# How to Compare a G-code Toolpath with Its CAD Model

Show the source model beside a G-code toolpath in CADProps. Check the revision, units, placement and layer before diagnosing an offset.

The toolpath looks like the part, but it sits off to one side. Before posting the job again, check that the program and model came from the same revision and that you are comparing them at the same origin. The [CADProps G-code viewer](https://www.cadprops.com/tools/gcode-viewer/index.md) can show both in the same view.

## Add the source model and check placement

1. Open the G-code program. Choose **Reference model → Choose model**, then select the model you sliced or used in CAM.
2. Open **Placement**. For a print, start with the model on the bed and centred on the printed extents. For CNC, compare the suggested placement with the work origin set in CAM. CADProps places the model using its bounding box.
3. Check **Top** and **Front**. Confirm a known span before adjusting position. If the origin is the only difference, enter the known offsets in **Move model (mm)**. Changing the placement preset resets those offsets.
4. Pause playback at a distinctive feature. Use **Cut at the current layer** or **Cut at the current Z step** to see the outline at that height. Turn the cut off when you need to see the whole part.

<figure class="workflow-evidence"><img src="/assets/evidence/gcode-2026-10-24/gcode-viewer-reference-model-1440.webp" alt="CADProps shows the chess rook print toolpath inside its translucent reference model at layer 234" width="1440" height="1000" loading="lazy"><figcaption>CADProps chess rook demo. The translucent model shows the remaining top of the rook while the path is paused below it.</figcaption></figure>

## Read the kind of mismatch

| What you see | What to check |
|---|---|
| Same shape and size, shifted in one direction | Compare the slicer origin or CAM work origin with the model placement. |
| One view looks aligned but the other does not | Check the Z origin and the reference model orientation. |
| The outlines differ by roughly a factor of 25.4 | Check the program units and the units assumed for the model. STL and OBJ are read as millimetres. |
| A local feature is missing or in a different position | Confirm the file pair and revision before moving the overlay. |

In a print, the line marks the nozzle centre. A CNC outside profile follows the cutter centre, so its line can sit outside the part by the tool radius. A small offset can be expected. An overlay helps you identify where to investigate, but it cannot establish fixture clearance or verify the material removed by a cutter. See [what the G-code viewer reads](https://www.cadprops.com/guides/open-gcode-file-online/index.md).

## Related reading

- [How to Inspect CNC G-code Rapids and Z Steps](https://www.cadprops.com/guides/check-cnc-gcode-rapid-moves/index.md)
- [How to Inspect the First Layer of a G-code Print](https://www.cadprops.com/guides/gcode-first-layer-preview/index.md)
- [G-code Files: What They Are and How to Open Them Online](https://www.cadprops.com/guides/open-gcode-file-online/index.md)

Original article: https://www.cadprops.com/guides/compare-gcode-toolpath-to-cad-model/

HTML page: https://www.cadprops.com/guides/compare-gcode-toolpath-to-cad-model/