STL to G-code: How to Convert Any STL File to .gcode

In this article

The short answer
STL vs G-code: what each file contains
Why converting STL to G-code means slicing
Online STL to G-code converters: what to check
How to convert STL to G-code in any slicer
G-code file extensions: .gcode, .bgcode and .gcode.3mf
How to view and simulate G-code before printing
Common G-code commands: a short list
Common problems after converting STL to G-code
The bottom line
Sources

To convert an STL to G-code (a .gcode file), you slice it. A slicer such as PrusaSlicer, OrcaSlicer, UltiMaker Cura or Bambu Studio cuts the model into layers and writes the moves, temperatures and fan commands your printer runs. Import the STL, choose your printer and filament profiles, slice, check the preview and export the file.

An STL only describes the shape of a part. Everything else, from layer height to nozzle temperature, comes from the slicer profile, which is why the same STL produces different G-code on different printers. If the model needs changes first, see how to edit STL files; if you design your own parts, start with how to make 3D models for printing.

The short answer

Question	Answer
Can an STL become G-code without settings?	No. The conversion is slicing, and it needs a printer and filament profile
What software do you need?	Any slicer; PrusaSlicer, OrcaSlicer, UltiMaker Cura and Bambu Studio are free
What file do you get?	.gcode on most printers, .bgcode on newer Prusa printers, .gcode.3mf from Bambu Studio and OrcaSlicer
Does one G-code file run on every printer?	No. It is written for one printer, nozzle and filament setup
Can G-code be turned back into an STL?	Not as an editable model; it holds toolpaths, not the original surface

STL vs G-code: what each file contains

Diagram of the same box as an STL surface made of triangles and, after slicing, as G-code with stacked layers, perimeter and infill toolpaths and commands such as G28, M109 S215 and G1 X50 Y25 E0.8

An STL file stores only the surface of a model as triangles, while a G-code file stores the instructions a printer follows line by line. The University of Maryland’s slicing guide calls G-code a list of machine instructions, not a description of geometry.

	STL	3MF	G-code
Contains	Surface geometry as triangles	Geometry plus units, color, material and slicer settings	Moves, extrusion, temperatures, fan and other machine commands
Units	None; measurements are unitless	Included	Millimeters by default in Marlin
Printer-specific	No	Only if printer settings are saved in it	Yes
Editable as a model	Yes, as a mesh	Yes	No
Made by	CAD or mesh software	CAD software and slicers	A slicer

G-code is plain text, one command per line. A line such as G1 X50 Y25 E0.8 tells a Marlin printer to move in a straight line to X50 Y25 while pushing filament, since E is the extruder axis. Slicers also add comments: PrusaSlicer marks feature types and layer changes, and OrcaSlicer writes a commented block that lists every resolved slicer setting.

Why converting STL to G-code means slicing

Converting STL to G-code is called slicing because the software cuts the model into horizontal layers and plans a toolpath for each one. Most prints are not solid: the slicer surrounds the part with perimeters and fills the inside with infill, as the University of Maryland guide explains. It also adds supports, sets speeds and temperatures, and wraps everything in your printer’s start and end routine.

That is why G-code is tied to one machine. OrcaSlicer asks for a G-code flavor (Marlin, Klipper, RepRapFirmware, Repetier or Marlin 2) and for the bed shape and origin, so coordinates match your printer. Prusa describes its own sample G-codes as tested and tailored specifically for each printer model.

So when you download a model, get the STL or 3MF and slice it yourself; a ready-made .gcode may carry the wrong bed size, nozzle or temperatures.

Online STL to G-code converters: what to check

Online STL to G-code converters run a slicer on a web server, which works for a quick test but gives you less control than a desktop slicer. Desktop slicers are free, so a converter mainly saves an installation. Check these points first:

Upload and rights: your model leaves your computer. Do not upload designs you are not allowed to share or confidential parts, and read how long the site keeps files.
Printer profile: a generic profile may not match your bed size, nozzle diameter, firmware flavor or start routine.
Filament settings: confirm the nozzle and bed temperatures match your spool’s data sheet.
Preview: if the site shows no layer preview, open the result in a G-code viewer before printing.
Output format: most converters write plain .gcode, which a printer that expects .bgcode or .gcode.3mf may not list.

Phone apps raise the same questions. Whatever produced the file, watch the first layer closely.

How to convert STL to G-code in any slicer

Nine-step flow for converting STL to G-code in a slicer: add your printer, import the STL, orient it, choose the filament profile, set the layer height, add supports, slice, check the preview, and export or send the file

Converting STL to G-code follows the same nine steps in every desktop slicer, even though the buttons have different names.

