Reading an ECG is not about memorizing a few shapes. It is about following a fixed order so you do not miss anything. This guide covers how to read a basic ECG in 8 steps, with a table of normal reference values. Every number is cited so you can check it.
This article is educational and does not replace clinical training or advice for a specific patient. All clinical decisions rest with the treating clinician.
What is an ECG (EKG)?
An ECG is a recording of the electrical activity of the heart over time, captured through electrodes on the skin. A standard recording uses 12 leads to view the heart from several directions. The goal when reading it is to decide whether the waves and intervals are normal or abnormal, and the best way to do that is to read systematically (StatPearls, Electrocardiogram).
On standard paper (speed 25 mm/second, calibration 10 mm/mV), one small square is 40 ms and one large square is 200 ms. Always confirm this calibration before you measure, otherwise every value below will be off.
Reading an ECG in 8 steps
The order below follows the systematic read recommended by StatPearls: rate, rhythm, axis, P wave, PR interval, QRS complex, ST segment and T wave, QT interval (StatPearls, Electrocardiogram).
Step 1. Heart rate
Divide 300 by the number of large squares between two R peaks, or 1500 by the number of small squares. A normal rate is about 60 to 100 beats per minute. Below 60 is bradycardia and above 100 is tachycardia (StatPearls, Electrocardiogram).
Step 2. Rhythm
Decide whether the rhythm is regular, and whether each QRS complex is preceded by a consistent P wave. Normal sinus rhythm has a P wave that is upright in lead II and precedes every QRS.
Step 3. Cardiac axis
A normal QRS axis lies between -30 and +90 degrees (StatPearls, Axis Deviation). Values outside this range suggest left or right axis deviation, which should prompt a search for a cause.
Step 4. P wave
The P wave reflects atrial depolarization. It is normally about 3 small squares wide (under 120 ms) and about 2.5 small squares tall (under 2.5 mm) (StatPearls, Electrocardiogram; StatPearls, P Wave).
Step 5. PR interval
Measure from the start of the P wave to the start of the QRS complex. Normal is 120 to 200 ms (3 to 5 small squares) (StatPearls, Electrocardiogram). A long PR interval suggests slowed atrioventricular conduction.
Step 6. QRS complex
This reflects ventricular depolarization. It is normally under 120 ms, usually 60 to 100 ms (StatPearls, Electrocardiogram). A wide QRS is seen in bundle branch block and other intraventricular conduction disturbances (AHA/ACCF/HRS, Part III).
Step 7. ST segment and T wave
The ST segment normally sits at the isoelectric baseline. ST elevation or depression, together with T wave changes, is noteworthy and must be read against the clinical picture (StatPearls, ST Segment; StatPearls, T Wave; AHA/ACCF/HRS, Part IV).
Step 8. QT interval
Measure from the start of the QRS complex to the end of the T wave. The normal QT interval is under about 400 to 440 ms (StatPearls, Electrocardiogram). In practice the rate-corrected QT (QTc) is used, and thresholds can differ by sex (AHA/ACCF/HRS, Part IV).
Normal ECG reference values
| Parameter | Normal value | Note |
|---|---|---|
| Heart rate | 60 to 100 bpm | 300 divided by large squares |
| P wave | under 120 ms, under 2.5 mm | atrial depolarization |
| PR interval | 120 to 200 ms | 3 to 5 small squares |
| QRS complex | under 120 ms | usually 60 to 100 ms |
| QRS axis | -30 to +90 degrees | outside this is axis deviation |
| QT interval | under about 400 to 440 ms | use rate-corrected QTc |
The reference values in this table follow StatPearls, Electrocardiogram and the AHA/ACCF/HRS standardization series. Exact thresholds can vary by reference source and by age and sex, so always check the guideline in use at your institution.
Common mistakes when starting out
- Forgetting to check paper speed and calibration before measuring, which throws off every interval.
- Not comparing with the patient’s previous ECG.
- Calling a rhythm sinus without checking that a P wave precedes each QRS.
- Estimating the axis and QT interval by eye instead of measuring on the grid.
When a clinician should review it urgently
Some patterns need urgent review by a clinician, for example new ST elevation, high-grade atrioventricular block, a clearly prolonged QT or QTc, and wide-complex tachycardia. These are illustrative examples, not management advice. Diagnosis and treatment must follow specialty guidelines and direct clinical assessment (AHA/ACCF/HRS, Part IV).
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References
- StatPearls, Electrocardiogram. NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK549803/
- StatPearls, P Wave. NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK551635/
- StatPearls, ST Segment. NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK459364/
- StatPearls, ECG T Wave. NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK538264/
- StatPearls, Electrical Right and Left Axis Deviation. NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK470532/
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram (Part I). Circulation. https://www.ahajournals.org/doi/full/10.1161/CIRCULATIONAHA.108.191096
- AHA/ACCF/HRS Recommendations, Part III: Intraventricular Conduction Disturbances. Circulation 2009. https://pubmed.ncbi.nlm.nih.gov/19228822/
- AHA/ACCF/HRS Recommendations, Part IV: ST Segment, T and U Waves, and the QT Interval. Circulation 2009. https://pubmed.ncbi.nlm.nih.gov/19281931/