When assessing an ECG tracing, you want to quickly determine whether it's normal or if there might be something wrong. One of the first things to spot at aglance is the pace. The heart rate (HR) can be determined easily using different strategies, not requiring the use of calipers or an ECG ruler.
1) On a standard 25mm/s ECG strip, count all the R-waves and multiply by 6. In the example given below there are nine QRS-complexes throughout the strip (9x 6 = 54 beats per minute (bpm)). However, when taking a closer look at the beginning of the strip the T-wave of the prior QRS-complex is captured, so you almost see 10 QRS complexes. 10x6 = 60 bpm.
2) Look at the R-R interval and count the big boxes (5mm). On a standard 25mm/s strip, if the R-R interval equals 1 big box the HR is ..300 bpm. 2 boxes equal 150bpm, and so on (see figure below).
3) Look at the R-R interval and count the big (5mm) and small boxes (1mm). On a standard 25mm/s ECG strip each large box equals 200ms, each small box equals 40ms. Now calculate 60.000 / R-R interval in ms. That will give you the beat-to-beat heart rate of the R-R interval that you have measured. In this case 60.000/1140 = 53 bpm.