How to Read an ECG: A Simple 10-Step Systematic Approach

An electrocardiogram (ECG or EKG) can look like an intimidating tangle of squiggly lines — but reading one becomes straightforward once you follow the same systematic order every single time. This guide breaks ECG interpretation into 10 simple steps, so you never miss an important finding.

First, Know the Waveform

Before the 10 steps, you must recognise the parts of a single heartbeat on the ECG. Every cardiac cycle produces a P wave, a QRS complex and a T wave, connected by segments and intervals.

What each part represents:

Reading the ECG paper (at standard 25 mm/s and 10 mm/mV):

• 1 small box = 0.04 seconds (horizontal) and 0.1 mV (vertical)
• 1 large box = 0.20 seconds (5 small boxes) and 0.5 mV

flowchart LR
    A[SA Node fires] --> B[P wave
Atria depolarise]
    B --> C[PR interval
AV node delay]
    C --> D[QRS complex
Ventricles depolarise]
    D --> E[ST segment
Plateau]
    E --> F[T wave
Ventricles repolarise]
    F --> G[Next beat]

    style B fill:#e3f2fd,stroke:#1565c0
    style D fill:#f3e5f5,stroke:#6a1b9a
    style F fill:#fff3e0,stroke:#e65100

Now let’s walk through the 10 steps.

How to Identify Each Wave on the Graph

When you look at a real ECG it helps to identify the waves in a fixed order rather than left to right. The trick is to find the tall spike first, then work outward.

Step-by-Step Identification

1. Find the R wave first — it is the tallest, sharpest upward spike. It is the easiest landmark on the whole trace. The distance between two R waves is the R-R interval.
2. Find the Q wave — the small downward dip immediately BEFORE the R spike. (Not always present.)
3. Find the S wave — the downward dip immediately AFTER the R spike.
4. Find the P wave — the small, smooth, rounded bump BEFORE the QRS complex. It represents the atria firing.
5. Find the T wave — the broader, rounded bump AFTER the QRS complex. It represents the ventricles resetting.

flowchart LR
    A[1. Locate
tall R spike] --> B[2. Dip before R
= Q]
    A --> C[3. Dip after R
= S]
    B --> D[4. Bump before QRS
= P]
    C --> E[5. Bump after QRS
= T]

    style A fill:#ffcdd2,stroke:#c62828
    style B fill:#ede7f6,stroke:#6a1b9a
    style C fill:#e0f7fa,stroke:#00838f
    style D fill:#e3f2fd,stroke:#1565c0
    style E fill:#e8f5e9,stroke:#2e7d32

Reading the Grid: What Each Box Measures

ECG paper is printed with a standardised grid. As long as the machine runs at the standard paper speed of 25 mm/s and calibration of 10 mm/mV, every box has a fixed meaning.

The Two Axes

• Horizontal axis = TIME (how long an event lasts). Determined by paper speed.
• Vertical axis = VOLTAGE / amplitude (how strong the electrical signal is, i.e. the height of a wave). Determined by calibration.

Box Values

How to Measure a Wave or Interval

1. Duration (time): count how many small boxes wide the wave/interval is, then multiply by 0.04 s.
   • Example: a QRS that is 2 small boxes wide = 2 × 0.04 = 0.08 s (normal).
2. Amplitude (voltage): count how many small boxes tall the wave is, then multiply by 0.1 mV.
   • Example: an R wave 10 small boxes tall = 10 × 0.1 = 1.0 mV.

The 10 Steps at a Glance

flowchart TD
    S1[1. Rhythm] --> S2[2. Rate]
    S2 --> S3[3. Axis]
    S3 --> S4[4. P wave]
    S4 --> S5[5. PR interval]
    S5 --> S6[6. Q wave]
    S6 --> S7[7. QRS complex]
    S7 --> S8[8. QT interval]
    S8 --> S9[9. ST segment]
    S9 --> S10[10. T wave]

    style S1 fill:#fce4ec,stroke:#c2185b
    style S2 fill:#fff3e0,stroke:#e65100
    style S3 fill:#fff8e1,stroke:#f9a825
    style S4 fill:#e8f5e9,stroke:#2e7d32
    style S5 fill:#e0f7fa,stroke:#00838f
    style S6 fill:#e3f2fd,stroke:#1565c0
    style S7 fill:#ede7f6,stroke:#5e35b1
    style S8 fill:#fce4ec,stroke:#ad1457
    style S9 fill:#fff3e0,stroke:#ef6c00
    style S10 fill:#e8f5e9,stroke:#388e3c

Step 1: Rhythm

The first question is always: is the rhythm regular or irregular? Compare the distance between consecutive R waves (the R-R interval). An easy trick is to mark two R-wave peaks on a strip of paper and slide it along the trace.

