G-Code Cheat Sheet: Most Common Codes Explained

A simple G-code cheat sheet covering the most commonly used CNC codes — G00, G01, G02/G03, G28, G90/G91 and more, explained for beginners.

Category: CNC Programming

Introduction

G-code is the language CNC machines understand. While CAM software generates most of it automatically, understanding the basics helps you read programs, troubleshoot issues, and make small manual edits with confidence. Here’s a practical cheat sheet covering the codes you’ll see most often.

Understanding the Basics

G-code is made up of lines called blocks, each containing commands. A typical line looks like this:

G01 X10.0 Y5.0 F150

This tells the machine: move in a straight line (G01) to position X10.0, Y5.0, at a feed rate of 150 (units per minute).

Most Common G-Codes

Code	Name	What It Does
G00	Rapid Positioning	Moves the tool quickly to a position (not cutting)
G01	Linear Interpolation	Moves the tool in a straight line while cutting, at a set feed rate
G02	Circular Interpolation (Clockwise)	Cuts a clockwise arc
G03	Circular Interpolation (Counter-clockwise)	Cuts a counter-clockwise arc
G20	Inch Units	Sets measurement units to inches
G21	Metric Units	Sets measurement units to millimeters
G28	Return to Home	Sends the machine to its home/reference position
G40	Cutter Compensation Off	Cancels tool diameter compensation
G54–G59	Work Coordinate Systems	Selects a specific work offset/origin point
G90	Absolute Positioning	All coordinates are measured from a fixed origin point
G91	Incremental Positioning	All coordinates are measured relative to the current position

Most Common M-Codes

Code	Name	What It Does
M00	Program Stop	Pauses the program
M03	Spindle On (Clockwise)	Starts the spindle rotating clockwise
M04	Spindle On (Counter-clockwise)	Starts the spindle rotating counter-clockwise
M05	Spindle Stop	Stops spindle rotation
M06	Tool Change	Initiates a tool change
M08	Coolant On	Turns coolant on
M09	Coolant Off	Turns coolant off
M30	Program End	Ends the program and resets to the beginning

A Simple Example Program

G21          (set units to millimeters)
G90          (absolute positioning)
G54          (select work coordinate system 1)
M03 S1000    (spindle on clockwise, 1000 RPM)
G00 X0 Y0    (rapid move to start position)
G01 Z-5 F100 (feed down into material)
G01 X50 F150 (cut a straight line to X50)
G00 Z10      (retract tool)
M05          (spindle off)
M30          (end program)

Frequently Asked Questions

Do I need to write G-code manually? Rarely. CAM software generates it automatically from your CAD design. Understanding the codes helps you read, troubleshoot, and make small edits — not write full programs from scratch.

What’s the difference between G90 and G91? G90 (absolute) measures every position from a fixed zero point. G91 (incremental) measures each move relative to the tool’s current position. Most beginners should stick with G90 until comfortable.

Why does my program throw an error? Common causes include missing coordinates, incompatible codes on the same line, or referencing a work offset (like G54) that hasn’t been set. See our Troubleshooting Common G-Code Errors guide for details.