Why does Cura say “Unknown file format” for my STL?

The extension is only a label. Check the detected signature and declared triangle count; a truncated download, an HTML error page saved as .stl or a renamed file is more likely than Cura refusing a valid STL.

Why is my model invisible in Cura even though the file is listed?

The file can contain zero triangles, or the geometry can be far outside the visible build plate. Check the triangle count and bounds first, then look at the model scale and position.

Can Cura open STEP, OBJ, 3MF and GLB files?

Cura's import support depends on the version and installed plugins. This checker accepts common mesh formats, but the safest path is to export a clean STL or 3MF in millimetres for the slicer you actually use.

Does Cura automatically repair non-manifold STL files?

Cura may offer repair or slice around some defects, but results vary with the defect and version. Closing boundaries and removing non-manifold edges first gives a more predictable result.

Does this page upload my model to a server?

No. The file is read with File.slice() and parsed inside a Web Worker in this tab. The signature, bounds and topology report are generated locally and no model data is sent anywhere.

Slicer import fixes

STL won't open in Cura? Diagnose the file locally

Cura is usually the messenger, not the cause. A file that appears in the queue but not on the build plate, an “Unknown file format” message, or a model that measures 2 mm instead of 200 mm usually points to a damaged header, an empty export, an open surface or a unit mismatch. This page reads the file itself and separates a slicer-import problem from geometry that needs repair.

Ultimaker CuraNo uploadSlicers · Windows / macOS / Linux

Typical symptoms

The file appears in the open dialog or queue but nothing appears on the build plate.
Cura reports an unknown or unsupported file format even though the extension is .stl.
The model loads at a tiny or enormous scale, often by a factor of 10, 25.4 or 1000.

File import check

No upload

Ultimaker Cura

The detected signature, bounds and topology are reported locally. These are triage signals, not a slicer validation.

File checks

The detected signature, bounds and topology are reported locally. These are triage signals, not a slicer validation.

Detected file signatureBoundary edges · Non-manifold edgesTypical symptoms

Drop the file that won't open

STL, OBJ, PLY, 3MF, GLB, glTF, or a .gltf with its .bin companions. The file is read locally.

Choose a file

Max file size: 500 MB · No upload

No upload · Worker

Typical symptoms

The file appears in the open dialog or queue but nothing appears on the build plate.
Cura reports an unknown or unsupported file format even though the extension is .stl.
The model loads at a tiny or enormous scale, often by a factor of 10, 25.4 or 1000.
Cura warns about an invalid mesh, repairs it, and the result still has holes or missing walls.

Common causes

The file is truncated or mislabeled

A binary STL declares its triangle count in the first 84 bytes. If the download stopped early, or a different file type was renamed to .stl, Cura may reject the file before it can inspect the mesh.

The export contains no triangles

An object can be hidden, outside the export selection or reduced to an empty scene. The file is valid enough to open, but there is no printable geometry to place on the plate.

Open boundaries or non-manifold edges

Cura can slice some imperfect meshes, but large missing faces and edges shared by more than two triangles may make repair fail or create missing layers in the preview.

The model units are not millimetres

STL and OBJ do not carry reliable print units. A design exported in metres or inches can load correctly and still be 1000× too small or 25.4× too large.

Export checklist

1
Export a binary STL or 3MF in millimetres and confirm the file size is not zero.
2
Make the object visible and selected before export; do not export the whole hidden scene.
3
Apply object transforms in the CAD tool so the STL contains the final scale and rotation.
4
Re-export from the original CAD file if the STL header or triangle count is damaged.

How it works

1
Drop the file here and read the detected signature and triangle count.
2
Check the bounds to catch a unit mismatch before changing any slicer scale setting.
3
Use the topology findings to decide between a quick clean, a hole fill and a full repair.
4
Re-export from CAD when the header or geometry is empty; repair cannot recover data that was not written.

Questions

References

https://ultimaker.com/software/ultimaker-cura
https://support.ultimaker.com/
UltiMaker Cura product page
UltiMaker support documentation
STL binary header and triangle-count convention

Specs verified: 2026-11-01

Each page checks the file header, triangle count, bounds and topology in this tab. No account, upload or slicer integration.

Other slicer checks

Bambu Studio

Diagnose an STL or 3MF that Bambu Studio will not open: header damage, empty geometry, open boundaries, non-manifold edges and scale errors. Local analysis, no upload.

Open diagnostic

PrusaSlicer

Check why PrusaSlicer shows no layers, a tiny model or an import error. Inspect headers, empty meshes, open boundaries, non-manifold edges and units locally before re-exporting.

Open diagnostic

OrcaSlicer

If OrcaSlicer fails to load a model, check the file signature, empty geometry, open boundaries, non-manifold edges and scale before trying another download. Local diagnosis, no upload.

Open diagnostic