📟 ECG Boxes to Seconds Calculator
Convert ECG box measurements to time in seconds and milliseconds. Supports both small and large boxes at standard paper speeds of 25 mm/s and 50 mm/s.
📊 ECG Conversion Result
How to Measure ECG Boxes
The best way to measure the number of ECG boxes between two points on an electrocardiogram is by using a caliper:
- Place one measuring tip at the first point (e.g., beginning of P wave).
- Put the other measuring tip at the second point. Keep the first tip in position.
- Without changing the angle, pick up the caliper and place the first tip at the beginning of any ECG box.
- Position the other tip to create a horizontal line between them.
- Count the number of boxes between the measuring tips.
How to Convert ECG Boxes to Seconds
A small ECG box measures 1 mm, while a large box has a length of 5 mm. They represent different duration times depending on the paper speed:
| Box Type | Size | @ 25 mm/s | @ 50 mm/s |
|---|---|---|---|
| Small box | 1 mm | 0.04 s (40 ms) | 0.02 s (20 ms) |
| Large box | 5 mm | 0.20 s (200 ms) | 0.10 s (100 ms) |
Normal ECG Values for Waves and Intervals
| Wave / Interval | Normal Duration |
|---|---|
| P wave | 80 ms |
| PR interval | 120–200 ms |
| QRS complex | 80–120 ms |
| ST segment | 80–120 ms |
| T wave | 160 ms |
| ST interval | 320 ms |
| QT interval | Varies (correct for HR) |
PR Interval
The PR interval represents the depolarization of atria and the conduction of signal from atria to ventricles through the atrioventricular (AV) node and the bundle of His. Its normal range is 120–200 ms.
To measure the PR interval, place one caliper tip at the beginning of the P wave and the other at where the QRS complex starts. If a Q wave is present, measure to the beginning of the Q wave.
First-Degree Heart Block
The PR interval is prolonged in first-degree heart block, where the electrical signal is conducted at a lower speed than usual. It is diagnosed when the PR interval exceeds 200 ms.
Wolf-Parkinson-White Syndrome
One cause of PR interval shortening is WPW syndrome, where an additional connection between atria and ventricles bypasses the AV node. This causes earlier ventricular depolarization, and the PR interval typically lasts less than 120 ms.
QRS Complex and QT Interval
Other routinely measured durations include the QRS complex and QT interval. Causes of QRS widening include:
- High blood potassium (hyperkalemia)
- Ventricular tachycardia
- Idioventricular rhythm
- Drugs (e.g., tricyclic antidepressants)
- Wolf-Parkinson-White syndrome
- Bundle branch blocks
- Premature ventricular contraction (PVC)
FAQs
How many small boxes make a large box?
A large ECG box contains 5 small boxes (5 mm ÷ 1 mm = 5).
What paper speed is standard?
The standard paper speed is 25 mm/s. A speed of 50 mm/s is sometimes used in pediatric ECGs or when finer time resolution is needed.