How to Read an ECG (Beginner's Walkthrough)

New to ECGs? This is the plain-English, step-by-step routine — with a worked example — that turns a wall of squiggles into a clear read.

First, understand the grid

ECG paper moves at 25 mm/s. A small box is 0.04 s wide; a large box (5 small) is 0.20 s. Vertically, 10 mm = 1 mV. Time runs left to right, voltage up and down. Everything you measure comes back to these boxes.

The five steps, in order

1. Rate — 300 ÷ large boxes between R waves (regular), or count in 6 s × 10 (irregular).
2. Rhythm — are the R–R intervals even? Is there a P before every QRS?
3. P waves — present, upright in lead II, one per QRS?
4. PR interval — 3–5 small boxes (0.12–0.20 s)?
5. QRS width — under 3 small boxes (< 0.12 s) = narrow.

A worked example

Say the R waves are 4 large boxes apart, every QRS has an upright P wave before it, the PR interval is 4 small boxes, and the QRS is 2 small boxes wide.

• Rate: 300 ÷ 4 = 75 bpm.
• Rhythm: regular, P before every QRS → sinus.
• PR: 4 × 0.04 = 0.16 s → normal.
• QRS: 2 × 0.04 = 0.08 s → narrow.

Read: normal sinus rhythm.

What to watch for once you have the basics

• No P waves + irregular → atrial fibrillation.
• Wide QRS → bundle branch block or ventricular origin.
• ST elevation → possible STEMI (urgent).
• Very slow or very fast rates with symptoms → act, don't just describe.

Go deeper with the full ECG interpretation guide and the cheat sheet.

Summary

• Learn the grid: 0.04 s per small box.
• Work rate → rhythm → P → PR → QRS every time.
• Practise on real strips until the routine is automatic.

Frequently asked questions

How do I read an ECG as a beginner?

Learn the grid (0.04 s per small box), then work five steps in order: rate, rhythm, P waves, PR interval, and QRS width. Practise the routine on real strips until it is automatic.

What is the normal heart rate on an ECG?

60–100 bpm. Below 60 is bradycardia and above 100 is tachycardia.

How do I know if a rhythm is sinus?

There is a P wave before every QRS, the P waves are upright in lead II, and the rhythm is regular.

How long should it take to read an ECG?

With practice, a systematic read takes well under a minute; the goal is consistency, not speed.