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ECG Heart Rate Calculator

Calculate heart rate from an ECG: the R-R interval, the number of large or small boxes between R waves, the number of R waves on a strip, or the distance in millimetres.

Formula

HR=60RR(RR in seconds)\mathrm{HR} = \dfrac{60}{\mathrm{RR}}\quad(\mathrm{RR}\ \text{in seconds})
RR=nboxes×5 mmpaper speed\mathrm{RR} = \dfrac{n_{\text{boxes}}\times 5\ \mathrm{mm}}{\text{paper speed}}
HR=300nlarge boxesat 25 mm/s\mathrm{HR} = \dfrac{300}{n_{\text{large boxes}}}\quad\text{at } 25\ \mathrm{mm/s}
HR=nR−1duration×60\mathrm{HR} = \dfrac{n_{\mathrm{R}} - 1}{\text{duration}}\times 60
HR\mathrm{HR}
heart rate, in beats per minute
RR\mathrm{RR}
the interval between two R waves, in seconds
paper speed\text{paper speed}
the speed the trace is recorded at, 25 mm/s as standard

How it works

The heart rate is the number of beats in a minute, which is 60 divided by the time between two beats. On standard ECG paper at 25 mm/s a large box (5 mm) is 0.2 s and a small box (1 mm) 0.04 s, which gives the familiar shortcuts: 300 divided by the number of large boxes, or 1,500 divided by the number of small boxes.

For an irregular rhythm a single interval is not representative. Counting the R waves over a longer strip and dividing by its duration gives an average rate; the tool counts the intervals between them, so the first R wave opens the first interval rather than counting as a beat of its own.

Worked example

There are 4 large boxes between two R waves at 25 mm/s.

  1. RR = 4 × 5 mm / 25 mm/s = 0.8 s.
  2. HR = 60 / 0.8 = 75 beats/min.

The heart rate is 75 beats per minute, and equally 300 / 4.

These are the values the calculator opens with, so you can check its output against this example.

Assumptions

  • A regular rhythm for the interval methods.
  • The paper speed and calibration are as stated.

Common mistakes

  • Using the 300 shortcut on a recording made at 50 mm/s, which halves the interval in boxes.
  • Taking one interval of an irregular rhythm as the rate.
  • Counting the R waves on a strip and dividing by the duration without allowing for the first wave.

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Related equipment

Service documentation, failure modes and parts for the instruments this calculation is used with.