Repair STL files online

You need an STL repair tool when a model looks fine on screen but your slicer struggles with it. Typical signs:

• the slicer warns that the model has errors, or says it "repaired" it automatically;
• some layers are missing, or a wall has a gap in the preview;
• infill pokes out of the model, or fills a space that should be hollow;
• the volume shows as zero or negative;
• a downloaded model has the right size but falls apart once sliced.

Broken STLs usually come from file conversions, 3D scans, exports from modeling software, or several parts glued into one file. This tool checks and repairs the mesh right in your browser; the file is never uploaded to a server. Besides STL, you can drop OBJ, 3MF and other mesh formats.

A slicer cuts your model with a horizontal plane at every layer. Each cut has to produce closed outlines, because those outlines decide where there is plastic and where there is air. For that the mesh must be manifold (watertight): every edge is shared by exactly two triangles, and every face points outward.

When that isn't true, the cut becomes ambiguous:

• at a hole (open edge) the outline doesn't close, so the slicer has to guess which side is inside;
• at a non-manifold edge more than two faces meet, so it isn't clear which body is on which side;
• a flipped normal points inward, which can swap inside and outside;
• in an inside-out shell every face points inward, and the volume comes out negative.

Slicers quietly handle many of these problems, but not always the way you'd want. Checking and repairing before you print saves failed prints.

The repair runs in a fixed order, each step building on the one before:

1. Weld vertices: vertices that sit on top of each other but are stored separately are merged. Many "holes" disappear here, because they were only split vertices.
2. Remove degenerate triangles: zero-area faces that have collapsed into a line or a point are deleted.
3. Remove duplicate triangles: faces that use the same three vertices twice are reduced to one.
4. Make orientation consistent: neighboring faces are turned the same way, and inside-out shells are flipped back.
5. Fill holes: a flat hole is closed with ear-clipping triangulation, a non-flat hole with a fan patch. You can turn this step off.
6. Re-check: the repaired mesh is tested the same way as the original.

At the end you get a report: how many vertices were welded, how many faces were removed or flipped, how many holes were filled with how many new triangles, and the triangle count before and after. If all is well, the mesh is closed and consistently oriented. If not, the remaining bad edges stay highlighted in red.

To be upfront: not every defect can be fixed safely and automatically.

• Non-manifold edges where more than two faces meet (for example, two cubes touching along one edge). The tool flags them but leaves them alone, because the right fix depends on intent: should the bodies be separated or merged? Slicers usually handle these well.
• Overlapping bodies: if two separate, individually clean bodies intersect, they aren't merged. The slicer combines them when it prints.
• Large, complex holes: if a hole can't be patched safely, it's skipped, and the report says so.
• Walls that are too thin, tiny details: these aren't mesh errors, and repair won't thicken them. Use the section view to check wall thickness.

If the model came from CAD software, the best fix is often to export it again from there with finer settings.

• Check the volume. On an open mesh the volume is only approximate. After repair it is the volume of a closed body, so compare it with what you expect.
• Look inside with a section. On the Section tab, cut the model along Z, X or Y. If a patch sits oddly, you'll see it there.
• Inspect patches in wireframe. The fan patch on large non-flat holes works, but isn't always pretty. On a visible surface, fix the source model instead.
• Save as 3MF. 3MF stores the unit and is more compact. STL is fine if that's what your slicer wants.
• Try the sample. The deliberately holey sample model lets you see what the repair does with no risk.

You can inspect the repaired model in the STL viewer or take it straight to the 3D print cost calculator.