How to Calculate the Electrical Axis of the Heart on an ECG, Step by Step

By the DecodeMed TeamOctober 19, 20266 min read

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Look at the QRS in lead I and aVF: if both are mainly positive, the axis is normal. If lead I is positive and aVF negative, check lead II: positive means still normal (0 to −30°), negative means left axis deviation. Lead I negative with aVF positive is right axis deviation. The normal axis runs from about −30° to +90°.

Normal 12-lead ECG in the DecodeMed ECG atlas, with the P waves, QRS complexes, ST segments and T waves coloured in every lead and a lead II rhythm strip — A real normal 12-lead ECG in the ECG atlas: each wave is coloured where cardiologists marked it. Tracing: LUDB, PhysioNet (ODC-BY 1.0). Image sources and licences.

What the axis is

The axis is the average direction of ventricular depolarisation in the frontal plane, measured on the hexaxial system formed by the six limb leads: I at 0°, II at +60°, aVF at +90°, III at +120°, aVL at −30° and aVR at −150°. A wave heading towards a lead is positive in it; heading away, negative; perpendicular, equiphasic.

Step 1: leads I and aVF

Lead I	aVF	Axis
Positive	Positive	Normal (0° to +90°)
Positive	Negative	Check lead II (step 2)
Negative	Positive	Right axis deviation (+90° to +180°)
Negative	Negative	Extreme axis (−90° to ±180°)

“Positive” means the QRS has more area above the baseline than below it.

Step 2: lead II separates normal from left deviation

If lead I is positive and aVF negative, the axis lies between 0° and −90°. Lead II sits at +60°, perpendicular to −30°: if the QRS in II is positive, the axis is between 0° and −30° (normal); if negative, it is beyond −30° (left axis deviation).

Step 3: refine with the isoelectric lead

Find the limb lead where the QRS is most equiphasic. The axis is perpendicular to it; pick the direction that agrees with the positive leads. For example, an equiphasic aVL (−30°) with a positive lead II places the axis near +60°.

Right bundle branch block in the DecodeMed ECG atlas: callouts point to the rsR′ in V1–V2 and the broad S wave in I and V6, with the diagnostic criteria in a box — Teaching view of a right bundle branch block in the ECG atlas: arrows show what changes and where to look. Tracing: LUDB, PhysioNet (ODC-BY 1.0). Image sources and licences.

Main causes of axis deviation

Left axis deviation (< −30°)	Right axis deviation (> +90°)
Left anterior fascicular block	Left posterior fascicular block
Inferior myocardial infarction	Lateral myocardial infarction
Left ventricular hypertrophy (sometimes)	Right ventricular hypertrophy
Ventricular pacing, some pre-excitation	Acute right heart strain, such as pulmonary embolism
	Normal in children and some thin young adults

Always rule out limb lead reversal first: a negative P, QRS and T in lead I with an upright aVR is more often swapped arm electrodes than dextrocardia.

Study it in RadioAnato AI

RadioAnato AI, the DecodeMed app, puts 3D anatomy, CT and MRI studies, histology slides and an AI tutor that answers on them in your pocket. Free to download for iPhone, iPad and Android.

Four screens of the RadioAnato AI app: a labelled PET scan, a brain MRI with a 3D brain, 3D lungs with CT slices, and a labelled lung CT — RadioAnato AI on a phone: PET, MRI and CT studies with every structure labelled, next to its 3D model. Download the app. Image sources and licences.

Practise

Estimate the axis on every ECG you read and compare it with the measured value. The ECG atlas shows the axis measured on each real tracing, so you can check yourself. Axis is step 3 of how to learn ECG from scratch.

Frequently asked questions

What is the normal ECG axis?

About −30° to +90° in adults. Some references extend the upper limit to +100°.

What is the fastest way to calculate the axis?

Look at the QRS in lead I and aVF. Both positive is normal; I positive and aVF negative needs lead II to decide; I negative and aVF positive is right axis deviation.