G43 activates tool length compensation in the positive direction. It tells the control to apply the length value stored in an H offset register, shifting the Z axis so the tip of that particular tool reaches the programmed Z depth. It is normally called right after a tool change.

G43 Tool Length Offset in G-Code, Explained

Q: What does the H number in G43 mean?

H points to the offset register that holds that tool's length. G43 H02 applies the length stored in register 02. The H number is independent of the tool number, but many shops set them to match (tool 2 uses H02) to avoid confusion.

Q: What is the difference between G43 and G49?

G43 turns tool length compensation on using an H value. G49 cancels it, so the Z axis goes back to uncompensated positions. G44 is the same idea as G43 but applies the offset in the negative direction and is rarely used.

Q: What is the best way to learn tool offsets and G43?

Drill the codes with active recall rather than re-reading a chart. A free app like G-Code Sprint quizzes G43 and the rest of the everyday codes as quick timed questions and repeats whichever ones you miss, so the offset codes stick before you are at the machine.

Every tool in the carousel is a different length. A long drill sticks far out of the holder; a short face mill barely pokes past it. G43 is how the control corrects for that difference so the same program cuts to the right depth no matter which tool is loaded.

The problem G43 solves

The program asks the tool to feed to, say, Z-5. (5 mm below the top of the part). But the control measures position to the spindle nose, not the tool tip. A tool that hangs 100 mm out of the spindle reaches the part long before one that hangs 60 mm out. Without a correction, swapping tools would change every depth.

Tool length compensation fixes this by storing each tool’s length and shifting the Z axis by that amount. The program keeps using the same Z-5., and the control makes the tip land there. The mechanism is documented in the LinuxCNC tool-length reference and the standard G43 compensation guides.

G43, the H register, G44, and G49

Four pieces make up the system:

Code / word	What it does
`G43`	Turns on tool length compensation, positive direction
`H`	Points to the offset register holding the length (for example `H02`)
`G44`	Same as `G43` but negative direction (rarely used)
`G49`	Cancels tool length compensation

A typical activation line looks like G43 Z25. H02: apply the length in register 02 and move to a safe Z of 25 with that correction active. The H number is independent of the tool number, but matching them (tool 2 uses H02) is the standard shop habit because it removes a whole class of mistakes.

Where G43 sits in the program

G43 almost always follows a tool change and the first move toward the part. The pattern is predictable:

Block	What it does
`T2 M06`	Change to tool 2
`G54`	Use the part’s work offset
`S2000 M03`	Start the spindle
`G00 X10. Y10.`	Rapid to the start point
`G43 Z25. H02`	Apply tool 2 length, rapid to safe Z
`G49`	Cancel it later, before the next change

That is why G43 follows an M06 tool change and often a G28 return to home: the tool comes home, gets changed, then its length is applied on the way back down.

G43’s place in the larger daily vocabulary of a vertical mill, alongside the cycles and M-codes it works with, is mapped in the printable VMC code list.

Tool length vs the other offsets

Beginners often blur three different corrections. They are separate tables doing separate jobs:

Offset	Code	Locates
Work offset	`G54`	The part on the table
Tool length offset	`G43` + `H`	Each tool’s length
Cutter (diameter) comp	`G41` / `G42` + `D`	The tool’s radius for sizing

The work offset (G54) locates the part, G43 locates the tool tip in Z, and cutter compensation (G41 vs G42) handles the tool’s radius in XY. Mixing them up is a common setup error, which is why G43 belongs on every list of common G-codes for CNC beginners and is worth knowing when you read a CNC program. The full address grammar behind these codes is summarized on the Wikipedia G-code page.

How the length gets measured

The stored value comes from measuring each tool: touching the tip off a known surface, using a tool-length setter on the table, or measuring on a presetter outside the machine. However it is measured, the number lands in the H register, and G43 does the rest. A wrong or missing length value is a classic crash cause, so it is verified before the first cut.

Bottom line

G43 applies a tool’s stored length from an H register so the tip reaches the programmed Z, whatever the tool’s length. G49 cancels it, G44 is the rare negative version, and the H number is separate from the tool number though shops match them. It is one of three offsets (work, tool length, cutter) that beginners must keep distinct.

Sources

Frequently asked questions

What is G43 in CNC?

G43 activates tool length compensation in the positive direction, applying the length stored in an H register so the tip of that tool reaches the programmed Z. It is normally called right after a tool change.

What does the H number in G43 mean?

H points to the offset register holding that tool’s length, so G43 H02 applies register 02. The H number is independent of the tool number, but many shops match them.

What is the difference between G43 and G49?

G43 turns tool length compensation on with an H value; G49 cancels it. G44 applies the offset in the negative direction and is rarely used.

What is the best way to learn tool offsets and G43?

Drill the codes with active recall. A free app like G-Code Sprint quizzes G43 and the rest of the everyday codes as quick timed questions and repeats whichever ones you miss.

G-Code Sprint is a study and practice tool only. Always follow your instructor, employer, machine manual, and shop safety procedures.