G-code Format: The Complete Guide for 3D Printing and CNC

What Is G-code?

G-code (also called RS-274 or CNC code) is the programming language used to control CNC (Computer Numerical Control) machines — including 3D printers, CNC milling machines, lathes, plasma cutters, laser engravers, and pick-and-place machines. It originated at MIT in the 1950s and was standardized as RS-274 by the Electronic Industries Alliance (EIA). The ISO version is ISO 6983.

For 3D printing specifically, G-code is the output of slicers (software that converts 3D models to print instructions): Cura, PrusaSlicer, Bambu Studio, Simplify3D, SuperSlicer. Every FDM (Fused Deposition Modeling) 3D printer reads G-code; laser cutters and CNC routers use dialect variants.

A G-code file is plain text — every line is either a command, a comment (enclosed in ;), or blank. Commands consist of a letter code followed by a number, with optional parameters.

G-code File Structure

; Generated by Cura 5.6.0
; Layer count: 142
; Filament used: 4.82m
; Print time: 2h 15min

M104 S215 ; Set extruder temperature to 215°C (no wait)
M109 S215 ; Set extruder temp and wait for it to reach target
M140 S60  ; Set bed temperature to 60°C (no wait)
M190 S60  ; Set bed temp and wait

G28       ; Home all axes (X, Y, Z to endstops)
G29       ; Auto bed leveling (if supported)
G1 Z5 F5000   ; Move Z to 5mm
G21           ; Set units to millimeters
G90           ; Absolute positioning mode
M82           ; Extruder absolute mode
G92 E0        ; Reset extruder position to 0

; === Start of actual print ===
G0 F9000 X50 Y50 Z0.2     ; Rapid move to start position, Z=0.2mm (first layer height)
G1 F1200 X100 Y50 E5.0    ; Feed move: print line to X100, extruding 5mm of filament
G1 F1200 X100 Y100 E10.0  ; Continue printing
; ... thousands more lines ...

M104 S0  ; Extruder off
M140 S0  ; Bed off
G91      ; Relative positioning
G1 Z10   ; Raise Z by 10mm
G90      ; Back to absolute
G1 X0 Y200 ; Move print head to front
M84      ; Disable motors

G-code Command Types

G Commands (Preparatory Functions)

M Commands (Miscellaneous)

Parameter Letters

Coordinate Systems

Absolute vs Relative Mode

• Absolute (G90): All coordinates are from machine home (0,0,0). G1 X50 moves to 50mm from origin.
• Relative (G91): All coordinates are relative to current position. G1 X10 moves 10mm in X direction from wherever the head is now.

Most slicers use absolute for XYZ and relative for E (extruder), or absolute for everything after G92 E0 (reset).

Coordinate Origin and Build Volume

The origin is typically the front-left corner of the print bed at Z=0 (bed surface). The build volume is defined in firmware: a Bambu Lab X1C has 256×256×256mm, a Creality Ender 3 has 220×220×250mm.

FDM 3D Printing Specific G-code

Extrusion Calculation

The E value controls how much filament is pushed through the extruder. In absolute mode, E accumulates across the print:

G1 X100 Y50 E5.2 F1200   ; Move to (100,50), extrude to E=5.2mm total
G1 X150 Y50 E8.7 F1200   ; Continue to (150,50), extrude to E=8.7mm total (3.5mm more)

Filament volume formula:

Volume (mm³) = π × (filament_diameter/2)² × E_distance
For 1.75mm filament: Volume = π × 0.875² × E ≈ 2.405 × E mm³

Retraction

Retraction pulls filament back before non-print moves to prevent stringing:

G1 E-4.5 F3600  ; Retract 4.5mm at 60mm/s (rapid retract)
G0 X80 Y80      ; Rapid travel move
G1 E0 F3600     ; Prime (push back) 4.5mm before next extrusion

Layer Changes

; Layer 12
;LAYER:12
G0 F9000 X44.5 Y44.5 Z1.4  ; Move to new layer height Z=1.4mm (layer 12 × 0.12mm layer height + first layer)

Slicers and G-code Generation

Slicers convert STL/3MF/OBJ files to G-code:

STL to G-code workflow

STL/3MF file → Slicer → G-code → USB/SD card/WiFi → Printer → Physical part

Key slicer settings that affect G-code:

• Layer height: Resolution (0.1–0.4mm typical)
• Print speed: Line/travel speed (30–300 mm/s)
• Infill: Internal structure density and pattern
• Support structures: For overhangs beyond 45°
• Temperature profile: Per-material extruder and bed temps
• Retraction: Distance and speed to prevent stringing

Inspecting and Editing G-code

Text editors

G-code is plain text — any text editor works. VSCode has G-code syntax highlighting extensions.

# View first 30 lines of G-code
head -30 print.gcode

# Count layers
grep -c "^;LAYER:" print.gcode

# Extract all temperatures set
grep "^M109\|^M104" print.gcode

# Find layer 50
grep -n "LAYER:50" print.gcode

G-code simulators / visualizers

• Repetier-Host: Desktop G-code host + visualizer
• Pronterface (Printrun): G-code sender with 2D preview
• gCodeViewer.com: Online 3D G-code visualizer
• NCViewer.com: Online CNC G-code viewer

G-code Dialects

Different firmware and machines use different G-code dialects:

Klipper uses macros in printer.cfg and passes G-code through Python for advanced motion planning. Marlin handles commands directly in firmware. A G-code file valid for Marlin may need modification for Klipper.

Converting STL to G-code

# CuraEngine command-line (Cura's slicer engine)
CuraEngine slice -j printer_profile.json -l model.stl -o output.gcode

# PrusaSlicer command-line
prusa-slicer --slice --load config.ini model.stl --output output.gcode

# Inspect the result
wc -l output.gcode  # count lines (a complex print = millions of lines)

G-code's plain-text format, its 70-year lineage, and its universality across industries from dental implants to automotive prototyping to aerospace tooling — all from the same fundamental command set — makes it one of the most durable technical formats in manufacturing history.

Related conversions

Frequent conversions across the catalogue:

• JPG to PNG
• MP4 to MP3
• PDF to DOCX
• PNG to JPG
• MP3 to WAV
• DOCX to PDF