What “Regular” and “Irregular” Mean

• Regular rhythm: The R-R intervals are all equal (or vary by only a tiny amount). The beats march out like a steady metronome. Normal sinus rhythm is regular.
• Irregular rhythm: The R-R intervals vary. This is further divided into:
  • Regularly irregular: The irregularity follows a repeating pattern (e.g., a pattern that recurs every few beats, as in some heart blocks or bigeminy).
  • Irregularly irregular: There is no pattern at all — the beats are completely random. The classic cause is atrial fibrillation.

flowchart TD
    A[Look at R-R intervals] --> B{All gaps equal?}
    B -->|Yes| C[REGULAR
e.g. Sinus rhythm]
    B -->|No| D{Is there a
repeating pattern?}
    D -->|Yes| E[Regularly irregular
e.g. 2nd degree AV block]
    D -->|No| F[Irregularly irregular
e.g. Atrial fibrillation]

    style C fill:#c8e6c9,stroke:#2e7d32
    style E fill:#fff9c4,stroke:#f9a825
    style F fill:#ffcdd2,stroke:#c62828

Step 2: Rate

Next, calculate the heart rate. A normal resting rate is 60-100 beats per minute (bpm).

• Below 60 bpm = bradycardia (slow)
• Above 100 bpm = tachycardia (fast)

The 300 Rule (for regular rhythms)

Count the number of large boxes between two R waves and divide 300 by that number:

Heart Rate (bpm) = 300 ÷ (number of large boxes between R-R)

The 6-Second Method (for irregular rhythms)

For irregular rhythms, count the number of QRS complexes in a 6-second strip and multiply by 10:

Heart Rate (bpm) = (QRS complexes in 6 seconds) × 10

Step 3: Axis

The cardiac axis describes the overall direction of the heart’s electrical depolarisation. A quick method uses Lead I and Lead aVF.

Step 4: P Wave

The P wave represents atrial depolarisation. Ask:

• Is a P wave present before every QRS?
• Is every QRS preceded by a P wave?
• Is the P wave upright in Lead II (normal sinus origin)?
• Is its shape and duration normal (< 0.12 s, < 2.5 small boxes tall)?

flowchart TD
    A[Examine P waves] --> B{P before
every QRS?}
    B -->|Yes, 1:1| C[Normal sinus activity]
    B -->|Absent / chaotic| D[AFib or junctional rhythm]
    B -->|More P than QRS| E[AV block]

    style C fill:#c8e6c9,stroke:#2e7d32
    style D fill:#ffcdd2,stroke:#c62828
    style E fill:#fff9c4,stroke:#f9a825

• Tall, peaked P waves → right atrial enlargement (P pulmonale)
• Wide, notched (M-shaped) P waves → left atrial enlargement (P mitrale)

Step 5: PR Interval

The PR interval is measured from the start of the P wave to the start of the QRS complex. It reflects the time taken for the impulse to travel from the atria through the AV node.

• Normal PR = 0.12-0.20 s (3-5 small boxes)
• Long PR (> 0.20 s) = first-degree AV block
• Short PR (< 0.12 s) = pre-excitation (e.g., Wolff-Parkinson-White)
• Progressively lengthening PR → dropped beat = Mobitz I (Wenckebach)

Step 6: Q Wave

A Q wave is the first downward deflection of the QRS before any upward (R) deflection. Small “septal” Q waves are normal, but pathological Q waves signal previous myocardial infarction.

A Q wave is pathological if it is:

• Wider than 0.04 s (1 small box), or
• Deeper than 25% of the height of the following R wave

flowchart LR
    A[Q wave present?] --> B{Width > 1 box
OR depth > 25% of R?}
    B -->|No| C[Normal septal Q]
    B -->|Yes| D[Pathological Q
Old MI / scar]

    style C fill:#c8e6c9,stroke:#2e7d32
    style D fill:#ffcdd2,stroke:#c62828

Step 7: QRS Complex

The QRS complex represents ventricular depolarisation. Assess both its width and height (amplitude).

Width:

• Normal (narrow) = < 0.12 s (< 3 small boxes) → impulse travelled normally through the fast conduction system
• Wide = ≥ 0.12 s → the beat originates in or is conducted abnormally through the ventricles (bundle branch block, ventricular rhythm, hyperkalaemia)

Height:

• Tall QRS → ventricular hypertrophy (e.g., left ventricular hypertrophy)
• Small QRS → pericardial effusion, obesity, COPD

flowchart TD
    A[Measure QRS width] --> B{< 0.12 s?}
    B -->|Yes, Narrow| C[Supraventricular origin
Normal conduction]
    B -->|No, Wide| D[Ventricular origin OR
Bundle branch block]

    style C fill:#c8e6c9,stroke:#2e7d32
    style D fill:#ffcdd2,stroke:#c62828

Step 8: QT Interval

The QT interval is measured from the start of the QRS to the end of the T wave. It represents the total time for the ventricles to depolarise and repolarise.

