ECG Boxes to Seconds Calculator

Why Convert ECG Boxes to Seconds?

On standard ECG paper, horizontal distance between two points represents time. Clinicians often count grid boxes with calipers when measuring PR intervals, QRS width, or other segments. Converting those box counts to seconds or milliseconds makes comparison with reference ranges easier. Pair this with the ECG Heart Rate Calculator and Max Heart Rate Calculator for full strip review.

ECG Paper Timing

Each small box is 1 mm wide. At 25 mm/s paper speed, one small box equals 0.04 seconds (40 ms). A large box spans five small boxes, so it equals 0.2 seconds (200 ms). When paper speed is 50 mm/s, divide those values by two: 0.02 s per small box and 0.1 s per large box.

Formula

Duration equals the number of boxes multiplied by the duration of one box at the selected paper speed:

$$ \text{duration (s)} = \text{boxes} \times \text{box duration (s)} $$

Multiply the result by 1000 to express the interval in milliseconds.

Common Reference Values

P wave: about 80 ms
PR interval: 120 to 200 ms
QRS complex: 80 to 120 ms
First-degree AV block: PR interval greater than 200 ms
WPW pattern: PR interval often under 120 ms

Frequently Asked Questions

How long is one small ECG box at 25 mm/s?

One small box (1 mm) equals 0.04 seconds, or 40 milliseconds, when paper speed is 25 mm/s.

How many seconds is one large box?

A large box covers 5 mm. At 25 mm/s that is 0.2 seconds (200 ms). At 50 mm/s it is 0.1 seconds (100 ms).

Why does paper speed matter?

Faster paper speed compresses the same time into fewer millimeters. You must match the speed setting on the ECG machine to convert boxes correctly.

Can I use this for PR interval measurement?

Yes. Count boxes from the start of the P wave to the start of the QRS complex, then convert to milliseconds and compare with the 120 to 200 ms normal range.

Does this replace clinical interpretation?

No. This tool only converts measured distance to time. Diagnosis requires full ECG review by a qualified clinician.

Report