Reading an EKG starts with understanding what the squiggly lines on that strip of graph paper actually represent: electrical signals moving through the heart with each beat. Once you learn the grid, the waves, and a handful of normal ranges, you can extract a surprising amount of information from a single tracing. Here’s how to break it down piece by piece.
Understanding the Grid
EKG paper is a grid of small 1-millimeter squares grouped into larger 5-millimeter squares (outlined with darker lines). The paper feeds through the machine at a standardized speed, so horizontal distance equals time and vertical distance equals voltage. Each small square represents 0.04 seconds horizontally and 0.1 millivolts vertically. Each large square (five small squares wide) represents 0.2 seconds horizontally and 0.5 millivolts vertically.
These numbers matter because every measurement you take on an EKG, from heart rate to the duration of specific waves, depends on counting squares. Five large squares equal exactly one second. Thirty large squares equal six seconds. Once this clicks, the rest of EKG reading is just pattern recognition.
The Waves and What They Mean
A single heartbeat produces a characteristic sequence of waves on the EKG. Each wave corresponds to a specific electrical event inside the heart.
- P wave: A small, rounded bump that represents the upper chambers (atria) firing. It’s the electrical signal that kicks off each heartbeat from the heart’s natural pacemaker, the sinus node.
- QRS complex: The tall, sharp spike (sometimes with a small dip before or after it) that represents the lower chambers (ventricles) firing. This is the main pumping action of the heart, so it produces the largest electrical signal.
- T wave: A broader, softer wave after the QRS that represents the ventricles resetting their electrical charge, preparing for the next beat.
Between these waves are intervals and segments that carry diagnostic information. The PR interval is the flat stretch from the start of the P wave to the start of the QRS. It tells you how long the electrical signal takes to travel from the upper chambers to the lower chambers. The ST segment is the stretch between the end of the QRS and the beginning of the T wave. It should be roughly flat, sitting at the same level as the baseline. Deviations here can signal serious problems like a heart attack.
Normal Ranges to Know
You don’t need to memorize dozens of numbers. A few key ranges cover most of what matters when scanning a tracing:
- Heart rate: 60 to 100 beats per minute for a normal resting adult.
- PR interval: 125 to 200 milliseconds (roughly 3 to 5 small squares). A longer PR interval suggests the signal is being delayed between the upper and lower chambers, a condition called heart block.
- QRS duration: 70 to 100 milliseconds (under 2.5 small squares). A wider QRS means the ventricles are taking longer than normal to conduct the signal, which can indicate a bundle branch block or other conduction problem.
- QT interval (corrected): 365 to 470 milliseconds. A prolonged QT interval raises the risk of dangerous heart rhythm disturbances.
How to Calculate Heart Rate
There are three common methods, and which one you use depends on whether the rhythm is regular or irregular.
The 300 Rule (Regular Rhythms)
Find two consecutive R waves (the tallest peaks of two QRS complexes in a row). Count the number of large squares between them and divide 300 by that number. If there are four large squares between R waves, the heart rate is 300 ÷ 4 = 75 beats per minute. This is the fastest method for regular rhythms.
The 1500 Rule (More Precise)
Same idea, but you count small squares instead of large ones and divide into 1500. If there are 18 small squares between R waves, the rate is 1500 ÷ 18 = 83 beats per minute. This gives you a more precise number when the R waves don’t land neatly on the dark grid lines.
The 6-Second Method (Irregular Rhythms)
When the spacing between beats varies, the 300 rule won’t work because there’s no single consistent interval to measure. Instead, count out 30 large squares (which equals 6 seconds), count how many QRS complexes fall within that stretch, and multiply by 10. If you count 7 QRS complexes in 6 seconds, the average rate is about 70 beats per minute.
A Step-by-Step Approach to Reading a Strip
Experienced readers follow a systematic checklist every time. Working through the same steps in the same order prevents you from jumping to a dramatic conclusion while missing something basic. Here’s a reliable sequence:
1. Rate. Calculate it using one of the methods above. Is it fast (over 100), slow (under 60), or normal?
2. Rhythm. Look at the spacing between R waves. Is it regular or irregular? A quick way to check: mark three consecutive R waves on a piece of paper, then slide the marks along the strip to see if they line up with subsequent R waves.
