and width, and at this speed, each small box corresponds to 0.04 seconds of time.

To break it down further, the larger boxes, which are composed of five small boxes, represent 0.2 seconds (0.04 seconds multiplied by 5). This understanding is critical for accurately calculating the heart rate and determining other time-dependent intervals on an ECG, such as PR interval and QT interval. Knowing the time each small box represents is essential for interpreting the rhythm and function of the heart as displayed on the ECG.