ECG Boxes to Seconds Calculator
Convert ECG box measurements to time duration in seconds and milliseconds. Enter number of boxes, box type, and paper speed for instant PR interval, QRS, and QT interval durations.
About This Calculator
The ECG Boxes to Seconds Calculator helps healthcare professionals, medical students, and cardiology enthusiasts convert lengths of ECG intervals and segments measured in boxes to their durations in seconds or milliseconds. This is essential for interpreting electrocardiograms and diagnosing cardiac conditions such as first-degree heart block, Wolf-Parkinson-White syndrome, bundle branch blocks, and QT prolongation.
On a standard ECG printout, each small box measures 1 mm in width, and each large box (comprising 5 small boxes) measures 5 mm. At the standard paper speed of 25 mm/s, one small box represents 0.04 seconds (40 ms) and one large box represents 0.2 seconds (200 ms). At a faster speed of 50 mm/s, these values are halved: 0.02 seconds (20 ms) per small box and 0.1 seconds (100 ms) per large box. The formula used is: duration = number of boxes × duration per box.
Normal ECG Values
- P wave: 80 ms (approximately 2 small boxes at 25 mm/s)
- PR interval: 120-200 ms (3-5 small boxes at 25 mm/s)
- QRS complex: 80-100 ms (2-2.5 small boxes at 25 mm/s)
- ST segment: 80-120 ms (2-3 small boxes at 25 mm/s)
- T wave: 160 ms (4 small boxes at 25 mm/s)
- QT interval: Varies with heart rate — use a QTc correction formula
How to Measure ECG Boxes
Place one measuring tip of a caliper at the first point (e.g., beginning of P wave) and the other tip at the second point (e.g., beginning of QRS complex). Without changing the angle, place the first tip at the beginning of an ECG box and count the number of boxes to the second tip. Enter that count into the calculator along with the box type and paper speed to get the duration in seconds and milliseconds.
Frequently Asked Questions
How long is one small ECG box at 25 mm/s?
At a standard paper speed of 25 mm/s, one small ECG box (1 mm) represents 0.04 seconds or 40 milliseconds.
How long is one large ECG box at 25 mm/s?
At a standard paper speed of 25 mm/s, one large ECG box (5 mm) represents 0.2 seconds or 200 milliseconds.
How do paper speed settings affect ECG box duration?
At 25 mm/s, a small box equals 0.04 s and a large box equals 0.2 s. At 50 mm/s, values are halved: a small box equals 0.02 s (20 ms) and a large box equals 0.1 s (100 ms).
What is the normal PR interval on an ECG?
The normal PR interval duration is 120 to 200 milliseconds. A PR interval longer than 200 ms may indicate first-degree heart block, while shorter than 120 ms may suggest Wolf-Parkinson-White syndrome.
What is the normal QRS complex duration?
The normal QRS complex duration is 80 to 100 milliseconds (80-120 ms according to some sources). A widened QRS complex may indicate ventricular conduction delays, bundle branch blocks, or other cardiac pathologies.
How many small ECG boxes equal 1 second at 25 mm/s?
At 25 mm/s, one second equals 25 small boxes (25 × 0.04 s) or 5 large boxes (5 × 0.2 s). Each large box contains 5 small boxes.
How do I measure the QT interval on an ECG?
Measure from the beginning of the QRS complex to the end of the T wave. Count the number of boxes and use this ECG boxes to seconds calculator to convert to milliseconds. The QT interval should be corrected for heart rate using a formula like QTc = QT / √(RR interval in seconds).
What is the normal ST segment duration?
The normal ST segment duration is 80 to 120 milliseconds. ST segment abnormalities such as elevation or depression can indicate myocardial ischemia or injury and require immediate medical evaluation.