What is the duration of one small box on an ECG?

One small box on an ECG paper represents a duration of 0.04 seconds. This is a fundamental measurement used in interpreting ECGs, as the paper speed is standardized at 25 millimeters per second. Each small box, which is 1 millimeter in height and width, corresponds to this time interval. Understanding this duration is crucial for measuring intervals such as PR, QRS, and QT, where accurate timing is essential for diagnosing dysrhythmias. The correct identification of time and intervals helps healthcare professionals evaluate the heart's electrical activity effectively.