G And M Codes

G-codes and M-codes are the instruction set CNC machines use to control tool movement, spindle speed, coolant, and other machine functions — G-codes handle geometric moves (rapid positioning, linear and circular cutting, canned cycles), while M-codes handle miscellaneous machine functions (spindle on/off, coolant, program stop, tool change). Together they form the low-level language that a CAM-generated program, or a manually written one, uses to drive the machine. The tables below give a comprehensive listing of common G-codes and M-codes for milling and turning, with an explanation and syntax example for each.

Codes can vary slightly between machine controllers (Fanuc, Siemens, Haas, etc.), so always check your specific controller's programming manual for any machine-specific variations before relying on a code's behaviour exactly as listed here.

'G' stands for Geometry; hence, the G-Code commands are responsible for the movements of the machine that create the geometry of the part.

'M' stands for Machine (or Miscellaneous), and the M-Codes are responsible for Machine commands that cause particular operations of the equipment. Unlike G-codes, which can appear multiple times on the same line, M-Code is limited to one code per line.

NOTE: Only one "M" code can be used per line. And the M-codes will be the last command to be executed in a line, regardless of where it's located in that line.

RELATED PAGES

• Drilling Information & Data
• Surface Speed Calculator
• Engineering Glossary

FREQUENTLY ASKED QUESTIONS

What's the difference between a G-code and an M-code?

G-codes primarily control tool motion and geometry (where and how the tool moves); M-codes control miscellaneous machine functions such as spindle control, coolant, and program flow. A typical program line often combines both, e.g. a G-code move alongside an M-code turning on coolant.

Are G-codes and M-codes standardised across all machines?

Many common codes (G00, G01, G02/G03, M03, M05, M08, etc.) are broadly consistent across most controllers, but not universally — some codes vary by manufacturer or are repurposed for different functions on different controllers. Always verify against your specific machine's programming manual, especially for less common codes.

Do I need to know G-code if I use CAM software?

Not in detail for everyday programming, since CAM software generates the code automatically, but understanding the basics is valuable for troubleshooting, editing programs by hand, and verifying that the post-processor output is correct for your machine.