What Are G Codes? List of Major Codes and How to Read Programs

Date Published:

18 March 2026

Machining with an NC machine tool requires an "NC program." One of the key elements of this program is the "G code." There are a huge number of G codes, but remembering the major ones will help you troubleshoot errors and improve work efficiency. This article introduces commonly used G codes and what they can do by category.

Contents

What Are G Codes?

Benefits of Using G Codes

How to Read Programs Using G Codes

List of Major G Codes

Example of Codes Used With G Codes

Example of Program Using G Codes

What to Watch Out for When Using G Codes

Understanding Major G Codes Improves Work Efficiency

What Are G Codes?

G codes are a type of code used to write programs for NC machine tools.

The codes used in programs can be broadly divided into two categories: G codes (preparatory functions) and M codes (miscellaneous functions). G codes are often used to issue commands related to machining, so you will encounter them more frequently than other codes.

A Type of Program Used to Operate NC Machine Tools

G codes are a type of NC machine tool program defined independently by the Japanese Industrial Standards (JIS), ISO, and machine tool manufacturers. G codes begin with G00. They are used to issue machining-related commands to NC machine tools, such as specifying the positioning and direction of materials, or specifying the position of tools.

Two Broad Types of G Codes

G codes can be broadly divided into "one-shot G codes" and "modal G codes."

One-shot G codes issue commands only on a specified line in a program. On the other hand, modal G codes remain valid until a command is issued by another G code in the same group.

Benefits of Using G Codes

If an error occurs in a program, remembering G codes allows you to swiftly deal with the issue by helping you to quickly identify the problem and adjust the program to resume machining. This also helps minimize downtime during machining.

In recent years, data for machining and other operations can be easily created using CAD/CAM, so NC machine tool operators are less likely to input data manually. As a result, the need to memorize G codes has decreased compared to the past.

However, even with such programs, machining may not always go as expected.

In cases where you need to enter a program manually, knowing G codes enables you to handle the situation smoothly.

How to Read Programs Using G Codes

Machining programs used with NC machine tools consist of a combination of G codes and other codes that specify coordinate positions (X, Y, Z), the type of tool to use, among others.

For example, the program "N1 G01 X100.Y50.F1000;" means "Move the tool in a straight line to the position with an X coordinate of 100 mm and a Y coordinate of 50 mm at a speed of 1000 mm/min."

Figure explaining the example program "N1 G01 X100.Y50.F1000;"

List of Major G Codes

This section presents a list of the most frequently used G codes and their functions.

Related to Basic Operations

Used for basic NC machine tool operations, such as moving and stopping the tool.

G-code type	Meaning	What it can do
G00	Positioning	Rapidly moves the tool to the specified position
G01	Linear interpolation	Moves the tool in a straight line
G02	Arc interpolation	Moves the tool clockwise
G03	Arc interpolation	Moves the tool counterclockwise
G04	Dwell	Stops the tool for a specified amount of time

Among these, G04 corresponds to the one-shot code mentioned above.

Specifying Tools and Coordinates

Used to specify which tool or plane to use when machining or moving a tool.

G-code type	Meaning	What it can do
G17	Plane selection	Specifies operation on the XY plane
G18		Specifies operation on the ZX plane
G19		Specifies operation on the YZ plane
G28	Return to zero point	Moves the tool to the zero point (reference point)
G40	Cutter compensation cancel	Cancels the specified cutter compensation
G41	Cutter compensation	Shifts the tool to the left in the direction of travel for machining
G42	Cutter compensation	Shifts the tool to the right in the direction of travel for machining
G43	Tool length compensation	Shifts the tool in the positive direction of the Z axis for machining
G44	Tool length compensation	Shifts the tool in the negative direction of the Z axis for machining
G49	Tool length compensation cancel	Cancels the specified tool length compensation
G54 to G59	Workpiece coordinate system selection	Selects one of up to six pre-registered workpiece coordinates

For G54 to G59, you can set one specific workpiece coordinate system as the machining zero point per code. G54 is selected when the power is turned on.

Related to Canned Cycles

Designed to allow the most frequently used machining cycles to be performed with short codes.

G-code type	Meaning	What it can do
G80	Canned cycle cancel	Cancels execution of the specified canned cycle
G81	Drill cycle	Drills a new hole to the specified position (drilling)
G82	Drill cycle	Drills a new hole to the specified position, then performs a dwell
G83	Peck drilling cycle	Drills a deep hole to the specified position
G84	Tapping cycle	Taps a pre-drilled hole
G85	Boring cycle	Drills a hole to the specified position (spot facing)
G86	Boring cycle	Drills a hole to the specified position, performs a dwell, then rapidly returns to retract point
G87	Back boring cycle	Machines the back side of an existing hole at the specified position (back counterbore)
G88	Boring cycle	Drills a hole to the specified position, then stops operation to allow for manual operation
G89	Boring cycle	Drills a hole to the specified position, performs a dwell, then returns to retract point at feed rate

Related to Setting Distance From Zero Point and Return Point

Used to specify the location for machining and the return location after the tool finished machining.

G-code type	Meaning	What it can do
G90	Absolute dimensions (absolute coordinates)	Moves the tool to the specified coordinates relative to the zero point, regardless of its current position
G91	Incremental dimensions (relative coordinates)	Moves the tool to the specified coordinates from its current position
G98	Canned cycle initial point level return	After the canned cycle ends, returns the tool to the Z-axis position it occupied when the canned cycle was commanded
G99	Canned cycle retract point level return	After the canned cycle ends, returns the tool to the Z-axis position it occupied when the feed rate began

G90 and G91 may appear similar at first glance, but these G codes operate in completely different ways, so a thorough understanding of each is essential. Also, be careful when using G98 and G99, as they may cause tool interference depending on the shape of the workpiece.

Example of Codes Used With G Codes

Other codes used in conjunction with G codes include the following.

Code type	Meaning	What it can do
N	Sequence number	Used as a marker to make programs easier to read
()	Control out-in	Used to annotate programs or create notes
;	End of block (EOB)	Indicates the end of the line in a program

Example of Program Using G Codes

In this section, we show an example of an actual program using G codes presented so far.
Example: A program for machining from the initial point to the final point with tool change, S = 3000 min^-1, F = 1800 mm/min

What to Watch Out for When Using G Codes

When using G codes, be sure to check the manufacturer and model of the NC machine tool first. This is because the same code may execute different commands depending on the NC machine tool used.

Some manufacturers create G codes tailored to specific NC machine tools. For example, "G100" on Brother's SPEEDIO is a unique code that enables tool change, positioning, and spindle start in a single line.

Understanding Major G Codes Improves Work Efficiency

G codes are indispensable for controlling NC machine tool operations. While several codes may appear similar at first glance, each one differs in what it can do, so it's important to understand these differences as you memorize them.

Start by gradually learning the codes related to basic operations. Once you have a grasp of the overall picture, I recommend staying informed about how to use different codes with similar functions.

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