Add your printer. Pick your model from the slicer’s vendor profiles. The printer profile holds the bed size, nozzle diameter, G-code flavor and start and end G-code.
Import the STL. Drag the file onto the plate or use File, Import. Bambu Studio accepts .3mf, .stl, .stp, .step, .amf and .obj files.
Orient and place it. Put a large flat face on the bed. OrcaSlicer’s auto orientation helps, but its wiki says to review the result, because strength varies with orientation.
Choose the filament profile. It sets nozzle and bed temperatures and cooling; for PLA starting values, see our PLA temperature guide.
Set the layer height. Bambu Lab calls 0.20 mm the norm for most prints and notes that thinner layers take longer. OrcaSlicer suggests 20–80% of the nozzle diameter, or 0.08–0.32 mm for a 0.4 mm nozzle. Our layer height guide shows how each step trades detail for print time.
Add supports under steep overhangs and bridges that the part cannot print in the air.
Slice. Press Slice (Bambu Studio: Slice plate). The slicer calculates every layer and toolpath.
Check the preview. Scroll through the layers, look at the first layer and the supports, and read the estimated time.
Export or send. Save the file to a USB stick or SD card, or send it over the network. In Bambu Studio, Export plate sliced file saves it for the SD card; Bambu Lab says to save it in the card’s root folder, not a subfolder.

A good profile only helps if the printer is tuned. Our guide on how to calibrate a 3D printer covers temperature, flow and retraction tests, and if the first layer fails, see 3D printing first layer problems.

G-code file extensions: .gcode, .bgcode and .gcode.3mf

A G-code file normally uses the .gcode extension and is plain text, but two newer formats compress or package it. Prusa notes that plain G-code is easy to read but stores data inefficiently, which makes files large.

Extension	What it is	Where you meet it	Notes
.gcode	Plain-text G-code	Most Marlin and Klipper printers	Opens in any text editor and G-code viewer
.bgcode	Binary G-code, encoded and compressed	Original Prusa MINI, MK4 and XL from firmware 5.1.0	About 70% smaller on average in Prusa’s testing; PrusaSlicer exports it by default since 2.7.0
.gcode.3mf	A 3MF archive with the G-code plus printer, material and process information	Bambu Studio and OrcaSlicer exports	A plain .3mf project is not sliced; the .gcode.3mf is

PrusaSlicer converts between plain and binary G-code under File, Convert. Prusa also warns that some external printer control programs, such as OctoPrint, may have trouble with binary files, in which case you can turn binary export off. The two ecosystems handle files differently, as our Prusa vs Bambu Lab comparison explains.

How to view and simulate G-code before printing

Every slicer shows a layer-by-layer preview after slicing, and a standalone G-code viewer lets you inspect files from other sources. Bambu Studio’s preview adds a chart of printing times for each part of the print, and PrusaSlicer’s legend shows the time share of perimeters, infill and other features.

For a file you did not slice yourself, the PrusaSlicer G-code Viewer works as a free gcode viewer and simulator. You can start it as its own application or from PrusaSlicer’s File, G-code preview menu, and it reads G-code from PrusaSlicer, Slic3r, Cura, ideaMaker, Simplify3D, Craftware and KISSSlicer. A horizontal time slider allows what Prusa calls “simulation of the print in time.” OrcaSlicer can also import a sliced G-code file to show its toolpaths.

Viewers have one limit: G-code lacks some information, so they estimate extrusion widths and feature types from slicer comments. Prusa notes that files from other slicers will most likely load, but some features may be missing. In any preview, check that:

the first layer is continuous and sits on the bed;
every overhang has support or a gentle angle below it;
no layers are missing or floating in the middle of the part;
the estimated time and filament use look plausible.

Common G-code commands: a short list

Nine cards explaining common G-code commands: G0 and G1 moves, G28 homing, G29 bed probing, G90 and G91 positioning, G92 set position, M82 and M83 extruder mode, M104 and M109 hotend temperature, M140 and M190 bed temperature, and M106 and M107 fan control

Most 3D printer G-code uses a small set of commands, and the list below covers the ones in almost every file. Descriptions follow the Marlin firmware documentation; Klipper supports the same standard commands, with the differences noted.

