To interpret ECG tracings one must first understand their natural habitat: a monotonous grid of orderly red squares. This will be important later when understanding rate, rhythm and ischemia.
The small red boxes on an ECG are 1 mm x 1 mm. The larger boxes with thicker lines are 5 mm x 5 mm.
The vertical axis of an ECG is the amplitude of the ECG waveform. Technically, it is a measure millivolts (mV), however you will often see things written in terms of mm (like “1 mm of ST elevation”).
With normal settings:
10 mm (two big boxes) = 1mV
1 mm (one small box)= 0.1 mV
The horizontal axis of an ECG is time. Older machines placed ink directly on moving paper like a seismograph. The standard “paper speed” that you will see on almost all ECGs is 25 mm/sec.
Therefore, on normal settings:
1 mm (small box) = 0.04 seconds and each 5 mm (big box) is 0.2 seconds.
Because time = distance/speed: 1 mm / 25 mm/sec = 0.04 sec. Likewise, 5 * 0.04 = 0.2
It follows then that:
5 big boxes = 1 second and an ECG tracing, being 50 large boxes long, lasts 10 seconds.
Minor Details / Caveats: The vast majority of ECGs will have these default scales, although you may get burned by the occasional ECG machine that has had the settings adjusted (sometimes to help tease out a rhythm). Both the paper speed (which affects the horizontal axis) and the mm/mV ratio will be displayed at the bottom of the ECG. Bear in mind that if the paper speed is slowed down to 10 mm/sec, 1 mm (a small box) of horizontal distance becomes 0.1 seconds and 5 mm (a large box) becomes 0.5 seconds. Likewise, if the mm/mV ratio is changed to 10/1 to 20/1, you will see the tracing amplitude in mm doubled even though the mV are the same.
To summarize, the most important things to remember are:
1 small box of (1 mm) horizontal distance = 0.04 seconds
1 large box (5 mm) = 0.2 seconds
5 large boxes = 1 second
Entire ECG = 10 seconds
1 small box vertical distance = 1mm = 0.1 mV