How to read & understand EKG strips

Updated July 19, 2017

Reading an electrocardiogram (EKG) is a skill that requires education and attention. With the proper amount of study and a dedication to dig in and use comparative analysis between several different iterations of the same cardiac rhythm, most anyone who can distinguish repetitive patterns can eventually learn how to read EKG tracings. As with many skills that depend on rote memorisation, remembering the rules of the basic wave forms and where they originate in the heart is a good starting point for beginners.

If you have EKG tracing paper, an EKG strip or the ability to print a strip of EKG paper from the Internet, please ready this item.

Locate the boxes with bold lines and borders on the EKG strip. Each of these boxes measures 0.20 milliseconds. Record this information and all numbers and measurements that follow on your notepaper.

Notice smaller boxes within each bold box. Each of these small boxes measures 0.04 milliseconds.

Locate the tic marks above and below the boxes on the EKG strip. Each of these measures one full second. You will notice five bold bordered boxes within the one second.

Obtain a 6-second EKG strip for use with interpretation by counting out the 1-second tic marks.

Assuming that the EKG strip you have is normal and regular, locate the first bump of on the strip before the complex that resembles a V shape. This first positive (upward) deflection is called a P-wave.

Locate the peaked, larger complex that is shaped like an upside down letter V.

Using your ruler, measure the length of where the P-wave starts by locating the box that begins to rise and make a bump. Measure until you reach the box closest to where the peaked complex deflects down. If no downward deflection is present, measure to the box where the peaked complex wave begins to rise.

Count the big boxes between these two measurement points. If only one big box or fewer is present, count how many little boxes are located between the two measured points. Follow this rule for any further wave formations.

Add the total number of boxes between the measured areas. This measurement is called the PRI, or P-wave-to-R-wave interval.

Locate the P-wave and find where the waveform begins to slope back down to the flat position before the upside-down V complex.

Locate where the complex makes a little divot before it slopes up into the peaked R-wave. If you see no divot before the complex, measure to the block that is closest to the positive deflection of the R-wave.

Count the boxes within this space. This measurement is called the PR segment.

Record the measurements that come between where the P-wave sloped up and the R-wave peaks is the section of the EKG strip that signifies the initiation of the heartbeat.

Make a notation for future reference: the P-wave begins to initiate a cycle known as polarity. It charges up the heart for the contraction. After the QRS (contraction), the heart depolarises until the next P-wave comes again.

Locate the tallest wave peak in the middle of the complex where you ended when measuring the PRI value.

Understand that this cluster of wave forms is referred to as the QRS complex. The tall middle wave is the R-wave.

Find the next R-wave in the next QRS complex on the EKG strip.

Count the number of bigger, bolder boxes between the peaks of the two R-waves.

Record the value (sum of boxes) you counted between the two R-wave peaks. This is known as the R-to-R interval.

Locate the next upward bump on the EKG strip after the QRS complex. This bump is the T-wave.

Go back and measure the width of the QRS complex. To do this, begin to measure the blocks from where the QRS either drops and then spikes upward. Or, if this little divot is not present, measure from where the R-wave begins to rise.

Measure from either start point in the previous step until the R-wave begins to fall back and makes a negative (downward) deflection. If the R-wave ends in a divot, stop measuring at the divot. If there is no divot, measure until the R-wave flattens out to a straight line before the T-wave deflects upward. This measurement will be the width of the QRS complex.

Measure the length in boxes from the beginning of the R-wave until the T-wave falls and is no longer bump-shaped. The T-wave may also look like a peaked wave instead of a bump. This measurement is called the Q-T interval.

Measure the bold and/or smaller boxes from where the QRS returns to a flat line on the right side of the R-wave, or until it makes another little divot until the T-wave begins to rise up and form a bump. The amount of boxes from the R-wave or the divot, which is called the S-wave, is a measurement known as the ST segment.

Refer to the value you measured and found as the PR interval. To be considered within normal limits, the PRI cannot be longer than 0.20 seconds (five little blocks or one bold block).

Refer to the value you measured and found as the PR segment. To be considered within normal limits, the PR segment cannot be longer than 0.12 seconds, or three little blocks.

Refer to the value you measured and found as the QRS complex. To be considered within normal limits, the QRS complex cannot be wider than 0.12 seconds, or three little blocks.

Refer to the value you measured and found as the ST segment. To be considered within normal limits, the ST segment cannot be longer than 0.12 seconds, or three little blocks.

Refer to the value you measured and found as the QT interval. To be considered within normal limits, the QT interval cannot be longer than 0.42 seconds (10 1/2 little blocks, or two bold blocks plus one little block).


Calipers are generally easier to use in taking measurements than a ruler. If you do not have a pre-existing heart tracing, you can use either EKG tracing paper or common graph paper to draw the waveforms. Applying the measurements in this article is easier if you have a pre-drafted P-, QRS- and T-wave diagram.


The measurements in this article are not meant to substitute the reading of a cardiology text, the advice and/or diagnosis of a medical physician, or as a means of self-diagnosis in the absence of medical staff well-versed in electrocardiogram interpretation. If you suspect you have a heart problem of any kind, seek the services of a health care professional. The measurements in this article are based on a healthy heart in normal sinus rhythm.

Things You'll Need

  • Pre-existing EKG tracing
  • EKG tracing paper
  • Graph paper
  • Diagram of cardiac wave formations
  • Ruler or calipers
  • Pen
  • Notebook paper
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About the Author

Michael Paul Maupin was an Indiana certified paramedic for 10 years, and coordinated an electrophysiology clinic at University of Louisville Healthservices for two years. He is now a college composition instructor and freelance writer, with work published in "Emergency Medical Services" magazine. Maupin holds a Bachelor of Arts in English and a Master of Liberal Studies, both from Indiana University.