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How to read an ECG rhythm strip, step by step

Read every strip the same way. First know the grid — each small box is 0.04 seconds, each large box is 0.20 seconds. Then run the same six-step scan: rate, rhythm, P waves, PR interval, QRS width, and ST segment. Doing it in that order, every time, is what makes rhythms recognizable.

VitalSim Rhythm's live ECG trainer on iPad, showing a frozen Normal Sinus rhythm on a calibrated grid with the heart rate and a rail of the five rhythm families color-coded

First, the grid

ECG paper (and a good on-screen grid) is calibrated: each small box is 1 mm = 0.04 seconds, and each large box is five small boxes = 0.20 seconds. Vertically, boxes measure amplitude. Knowing the boxes is what lets you turn a shape into a measurement — a PR interval of "four small boxes" is 0.16 seconds.

The six-step scan

  • 1 — Rate. Is it fast, slow, or normal? (See "calculating the rate" below.)
  • 2 — Rhythm. Are the R–R intervals regular or irregular? "March out" the R waves — mark two on a paper edge and slide along; if they keep landing on R waves, it's regular.
  • 3 — P waves. Is there a P wave? Do they all look alike? Is there one P before every QRS?
  • 4 — PR interval. From the start of the P to the start of the QRS — normally 0.12–0.20 seconds (three to five small boxes).
  • 5 — QRS width. From the start to the end of the QRS — normally narrow, about ≤0.10–0.12 seconds. A wide QRS points toward the ventricles.
  • 6 — ST segment. Is it flat, elevated, or depressed?

Calculating the rate

Two common ways. On a six-second strip, count the QRS complexes and multiply by 10. For a regular rhythm, use the big-box method: divide 300 by the number of large boxes between two R waves. Both are quick estimates — the app's Analyze does the exact math for you.

Practice the scan on a live strip

Reading gets fast when you can drive the strip. In VitalSim Rhythm you pick a rhythm, freeze it, and measure it with calipers on a calibrated grid — then Analyze shows the rate and intervals so you can check your six-step read against the numbers.

Analyze mode measuring a strip: heart rate, RR, PR, QRS, QT, and QTc intervals with an auto-interpretation of Normal Sinus Rhythm and its supporting reasons

FAQ

What is the systematic way to read an ECG?

Read every strip the same way: rate, rhythm, P waves, PR interval, QRS width, then ST segment. A consistent order is what makes rhythms recognizable.

How do you calculate heart rate on an ECG?

Count the QRS complexes in a six-second strip and multiply by 10, or — for a regular rhythm — divide 300 by the number of large boxes between two R waves.

What do the boxes on ECG paper mean?

Each small box is 0.04 seconds and each large box (five small) is 0.20 seconds, so you can turn intervals into times.

VitalSim Rhythm