ECG Rate Interpretation

Understanding paper speeds
  • Paper output speed is the rate at which the ECG machine produces a trace
  • Standard output is 25mm per second
  • If a different paper speed is used, standard rate calculations will have to be modified appropriately (see other examples below)
The standard paper speed is 25mm/sec:
  • 1mm (small square) = 0.04 sec (40ms)
  • 5mm (large square) = 0.2 sec (200ms)
ECG RATE 25 mm sec standard paper speed

Estimate the rate

At a paper speed of 25 mm/second
  • 1 SMALL square = 0.04 seconds
  • 5 SMALL squares = 1 LARGE square = 0.2 seconds
  • 5 LARGE squares = 1 second
  • ECG rhythm strip:
    • = 250 SMALL squares = 50 LARGE squares = 10 seconds
  • To calculate beats per minute (bpm):
    • 1500 SMALL squares = 300 LARGE squares = 1 minute
There are multiple methods to estimate the rate:

We can calculate the beats per minute (bpm) by dividing 1500 by the number of SMALL squares between two R waves (R-R interval = one beat)

We can calculate the beats per minute (bpm) by dividing 300 by the number of LARGE squares between two R waves (R-R interval = one beat)

REGULAR rhythms

  • Rate = 300 / number of LARGE squares between consecutive R waves.

Very FAST rhythms:

  • Rate = 1500 / number of SMALL squares between consecutive R waves.

SLOW or IRREGULAR rhythms:

  • Rate = Number of R waves X 6
  • The number of complexes (count R waves) on the rhythm strip gives the average rate over a ten-second period. This is multiplied by 6 (10 seconds x 6 = 1 minute) to give the average Beats per minute (bpm)

Example of 1500 (small squares) versus 300 (large square) method

ECG RATE calculation 300 1500 rules 25 mm sec
1500 (small square) and 300 (large square) methods

Now adding the R wave (10 second rhythm strip)

ECG RATE rhythm strip 10 seconds 300 1500 rules ECG


  • Calculate atrial and ventricular rates separately if they are different (e.g. complete heart block)
  • The machine reading can also be used and is usually correct — however, it may occasionally be inaccurate in the presence of abnormal QRS/T-wave morphology, e.g. may count peaked T waves as QRS complexes or miss QRS complexes with reduced amplitude.

Interpretation (adults)

  • Normal: 60–100 beats/min
  • Tachycardia: >100 beats/min
  • Bradycardia: <60 beats/min

Normal Heart Rates in Children

  • Newborn: 110 – 150 bpm
  • 2 years: 85 – 125 bpm
  • 4 years: 75 – 115 bpm
  • 6 years+: 60 – 100 bpm

Other paper speeds: 50mm/sec

Doubling the standard rate will cause the ECG to appear drawn out or wider complex than 25mm/sec paper speeds

  • 1mm (small square) = 0.02 sec (20ms)
  • 5mm (large square) = 0.1 sec (100ms)
ECG RATE 50 mm sec ECG measurement RATE

Why use 50 mm/second?

Doubling the standard rate can reveal subtle ECG findings hidden at the slower rates, in particular atrial flutter waves in a 2:1 block:

ECG RATE 25 versus 50 mm sec show p waves flutter
At 50mm/sec, P waves are more clearly discernible in a 2:1 ratio to QRS complexes

C. Paper speed: 10mm/sec

  • 1mm (small square) = 0.1 sec (100ms)
  • 5mm (large square) = 0.5 sec (500ms)


Advanced Reading



LITFL Further Reading




Emergency physician MA (Oxon) MBChB (Edin) FACEM FFSEM with a passion for rugby; medical history; medical education; and asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books | Twitter |

MBBS (UWA) CCPU (RCE, Biliary, DVT, E-FAST, AAA) Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor-in-chief of the LITFL ECG Library. Twitter: @rob_buttner


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