Repair STL Online

Open the file and click Repair mesh. The tool joins corners that almost touch, removes zero-area and duplicate triangles, turns flipped triangles and inside-out parts so every normal points out of the solid, and fills simple holes. Then it checks the result again and shows the numbers before and after, so you can see whether the model is now watertight before you download it.

Slicers need a closed (watertight) surface to know what is inside the part. Exporters often leave tiny gaps where two surfaces meet, triangles facing the wrong way, missing faces, or triangles that collapsed to a line. Each of those shows up here as open edges, flipped normals or zero-area triangles. Most of them can be fixed automatically; the table tells you what is left.

Corners closer together than this distance are treated as the same point, which closes hairline cracks between triangles. 0.01 mm closes typical export rounding errors and is far below what a printer can resolve. Use 0.1 mm for models with visibly cracked seams, and 0.001 mm for tiny, very detailed parts. Setting it larger than your smallest detail can collapse that detail.

Each hole's border is followed around and closed with a fan of triangles around its center point, facing the same way as the surface around it. That is exact for flat holes such as a missing cap or bottom. On a curved surface the patch is a shallow cone rather than the original curve. Holes with more than 256 edges, or whose border touches itself, are left open and reported, so a large opening is never covered with a wrong surface.

It does not fix edges shared by three or more triangles (usually overlapping or touching parts), surfaces that pass through each other, walls that are too thin to print, or large and tangled holes. Those are listed after the repair. Fix them in the source CAD file, or combine overlapping parts with union() in a CAD tool such as the OpenSCAD playground.

No. Each closed shell is checked with a ray test to see whether it sits inside another one. An inner wall (the cavity of a hollow part) is kept facing inward, so the cavity stays a cavity, while outer surfaces face outward.

No. Files are read and processed by JavaScript in your browser tab, and downloads are generated locally. Nothing is sent to a server, so it also works with private or unreleased designs.