# G-code Files: What They Are and How to Open Them Online

A .gcode or .nc file is a list of machine moves from a slicer or CAM program. Learn what it contains, how to preview 3D printing and CNC G-code online and why some files will not draw.

A G-code file is a plain-text program for a machine. Each line is a command such as `G1 X40 Y12.5 F1800`: move in a straight line to X 40, Y 12.5 at 1,800 mm per minute. 3D printer slicers (Cura, PrusaSlicer, OrcaSlicer, Bambu Studio, Simplify3D) write it from an STL or 3MF model, and CAM software (Fusion, Mastercam, FreeCAD Path, Estlcam and many others) writes it for mills, routers, lathes and lasers. The extension varies: `.gcode` and `.gco` are common for printers, while CNC programs are often saved as `.nc`, `.ngc` or `.tap`.

You can open one in the [CADProps G-code viewer](https://www.cadprops.com/tools/gcode-viewer/index.md). Your browser reads the program and draws every move as a line, with a layer slider, colours by line type and the extents and path lengths of the job. The file is also uploaded so you can reopen or share it, but the drawing itself does not wait for the upload.

## What the commands mean

Most of a file is made of a handful of commands:

| Command | Meaning |
|---|---|
| G0 | Rapid or travel move, not cutting or printing |
| G1 | Straight move at the feed rate; on a printer, with an E value it extrudes |
| G2, G3 | Clockwise and counter-clockwise arcs, given by a centre (I, J, K) or a radius (R) |
| G20, G21 | Coordinates in inches or millimetres |
| G90, G91 | Absolute or incremental coordinates |
| G92 | Set the current position to a new value, often used to reset the extruder |
| M3, M5 | Spindle on and off |
| M104, M109, M140 | Nozzle and bed temperatures |
| T1, M6 | Select and change tool |

Comments start with `;` or sit in parentheses. Slicers use them for useful markers such as `;LAYER:12` and `;TYPE:Outer wall`, which is how a viewer can colour walls, infill and supports differently.

## A 3D printing file versus a CNC program

The two look alike but are read differently.

In a **3D printing file**, the E axis is the filament. A G1 move whose E value grows lays down plastic; a move without new extrusion is travel. The model is built bottom-up, so layers are the natural way to inspect it: stepping through them shows where supports start, how infill is laid out and whether the first layer covers the bed as expected. The total net E movement tells you how much filament the job feeds, as a length.

In a **CNC program**, there is no extrusion. G0 rapids position the tool above the work and G1, G2 and G3 moves cut. Z steps down pass by pass, so stepping through Z levels shows the step-down and the order of operations. The lines are the centre of the tool, not the edge of the cut: a 6 mm end mill following a line removes 3 mm on each side of it.

CADProps decides which kind a file is from its content. A file with extrusion moves is treated as a print; anything else is treated as a CNC or laser program.

## How to open a G-code file online

1. Open the [G-code viewer](https://www.cadprops.com/tools/gcode-viewer/index.md) and select or drop the file.
2. Drag the progress bar to move through the program; the layer or Z step follows it. The arrow buttons beside the bar step one whole layer at a time, and **This layer only** shows a single layer or Z step.
3. Press play to watch the program run in file order.
4. Use **Line types** to hide categories, and **Travel** or **Rapids** to show non-printing moves.
5. Press **Reference model** and choose the model the program was made from: STEP, IGES, a native CAD part or a mesh such as STL or 3MF. It is placed against the toolpath and cut at the current layer as you play, so you can see whether the path matches the part.
6. Check the extents and lengths in the details panel. The units come from G20 or G21; if the file has neither, millimetres are assumed and the panel says so.

Large files stay quick because nearly straight runs of short moves are merged before drawing. The numbers in the panel are still calculated from every move.

## Why a G-code file might not draw correctly

- **It is binary or resin output.** Prusa `.bgcode` and resin printer slices (`.ctb`, `.sl1` and similar) are not text G-code. Export plain G-code from the slicer instead.
- **It relies on macros.** LinuxCNC and Fanuc macro programs that compute positions with `#` variables, expressions or subprogram calls cannot be drawn line by line. Those lines are skipped and a warning appears; the controller's own preview is the right tool for them.
- **It uses drilling cycles.** G81 to G89 are drawn as a simple plunge and retract at each hole, without pecks or dwells.
- **It uses work offsets.** Moves under G54 to G59 are drawn in one coordinate system, so a program that machines several fixtures may show them overlapping.
- **The scale looks wrong.** A program without G20 or G21 is assumed to be in millimetres. If a part looks 25.4 times too small, the program was probably written in inches; see [the scale factor guide](https://www.cadprops.com/guides/cad-model-wrong-scale-25-4/index.md).

## What a preview cannot tell you

A G-code preview shows the path the program asks for. It does not simulate material removal, check the tool against clamps or machine limits, or know your printer's firmware settings. Treat it as a quick check that the right file was sliced or posted, and use your slicer, CAM software or machine controller for anything that decides whether the job is safe to run.

If you are preparing parts for a shop rather than running them yourself, send the model, not the G-code: shops program their own machines. The [CNC quote checklist](https://www.cadprops.com/guides/prepare-step-file-for-cnc-quote/index.md) covers what to include, and the [3MF versus STL comparison](https://www.cadprops.com/guides/3mf-vs-stl/index.md) explains which model file suits a print service.

## Related reading

- [3MF vs STL: Units, Multiple Parts and Mesh Handoff](https://www.cadprops.com/guides/3mf-vs-stl/index.md)
- [STEP File Scale Wrong by 25.4? Fix Inch and mm Units](https://www.cadprops.com/guides/cad-model-wrong-scale-25-4/index.md)
- [IGES File (.igs, .iges): What It Is and How to Open It](https://www.cadprops.com/guides/iges-file-what-it-is-how-to-open/index.md)
- [Open a Rhino 3DM File Without Rhino](https://www.cadprops.com/guides/open-3dm-without-rhino/index.md)
- [Open CAD Files on Mac: STEP Viewers and Format Options](https://www.cadprops.com/guides/open-cad-files-on-mac/index.md)
- [Prepare a STEP File for a CNC Quote: A Shop-Ready RFQ Checklist](https://www.cadprops.com/guides/prepare-step-file-for-cnc-quote/index.md)
- [STL Export Settings: Mesh Resolution, Units and Scale](https://www.cadprops.com/guides/stl-export-settings/index.md)

Original article: https://www.cadprops.com/guides/open-gcode-file-online/

HTML page: https://www.cadprops.com/guides/open-gcode-file-online/