Free Online STL Repair Tool

Slicer says your model has errors? Drop the STL here. See exactly what's broken — holes, flipped faces, duplicate vertices, degenerate triangles — get the fixable parts repaired, and download the result. Everything runs in your browser.

Drop an STL file here

Max file size: 100MB

No credits · files stay on your device

STL preview

Upload or enter content to preview the printable mesh.

01Free diagnostic repair tool

Diagnose first, then repair

Your slicer flags errors when it can't tell what's inside your model and what's outside — and useful STL repair starts by showing you exactly which problem your file has. A printable STL describes one or more closed triangle surfaces; slicers don't need CAD history, colors, textures, or units, just topology that reads as solid material. Watertight means every printable surface edge is shared by exactly two triangles. When your STL has boundary edges, the model has a hole or open seam. When an edge is non-manifold — three or more triangles sharing it — the model may hide overlapping shells, internal faces, or self-intersections. Degenerate faces are triangles with repeated vertices or zero area: they add noise without describing real surface, and they can confuse normal calculation and repair tools.

The watertight rule

Every printable surface edge must be shared by exactly two triangles — boundary edges mean holes.

━ shared by two triangles┅ boundary edge = hole

What the pass does — and won't

You see the counts before anything changes — and again after.

Fixed automatically

welds duplicate vertices
removes degenerate triangles
recalculates orientation
caps small boundary rings

Beyond automatic repair

large missing regions
non-manifold intersections
inverted internal parts
paper-thin walls

Diagnose → repair → verify

Use the result as practical cleanup, not proof of printability.

Diagnose

boundary, non-manifold, degenerate, duplicate counts

Repair

weld, clean, re-orient, fill small holes

Verify

inspect the layer preview in your slicer

02How it works

How to repair an STL file

Upload and see what's wrong

Drop your local STL into the tool. Before anything changes, you see the counts: boundary edges, non-manifold edges, degenerate triangles, duplicate vertices.

Run automatic repair

Click Repair STL to weld duplicate vertices, remove degenerate faces, align triangle winding, recalculate normals, and fill small boundary loops.

Compare and download

Check the before-and-after metrics, confirm whether your mesh is watertight, preview the result, and download the repaired STL for your slicer.

03Deep dive

What STL repair can and cannot fix

3.1See what was broken — before anything changesYou see the damage first. The tool diagnoses your mesh and shows the counts before touching a single triangle, then again after repair, so you know exactly what changed — not just that 'something was fixed'.

You see the damage first. The tool diagnoses your mesh and shows the counts before touching a single triangle, then again after repair, so you know exactly what changed — not just that 'something was fixed'. The repair pass welds vertices that sit in the same position within a small tolerance, removes degenerate triangles, builds triangle adjacency, and uses a directed flood-fill so neighboring faces agree on winding. The largest coherent shell then sets the outside direction by signed volume, and normals are recalculated from the repaired triangles. Finally, boundary edges that form small closed loops are grouped into rings; rings with 32 vertices or fewer are capped with a centroid triangle fan, so tiny missing triangles and small holes become closed surfaces.

3.2What automatic repair cannot fixYour file may need more than an automatic pass — better to know that up front.

Your file may need more than an automatic pass — better to know that up front. Automatic STL repair handles common export damage well: duplicated STL vertices, a few flipped facets, accidental zero-area triangles, and small open holes from mesh edits or format conversion. It is not a full geometric solver. Large missing regions are reported but not filled, because a centroid cap would invent a flat patch that may destroy the shape. Non-manifold intersections, overlapping duplicate shells, inverted internal parts, paper-thin walls, and badly self-intersecting scans often require manual modeling decisions. And a slicer may still reject a technically watertight model if the walls are thinner than your nozzle, resin pixel size, or material process can print.

3.3After repair, verify in your slicerYour repaired STL is a practical cleanup result, not proof that the part will print perfectly.

Your repaired STL is a practical cleanup result, not proof that the part will print perfectly. Open it in Cura, PrusaSlicer, Bambu Studio, Lychee, Chitubox, or your preferred slicer and inspect the layer preview, scale, wall thickness, islands, and supports. If holes remain, the report tells you how many boundary loops were too large for automatic capping. Repair those in Blender, MeshLab, Fusion, or a CAD tool where you can decide what the real surface should be — or regenerate the model with a workflow that produces a watertight printable mesh from the start.

04Beyond this tool

Complex model still not repairable?

When your STL has missing surfaces, severe scan damage, or topology that needs more than automatic cleanup, use the AI generator to create a new watertight, print-ready model instead.

Generate a watertight model

05FAQ

STL repair FAQ

Answers about slicer errors, watertight meshes, what automatic repair can't do, and file privacy.

My slicer says the model has errors — why won't my STL print?

Usually because the mesh can't be read as a closed solid: open boundary edges, non-manifold edges, flipped faces, zero-area triangles, self-intersections, disconnected shells, or walls too thin for the printer process. Run the diagnosis to see which of these your file actually has.

Can I fix an STL without learning Blender or Meshmixer?

Yes, for common export damage — duplicate vertices, flipped facets, zero-area triangles, and small holes — automatic STL repair handles it in your browser. Large missing regions and badly self-intersecting geometry still need a mesh editor or CAD tool.

Is repair lossless?

No. Welding vertices, deleting degenerate faces, recalculating normals, and filling holes all change topology. STL repair gives you a more printable mesh, not a byte-for-byte or edit-history preserving conversion.

Are my STL files uploaded?

No. Diagnosis, repair, preview, and STL export run locally in your browser. Your file does not leave your device for this tool.

What does watertight mean?

Watertight means every edge in the printable surface is shared by exactly two triangles. That lets a slicer infer a closed solid volume instead of an open shell.

Why were some holes not filled?

Because they were too large to fill safely. The tool only caps small boundary loops; large holes need shape-aware decisions, so they are reported instead of being patched with a potentially wrong flat surface.

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