ECG Basics · Lesson 4 · Reading the paper
The grid and calibration
Every measurement on an ECG relies on two settings: paper speed and gain. Check them first, then the grid turns into a ruler.
Time runs left to right
At the standard speed of 25 mm/s:
- One small box (1 mm) = 0.04 s (40 ms).
- One large box (5 mm) = 0.20 s (200 ms).
- Five large boxes = 1 second; a 10-second strip has 50 large boxes.
Voltage runs bottom to top
At the standard gain of 10 mm/mV, one small box = 0.1 mV and two large boxes = 1 mV. Many machines print a calibration pulse at the start: a rectangle 10 mm tall (1 mV) and 5 mm wide.
When the settings change
| Setting | Effect | Why it is used |
|---|---|---|
| 50 mm/s | 1 small box = 20 ms; everything looks twice as wide | Spreading out fast rhythms or fine detail |
| 12.5 mm/s | 1 small box = 80 ms; complexes look crowded | Long rhythm overviews |
| 5 mm/mV | Half height; calibration pulse 5 mm tall | Very tall QRS complexes |
| 20 mm/mV | Double height | Small complexes |
If a tracing looks oddly fast, slow, tall or flat, check the speed and calibration labels before calling anything abnormal.
Here is a real-scale strip from the engine: count the large boxes between two R waves and you already have the rate (next lessons).
The live simulator lets you switch between 12.5, 25 and 50 mm/s and between 5, 10 and 20 mm/mV while the same rhythm keeps running. Open Settings → Display to try it.
See it move in the live simulatorOpen with normal sinus rhythm
References
- Kligfield P, et al. Recommendations for the standardization and interpretation of the electrocardiogram, Part I: the electrocardiogram and its technology. Circulation 2007;115:1306–1324.