ECG Boxes to Time
Enter the number of small (1 mm) and large (5 mm) boxes measured on the ECG tracing. Select the paper speed (usually 25 mm/s) to convert to seconds and milliseconds.
Seconds
0s
Milliseconds
0 ms
Paper speed
25 mm/s
At 25 mm/s, one small box equals 0.04 seconds (40 ms), and one large box (5 small boxes) equals 0.2 seconds.
How to Use This Calculator
Measure boxes on the ECG
Count small 1 mm boxes and large 5 mm boxes corresponding to your interval (e.g., PR, QRS, QT, RR).
Select paper speed
Standard paper speed is 25 mm/s. Some monitors use 50 mm/s—switch if applicable.
Review interval duration
The calculator converts the total boxes into seconds and milliseconds to aid interpretation.
Formula
Time (s) = (Small boxes + 5 × Large boxes) ÷ Paper speed (mm/s)
At 25 mm/s: 1 small box = 0.04 s, 1 large box = 0.20 s. At 50 mm/s: 0.02 s and 0.10 s respectively.
Full Description
ECG paper speed defines the relationship between distance and time. At the standard 25 mm/s, each small box equals 40 ms and each large box equals 200 ms. Accurate measurement of ECG intervals (PR, QRS, QT, RR) is essential for diagnosing conduction abnormalities, arrhythmias, and repolarisation disorders.
Frequently Asked Questions
What if my ECG uses 50 mm/s?
Select the 50 mm/s option. Each small box then represents 20 ms and each large box 100 ms.
How do I measure fractional boxes?
Estimate partial boxes visually; enter decimals (e.g., 2.5 small boxes) for improved precision.
Can this help measure heart rate?
Yes. Convert RR interval boxes to seconds, then HR = 60 / RR seconds.
Does the tool account for calibration errors?
It assumes correct calibration. Verify ECG recording calibration (1 mV = 10 mm, paper speed noted on strip).