Measuring an EKG strip starts with understanding the grid. Every EKG prints on standardized paper where the horizontal axis represents time and the vertical axis represents voltage. Once you know what each square means, you can calculate heart rate, measure intervals, and identify abnormalities with nothing more than the strip itself and a pair of calipers.
What the Grid Lines Mean
EKG paper runs at a standard speed of 25 mm per second. Each small square on the grid is 1 mm wide and represents 0.04 seconds (40 milliseconds). Five small squares make up one large square, which is 5 mm wide and represents 0.2 seconds. This means five large squares equal exactly one second, and 30 large squares span a full 6-second strip.
On the vertical axis, the machine is calibrated so that 1 millivolt of electrical activity produces 10 mm (1 cm) of deflection. Each small square vertically equals 0.1 millivolt. You’ll usually see a calibration box printed at the beginning of the strip: a rectangular signal that should be exactly 10 mm tall. If it isn’t, the gain has been adjusted and your voltage measurements need to account for that.
Checking Rhythm Regularity
Before measuring anything else, determine whether the rhythm is regular or irregular. Find the tall, sharp peaks on the strip. These are the R waves, the most prominent part of the QRS complex. Measure the distance between two consecutive R waves. This is the R-R interval.
The easiest way to check regularity is with calipers or a simple piece of paper. Place one caliper point on an R wave and the other on the next R wave to fix the distance. Then walk the calipers across the strip, comparing that fixed distance to each successive R-R pair. If the points consistently land on R waves, the rhythm is regular. If they drift off, the rhythm is irregular. Without calipers, you can mark two R waves on the edge of a piece of paper and slide it across the strip the same way.
Three Ways to Calculate Heart Rate
The 1500 Method (Most Precise)
Count the number of small squares between two consecutive R waves. Divide 1500 by that number. For example, if there are 20 small squares between R waves, the heart rate is 1500 ÷ 20 = 75 beats per minute. This works because there are 1500 small squares in one minute of EKG paper at standard speed. This method is the most accurate for regular rhythms.
The Large-Box Method (Quick Estimate)
Count the number of large squares between two R waves and divide 300 by that number. If there are 4 large squares, the rate is 300 ÷ 4 = 75 beats per minute. An even faster version is the “count off” sequence: memorize the numbers 300, 150, 100, 75, 60, 50. Find an R wave that lands on a dark gridline, then count large squares to the next R wave. One large square away means a rate of 300. Two means 150. Three means 100. Four means 75. Five means 60. Six means 50.
The 6-Second Method (Best for Irregular Rhythms)
When the rhythm is irregular, single R-R intervals aren’t representative. Instead, count 30 large squares across the strip (that’s 6 seconds). Count the number of R waves in that span and multiply by 10 to get the rate per minute. This averages out the variation and gives a more reliable number for rhythms like atrial fibrillation.
Measuring the PR Interval
The PR interval reflects the time it takes for an electrical signal to travel from the upper chambers of the heart through the conduction system to the lower chambers. To measure it, find the very beginning of the P wave, the small rounded bump before the QRS complex. Place one caliper point there. Place the other at the very start of the QRS complex, whether that begins with a Q wave or goes straight into the R wave. The distance between those two points is your PR interval.
A normal PR interval is 0.12 to 0.20 seconds, which translates to 3 to 5 small squares on the grid. A PR interval shorter than 3 small squares suggests the signal is bypassing the normal conduction pathway. A PR interval longer than 5 small squares indicates a delay, which is the hallmark of first-degree heart block.
Measuring the QRS Duration
The QRS complex represents the electrical activation of the heart’s lower chambers. Measure from the very first deflection of the Q wave (or R wave if no Q is present) to the end of the S wave, where the tracing returns to baseline. Normal QRS duration is 0.04 to 0.10 seconds, or 1 to 2.5 small squares wide.
A QRS wider than 0.12 seconds (3 small squares) is considered abnormally wide and can indicate a bundle branch block or other conduction problem. When measuring, look at multiple leads on a 12-lead EKG and use the widest QRS you find, since some leads may not show the full beginning or end of the complex.
Measuring the QT Interval
The QT interval captures the full cycle of the lower chambers activating and then resetting. Measure from the start of the QRS complex to the end of the T wave. This can be tricky because the T wave often blends gradually into the baseline rather than ending sharply.
The QT interval changes with heart rate: faster rates produce shorter QT intervals, and slower rates produce longer ones. To account for this, clinicians use a corrected QT (QTc). The simplest practical method adds a correction factor based on heart rate. Normal QTc is up to 0.45 seconds in men and 0.46 seconds in women. A prolonged QTc increases the risk of dangerous heart rhythms, so getting this measurement right matters.
Evaluating the ST Segment
The ST segment is the flat stretch between the end of the QRS complex and the beginning of the T wave. To measure it, first find the J-point, which is the exact spot where the QRS complex ends and the ST segment begins. Then compare the height of the ST segment at the J-point to a baseline reference.
The best baseline reference is the PR segment, the flat line between the end of the P wave and the start of the QRS. Some sources recommend using the TP segment (the flat line between the T wave and the next P wave), but at faster heart rates the P wave often overlaps the T wave, making the TP segment impossible to identify. The PR segment is more reliably present.
Any displacement above baseline is ST elevation; any displacement below is ST depression. The thresholds that matter clinically are remarkably small. In some leads, a deviation as low as 0.5 mm (half a small square) can be significant depending on the patient’s age, sex, and which lead you’re looking at. This is why baseline stability is so important. A wandering baseline caused by patient movement, breathing, or poor electrode contact can shift the entire tracing up and down, making it nearly impossible to judge true ST changes. If the baseline is drifting, the strip needs to be repeated with better electrode placement and a still patient before you can trust ST measurements.
Putting It All Together
A systematic approach prevents you from missing findings. Work through the strip in the same order every time: rate, rhythm, PR interval, QRS duration, QT interval, ST segment. For each measurement, count small squares and multiply by 0.04 seconds to convert to time. For voltage measurements, count small squares vertically and multiply by 0.1 millivolt.
Physical calipers make every measurement more consistent, but the paper-edge technique works in a pinch. Place the strip on a flat surface under good light. For intervals that are hard to judge, look at the same interval across multiple leads. The lead that shows the earliest onset and latest offset of a wave gives you the truest measurement. With practice, reading a strip takes less than a minute, and the grid does most of the math for you.