STL Repair Online

STL repair fixes the topology of a triangle mesh so it forms a closed, consistently oriented surface that slicers can turn into toolpaths. Typical fixes are closing holes, removing duplicate or zero-area triangles, welding loose vertices and making face orientation consistent.

Yes. Checking and repairing STL files is free, with no account, watermark or download limit.

No. The file is read and processed locally in your browser; the model and its name are never sent to a server. Anonymous usage statistics record only summary numbers, such as triangle and issue counts.

A non-manifold edge is shared by more than two triangles, something a real surface cannot do; a non-manifold vertex joins parts of the surface that touch at a single point. Both usually come from overlapping parts or internal faces. FixSTL counts them and repairs the ones caused by duplicate faces, unwelded seams, zero-thickness flaps and small damaged patches it can cut out and fill again. Internal walls and parts that touch along an edge or at one corner stay as they are; they need a boolean union or manual cleanup in a modelling tool.

FixSTL repairs self-intersections confined to a small area: it cuts out the crossing faces with a ring of faces around them and fills the opening, as long as the patch spans less than 15% of the model and the result stays within 1% of the model’s size of your original surface. It does not split parts along the line where they cross, so separate parts pushed into each other and boolean leftovers are reported and left unchanged. It estimates wall thickness and flags zero-thickness surfaces, but cannot thicken them. Use a modelling tool such as Blender or Meshmixer for those, or regenerate the model. If your browser cannot run the WebAssembly checker, the basic checker used instead does not look for self-intersections; the report says which ran.

Most repairs leave the surface where it is: welding, cleanup, reorienting and stitching may move it by at most 0.1% of the model’s bounding-box diagonal. Filling a hole adds surface across the opening. Rebuilding a damaged patch replaces the surface there; the result stays within 1% of the diagonal of your original surface, and the number of parts does not change. A repair beyond these limits is discarded and the original geometry is kept. Dimensions and units are never rescaled. When a patch is rebuilt, the report says how many faces were replaced and the largest measured surface shift, but not where on the model, so check critical dimensions on functional parts.

Automatic repair leaves what it cannot fix within its limits: openings larger than about a third of the model, overlapping parts and boolean leftovers, internal walls, parts touching along an edge, and damaged areas too large to rebuild locally. The result report shows what remains. Those need manual modelling or a rebuild.

ASCII and binary STL files up to 100 MB and 2,097,150 triangles. Checking a 100 MB mesh takes seconds; repairing it can take several minutes, with progress shown and cancel available. The download is a binary STL, or in rare cases an ASCII STL when binary rounding would undo the repair. You can drop up to 20 files (200 MB in total) at once: they are checked and repaired one at a time, and the repaired ones download together as a zip with a CSV summary of every file.

No. Watertight means the surface is closed. Printability also depends on wall thickness, overhangs, scale, supports and material, so always check the slicer preview.