ECG Interpretation: A Step-by-Step Guide

Read any ECG with confidence using one repeatable method. This guide walks through rate, rhythm, axis, intervals, and the abnormalities you must not miss — then lets you practise on real strips.

What an ECG shows

An electrocardiogram (ECG) records the heart's electrical activity as waves on gridded paper. Each small box is 0.04 s wide (large box 0.20 s) at the standard 25 mm/s, and 10 mm of height = 1 mV. Every heartbeat produces a P wave (atrial depolarization), a QRS complex (ventricular depolarization), and a T wave (ventricular repolarization). Interpretation is simply reading those deflections in a fixed order.

The systematic approach (never skip a step)

Read every ECG the same way so nothing slips past you:

1. Rate — is it fast, slow, or normal?
2. Rhythm — regular or irregular, and what is the underlying pattern?
3. Axis — normal, left, or right?
4. Intervals — PR, QRS, and QT.
5. Morphology — P waves, Q waves, ST segments, T waves.

For a beginner-friendly walk-through with a worked example, see how to read an ECG.

Step 1 — Rate

• 300 method (regular): 300 ÷ large boxes between R waves → 300, 150, 100, 75, 60, 50.
• 1500 method (precise): 1500 ÷ small boxes between R waves.
• 6-second method (irregular): QRS complexes in 6 seconds × 10.

Normal is 60–100 bpm; below is bradycardia, above is tachycardia.

Step 2 — Rhythm

Decide whether the R–R intervals are regular, then whether the QRS is narrow or wide. That two-way split points straight at the diagnosis:

	Narrow QRS	Wide QRS
Regular	Sinus, SVT, atrial flutter	VT, SVT with aberrancy
Irregular	Atrial fibrillation, MAT	AFib with aberrancy, polymorphic VT

Deep-dive: ECG rhythm interpretation.

Step 3 — Axis

The QRS axis is the net direction of ventricular depolarization. A fast bedside check uses leads I and aVF:

Lead I	Lead aVF	Axis
Up	Up	Normal (−30° to +90°)
Up	Down	Left axis deviation
Down	Up	Right axis deviation

Step 4 — Intervals

Interval	Normal	If abnormal
PR	0.12–0.20 s	Long = AV block; short + delta = WPW
QRS	< 0.12 s	Wide = BBB or ventricular origin
QT (QTc)	< 0.44 s	Long = drugs/electrolytes, torsades risk

Step 5 — Morphology & common abnormalities

• ST elevation in contiguous leads → acute STEMI. See STEMI ECG interpretation.
• Pathologic Q waves → prior infarction.
• Tall, tented T waves → hyperkalemia; flattened T + U waves → hypokalemia.
• Delta wave + short PR → Wolff-Parkinson-White.
• Peaked P (P pulmonale) / wide notched P (P mitrale) → atrial enlargement.

The 12 leads and what they see

The 12 leads view the heart from different angles, which is how you localise disease. Inferior = II, III, aVF; septal = V1–V2; anterior = V3–V4; lateral = I, aVL, V5–V6. Full breakdown: 12-lead ECG interpretation.

Recognise the core rhythms

Practise on our ECG strip identification questions and ECG MCQs.

Summary & key takeaways

• Always work the same 5 steps: rate → rhythm → axis → intervals → morphology.
• Rate: 300 method for regular, 6-second method for irregular.
• Regular/irregular × narrow/wide localises most rhythms.
• Know normal intervals and what abnormal values mean.
• ST elevation in contiguous leads is a STEMI until proven otherwise.

Frequently asked questions

How do you interpret an ECG step by step?

Read it in a fixed order: rate, rhythm, axis, intervals (PR, QRS, QT), then morphology (P waves, Q waves, ST segments, T waves).

What is the fastest way to calculate heart rate on an ECG?

For a regular rhythm, divide 300 by the number of large boxes between two R waves. For an irregular rhythm, count QRS complexes in a 6-second strip and multiply by 10.

What are the normal ECG intervals?

PR 0.12–0.20 s, QRS under 0.12 s, and QTc under 0.44 s.

How do you tell a normal QRS axis?

If the QRS is upright in both lead I and lead aVF, the axis is normal.

What does ST-segment elevation mean?

ST elevation in two or more contiguous leads suggests an acute ST-elevation myocardial infarction (STEMI) and needs urgent evaluation.

Where can I practise ECG interpretation?

Use our free ECG strip identification questions and ECG MCQs, each with instant feedback and explanations.