G02 vs. G03: A Beginner’s Guide to Circular Interpolation (With Examples)

If you are learning CNC programming, linear moves (G01) are easy to understand. You go from point A to point B in a straight line. Simple.

But as soon as you need to cut a curve, a radius, or a full circle, things get a bit tricky. You are introduced to Circular Interpolation, specifically G02 and G03.

A common question I hear from students at Machining Tutor is: "I always mix them up! How do I remember which one is which?"

In this guide, we’ll break down the difference, give you a fool-proof way to remember them, and show you how to write the code using both the Radius (R) method and the Arc Center (I, J) method.

The Quick Answer: What is the difference?

At the most basic level, the difference comes down to the direction of the cutter path:

G-Code	Direction	Terminology	Memory Trick
G02	Clockwise	Circular Interpolation CW	Like a standard clock (12 to 3)
G03	Counter-Clockwise	Circular Interpolation CCW	Reverse clock direction

(Tip: Imagine placing an analog clock face directly on top of your workpiece to visualize the move.)

How to Visualize It (So You Never Forget)

• If your cutter is moving from 12 o'clock towards 3 o'clock, that is G02.
• If your cutter is moving from 3 o'clock towards 12 o'clock, that is G03.

Pro Tip: This rule applies when looking at the machining plane from the "positive" direction. On a standard 3-axis mill, you are looking down from the spindle (Z-axis). On a lathe, the perspective changes, but the clock logic remains the same relative to the tool's movement.

The Syntax: R vs. I, J, K

Knowing the direction is step one. Step two is telling the machine how to cut the arc. There are two ways to do this in G-code.

Method 1: The "R" Word (Radius)

This is the modern, beginner-friendly method. You simply tell the machine the endpoint and the radius of the arc.

Format:
G02 X[End X] Y[End Y] R[Radius] F[Feedrate]

• Pros: Easy to read and write manually.
• Cons: Cannot cut a full 360-degree circle in one line (the machine gets confused).

Method 2: The "I and J" Method (Arc Center Offsets)

This is the classic method. Instead of a radius, you tell the machine where the center of the arc is relative to the start point.

• I: Distance from Start Point to Arc Center along the X-axis.
• J: Distance from Start Point to Arc Center along the Y-axis.

Format:
G03 X[End X] Y[End Y] I[X distance] J[Y distance] F[Feedrate]

Real-World Examples

Let’s say we are cutting a 90-degree corner with a 10mm Radius.

• Start Point: X0 Y0
• End Point: X10 Y10

Example 1: Cutting Clockwise (G02) using "R"

We want to arc "in" towards the part.

Example 2: Cutting Counter-Clockwise (G03) using "I and J"

Now let's do the opposite curve using arc centers.

Why I-10.0? Because the center of our imaginary circle is 10mm to the left (negative direction) of our start point.

Common Mistakes to Avoid

1. Modal Confusion: G02 and G03 are modal. This means if you write a G02 line, the next line will also be a G02 unless you switch back to G01 (linear move).
2. Wrong Plane: G02/G03 defaults to the XY plane (G17). If you are doing advanced surfacing on XZ or YZ planes, you need to call G18 or G19 first.
3. Missing the Negative: When using I and J, the direction is relative to the start point. Forgetting a minus sign will cause the machine to throw an "Invalid Arc" alarm.