3. P waves. Is there a P wave before every QRS? Do they all look the same shape? In normal sinus rhythm, P waves should be upright (pointing up) in leads I and II and should appear consistently before each QRS complex. If P waves are missing, oddly shaped, or outnumber QRS complexes, the rhythm is not originating normally from the sinus node.
4. PR interval. Measure from the beginning of the P wave to the beginning of the QRS. Is it within 125 to 200 milliseconds? Is it the same for every beat?
5. QRS width. Is it narrow (under about 100 milliseconds) or wide? A narrow QRS means the ventricles are being activated through the normal conduction pathway. A wide QRS means the signal is taking an abnormal route.
6. ST segment and T wave. Is the ST segment flat, elevated, or depressed relative to the baseline? Are T waves upright where they should be, or inverted? ST changes and T wave abnormalities are the primary clues for ischemia (reduced blood flow to the heart muscle).
What ST Changes Look Like
ST segment changes are among the most clinically important things to spot on an EKG, because they can indicate an active heart attack. The 2025 guidelines from the American College of Cardiology and American Heart Association define specific thresholds.
ST elevation of 1 millimeter or more (one small square) in at least two neighboring leads suggests a major blockage cutting off blood flow to the heart muscle. In leads V2 and V3, the threshold is higher: 2 millimeters for men aged 40 and older, 2.5 millimeters for men under 40, and 1.5 millimeters for women. These differences exist because those leads naturally show slightly higher ST segments in younger men.
ST depression of 0.5 millimeters or more in two or more neighboring leads, especially if the segment slopes downward, can indicate a different type of acute coronary event. T wave inversion deeper than 1 millimeter in two neighboring leads is another warning sign. It’s worth noting that a completely normal-looking EKG does not rule out a heart problem. Many patients with acute coronary syndromes have only subtle changes or even a normal tracing.
The 12 Leads and What They See
A standard clinical EKG uses 10 electrodes (stickers on the skin) to generate 12 different “views” of the heart’s electrical activity. Four limb electrodes go on the arms and legs (or shoulders and lower abdomen). Six chest electrodes, labeled V1 through V6, wrap around the left side of the chest. V1 and V2 sit on either side of the breastbone at the fourth rib space. V4 goes at the fifth rib space in line with the middle of the collarbone. V3 sits between V2 and V4. V5 and V6 continue along the same horizontal line toward the left side of the chest.
Each lead captures the heart’s electrical signal from a different angle. The limb leads (I, II, III, aVR, aVL, aVF) look at the heart in the vertical plane, while the chest leads (V1 through V6) look at it in the horizontal plane. This matters because changes in specific groups of leads point to specific regions of the heart. ST elevation in leads II, III, and aVF, for example, suggests a problem with the bottom (inferior) wall of the heart. Changes in V1 through V4 point to the front (anterior) wall.
Common Beginner Mistakes
The most frequent error is mistaking artifact for a real finding. Artifact is any electrical noise on the tracing caused by muscle movement, loose electrodes, or electrical interference. It can mimic dangerous rhythms or distort the ST segment enough to suggest a heart attack that isn’t happening. If something looks alarming but the patient seems fine, check the electrode connections and get another tracing before drawing conclusions.
Another common mistake is confusing T waves with QRS complexes, which effectively doubles your calculated heart rate. This tends to happen with very fast or very slow rhythms where wave spacing is unusual. Automated EKG machines make this same error surprisingly often. Studies of automated interpretation algorithms show that ST segment analysis and arrhythmia identification are the two areas most prone to errors, particularly when artifacts are present or the heart rate is unusually fast or slow. The printed interpretation on an EKG machine should always be treated as a suggestion, not a diagnosis.
Smartwatch EKGs vs. Clinical 12-Lead
Consumer devices like the Apple Watch and Samsung Galaxy Watch can record a single-lead EKG, primarily designed to detect atrial fibrillation. A 2025 meta-analysis found that when a clinician manually reviews a smartwatch tracing, sensitivity reaches 96% and specificity 95% for detecting atrial fibrillation. The watch’s built-in algorithm performs slightly lower, at 86% sensitivity and 94% specificity.
These are strong numbers for a screening tool, but a single-lead tracing captures only one electrical angle. It cannot identify which part of the heart is affected, detect ST elevation reliably, or evaluate the full range of conduction abnormalities that a 12-lead EKG reveals. A smartwatch EKG is useful for catching irregular rhythms that come and go, but it is not a substitute for a clinical recording when something specific needs to be diagnosed.