CNC G Code List & Meanings

G and M codes form the foundation of programming CNC machines. These codes are a universal language that tells the machine commands like “go here,” “go this fast,” and “run this function.” In this article, we’ve compiled the most commonly used G and M codes, their meanings, and how they’re used with simple examples.

What Is a G Code?

G codes (Geometric/Preparatory Function) are commands that tell the CNC machine how the cutting tool should move. They define geometric movements such as linear motion, circular motion, and rapid positioning. Each G code activates a specific motion mode in the machine’s control unit, and that mode stays in effect until it’s changed by another G code.

Most Commonly Used G Codes

Code	Meaning	Description
G00	Rapid positioning	The tool moves to the specified point at the fastest speed, without touching the workpiece.
G01	Linear interpolation	The tool cuts along a straight line at the specified feed rate.
G02	Circular interpolation (clockwise)	The tool cuts following a circular path in the clockwise direction.
G03	Circular interpolation (counterclockwise)	The tool cuts following a circular path in the counterclockwise direction.
G04	Dwell	Keeps the tool stationary for the duration specified in the program.
G17/G18/G19	Plane selection	Activate the XY, XZ, and YZ planes, respectively, for circular interpolation.
G20/G21	Unit selection	G20 activates inch units, G21 activates millimeter units.
G28	Return to reference point	Sends the tool to the machine’s defined reference (home) point.
G40/G41/G42	Tool radius compensation	G40 cancels compensation, G41 activates left compensation, G42 activates right tool radius compensation.
G54-G59	Workpiece coordinate system	Used to define different workpiece zero points.
G90/G91	Absolute / incremental programming	G90 defines coordinates relative to the zero point, G91 defines them relative to the previous point.
G92	Coordinate system setting	Sets the tool’s current position to a specific coordinate value (lathe).

What Is an M Code?

M codes (Miscellaneous/Machine Function) deal not with the machine’s motion but with its auxiliary functions: spindle rotation, turning coolant on/off, stopping the program, and so on. While G codes say “how to move,” M codes say “which additional function to run.”

Code	Meaning
M00	Stop program (manual resume)
M02	End of program
M03	Rotate spindle clockwise
M04	Rotate spindle counterclockwise
M05	Stop spindle
M06	Tool change
M08	Turn coolant on
M09	Turn coolant off
M30	End of program and return to start

A Simple Example Program

Below is a simplified example program written to mill a straight slot on a milling machine:

G21 G90 G17         (unit: mm, absolute coordinates, XY plane)
G54                  (workpiece coordinate system)
M03 S1500            (spindle clockwise, 1500 rpm)
G00 X0 Y0 Z5         (rapid move to start point)
G01 Z-2 F100         (plunge 2 mm into part, 100 mm/min feed)
G01 X50 F200         (50 mm linear cut)
G00 Z5                (raise the tool)
M05                   (stop spindle)
M30                   (end of program)

Things to Watch Out for When Learning G Codes

Most G codes can vary slightly depending on the machine’s control system (Fanuc, Siemens, Heidenhain, etc.); always check the programming manual for the control unit of the machine you’re using.
Before running a new program, always check it with a simulation or a “dry run.”
When moving rapidly with G00, make sure the tool doesn’t collide with the workpiece or fixtures.
Learn to distinguish between modal (persistent) and non-modal (single-line) codes; this saves a lot of time when reading code.

Conclusion

G and M codes are the basic alphabet of CNC programming. Learning these codes by understanding their logic, rather than memorizing them, lets you read programs generated by CAM software, spot errors, and make small manual edits when needed. Practicing by programming simple parts with the basic codes is the fastest way to progress in this field.