Because the QT shortens as heart rate rises, we use the corrected QT (QTc), most commonly with Bazett’s formula:

QTc = QT ÷ √RR

where RR is the R-R interval in seconds.

• Normal QTc ≈ < 0.44 s (men) and < 0.46 s (women)
• Prolonged QT → risk of a dangerous arrhythmia (torsades de pointes); caused by some drugs, low potassium/magnesium, or congenital long-QT syndrome
• Short QT → hypercalcaemia, congenital short-QT syndrome

Step 9: ST Segment

The ST segment is the flat section between the end of the QRS (the J point) and the start of the T wave. Normally it sits level with the baseline (isoelectric line).

• ST elevation → acute myocardial infarction (STEMI), pericarditis
• ST depression → myocardial ischaemia, strain

flowchart TD
    A[Look at ST segment] --> B{Position vs baseline?}
    B -->|On baseline| C[Normal]
    B -->|Elevated| D[STEMI / Pericarditis]
    B -->|Depressed| E[Ischaemia / Strain]

    style C fill:#c8e6c9,stroke:#2e7d32
    style D fill:#ffcdd2,stroke:#c62828
    style E fill:#fff9c4,stroke:#f9a825

Step 10: T Wave

The T wave represents ventricular repolarisation (the ventricles relaxing and resetting). Normally it is upright in most leads and smoothly rounded.

• Tall, peaked T waves → hyperkalaemia (high potassium) or very early MI
• Flattened T waves → hypokalaemia, ischaemia
• Inverted T waves → ischaemia, prior infarction, strain (can be normal in some leads)

flowchart TD
    A[Examine T waves] --> B{Shape?}
    B -->|Upright, rounded| C[Normal]
    B -->|Tall & peaked| D[Hyperkalaemia]
    B -->|Flat| E[Hypokalaemia / Ischaemia]
    B -->|Inverted| F[Ischaemia / Old infarct]

    style C fill:#c8e6c9,stroke:#2e7d32
    style D fill:#ffcdd2,stroke:#c62828
    style E fill:#fff9c4,stroke:#f9a825
    style F fill:#ffe0b2,stroke:#e65100

Putting It All Together

Run through the same 10 steps in the same order every time, and no major abnormality will slip past you.

flowchart LR
    A[ECG in hand] --> B[Apply 10 steps
in order]
    B --> C{All normal?}
    C -->|Yes| D[Normal ECG]
    C -->|No| E[Identify the
abnormal step]
    E --> F[Correlate with
patient clinically]

    style D fill:#c8e6c9,stroke:#2e7d32
    style E fill:#fff9c4,stroke:#f9a825
    style F fill:#e3f2fd,stroke:#1565c0

Worked Example: Why This 12-Lead ECG Is Normal

Let’s apply the 10 steps to a real 12-lead ECG printed at the standard settings shown along the bottom: Speed 25 mm/s, Limb leads 10 mm/mV, Chest leads 10 mm/mV. These are exactly the standard calibration values, so every small box = 0.04 s / 0.1 mV.

Running the 10 Steps

The Verdict

Every one of the 10 checks lands in the normal range, so this tracing is a normal sinus rhythm — a regular rhythm at ~75 bpm, originating from the SA node (P before every QRS), conducting normally (normal PR and narrow QRS), with a normal axis and no signs of ischaemia (flat ST, upright T).

flowchart LR
    A[Regular R-R] --> B[Rate ~75 bpm]
    B --> C[P before
every QRS]
    C --> D[Normal PR
& narrow QRS]
    D --> E[Flat ST
Upright T]
    E --> F[NORMAL
SINUS RHYTHM]

    style F fill:#c8e6c9,stroke:#2e7d32,stroke-width:2px

Conclusion

ECG interpretation stops being scary the moment you replace guesswork with a repeatable system. Master these 10 steps — rhythm, rate, axis, P wave, PR interval, Q wave, QRS complex, QT interval, ST segment and T wave — and you will be able to read any ECG methodically and confidently.

Practise on as many real tracings as you can, and always interpret the ECG in the context of the patient, never in isolation.

References and Further Reading:

• Standard 12-lead ECG interpretation guidelines
• Bazett’s formula for QT correction
• AV block classification (first-degree, Mobitz I & II, third-degree)
• Axis determination using the hexaxial reference system