Identify the start of the QRS complex at the first deflection from the baseline
Identify the end of the QRS complex at the point where the waveform returns to the baseline or where the S wave ends
Use the ECG paper speed to determine time
At 25 mm/s, each small box equals 0.04 seconds
Count the number of small boxes from the start to the end of the QRS complex
Multiply the number of small boxes by 0.04 seconds
Measure in the lead where the QRS complex is widest
If the QRS is less than 3 small boxes, it is under 0.12 seconds
If the QRS is 3 small boxes or more, it is 0.12 seconds or longer