Command	What it does	Klipper note
G0 / G1	Straight-line move; with E it extrudes filament, and F sets the feedrate in units per minute	Same
G28	Home one or more axes; with no parameters it homes all	Same
G29	Probe the bed and enable leveling compensation	Not a standard command; use BED_MESH_CALIBRATE or a macro
G90 / G91	Absolute or relative positioning	Same
G92	Set the current position, for example G92 E0 to reset the extruder	Same
M82 / M83	Extruder in absolute or relative mode	M83 forces relative extrusion; M82 only cancels that
M104 / M109	Set the hotend temperature; M109 waits until it is reached	Same
M140 / M190	Set the bed temperature; M190 waits until it is reached	Same
M106 / M107	Turn a fan on at speed S0–S255, or turn it off	Same

A start routine strings these together. The sketch below follows the pattern of OrcaSlicer’s example start G-code; the temperatures are example values for PLA, within the ranges in our PLA temperature guide:

G90          ; absolute coordinates
M83          ; relative extrusion
M140 S60     ; start heating the bed
M104 S150    ; warm the nozzle below printing temperature to limit oozing
G28          ; home all axes
M190 S60     ; wait for the bed
M109 S215    ; heat the nozzle to printing temperature and wait
G92 E0       ; reset the extruder position

You can edit a .gcode file in a text editor, but a wrong temperature or coordinate goes straight to the machine, and OrcaSlicer advises testing custom G-code thoroughly. Change the profile and slice again instead.

Common problems after converting STL to G-code

Most failed conversions trace back to a profile that does not match the printer, or to the model’s units.

Symptom	Likely cause	Fix
Model is tiny or huge after import	STL has no units; it was exported in inches	Scale it, or export again in millimeters
Printer does not list the file	Wrong format or folder, for example .bgcode on a host that cannot read it	Export plain .gcode, or save to the card’s root folder
Print starts off the bed or hits a limit	Wrong printer profile or bed origin	Select the correct printer and slice again
Unknown or ignored commands	Wrong G-code flavor	Match the flavor to your firmware
First layer does not stick	Leveling, Z offset or a dirty plate	Fix the first layer before changing slicer settings

The bottom line

Converting STL to G-code is slicing: a slicer turns a unitless triangle mesh into printer-specific instructions, using your printer, filament and process profiles. Prefer a free desktop slicer, treat online converters and downloaded .gcode files with caution, and check the preview before printing. With the three file types and the commands above, you can read almost any printer’s G-code.

Sources

University of Maryland: Slicing and 3D Printing File Types Overview (slicing, unitless STL, 3MF contents, G-code as instructions, infill)
Prusa Knowledge Base: Binary G-code, PrusaSlicer G-code viewer and Sample G-codes (.bgcode size, versions, conversion, OctoPrint note, viewer features and limits, sample files)
Bambu Lab Wiki: Bambu Studio quick start guide and printing from the SD card and third-party integration (import formats, presets, 0.20 mm layers, preview, export to the SD card root, .gcode.3mf)
OrcaSlicer Wiki: Import and export, Layer height, Auto orientation, Advanced printer settings, Printable space and Machine G-code (Gcode.3MF, G-code import, OBJ toolpaths, 20–80% layer height, flavors, bed origin, config block, start G-code)
Firmware documentation: Marlin G-code index, G0-G1, G28, M106 and M109; Klipper G-Codes (command meanings, units, fan range, Klipper support and differences)

Frequently asked questions

Are STL files G-code?

No. An STL file only describes the outside surface of an object as a mesh of triangles, with no units and no print settings. G-code is a list of machine instructions, such as moves, extrusion amounts and temperatures, written for one specific printer. A slicer creates the G-code from the STL, so you always need that step in between before a printer can use the model.

What is a G-code file extension?

The standard extension is .gcode, a plain-text file you can open in any text editor. Newer Prusa printers also accept .bgcode, a compressed binary version that PrusaSlicer exports by default since version 2.7.0. Bambu Studio and OrcaSlicer can export .gcode.3mf, an archive that bundles the G-code with printer, material and process information. All three hold the same kind of instructions.

How do I convert an STL to G-code for a CNC machine?

Use CAM software rather than a 3D printer slicer. A CNC mill removes material with a spinning cutter, so its toolpaths depend on the tool diameter, the stock and cutting depths, none of which a printer slicer knows. A printer slicer adds material layer by layer and writes extrusion and heater commands. Printer G-code is not meant for a mill and will not produce the part.

Can you convert G-code back to an STL?

Not into a usable model. G-code stores toolpaths, not the original surface, so any conversion only approximates the printed lines. OrcaSlicer can export the toolpath preview as an OBJ mesh, which is useful for visualizing a print but poor for editing. If you need the model, ask the file's author for the STL, 3MF or CAD source, which slices far more cleanly.

Is G-code the same for every 3D printer?

The basic commands are shared, but a finished file is not portable. Marlin and Klipper both understand moves such as G1 and heater commands such as M104, yet bed size, start routine, nozzle and firmware extensions differ between machines. That is why slicers ask for a G-code flavor, such as Marlin or Klipper, and why you should slice for your own printer profile.