Repair an STL file online

To repair an STL file, run it through a mesh repair tool that closes holes, turns flipped faces outward and removes duplicate and zero-area triangles, then save it again as a watertight STL. The tool above does this in your browser: drop the file, click Repair and download a result that has been checked again from scratch. It also lists anything it could not fix, such as self-intersections, with what to try next.

What each problem means

• Holes. Triangles are missing, so the surface has a gap. Each hole is a loop of open edges that only one triangle uses.
• Non-manifold edges. Three or more triangles meet at one edge. A clean surface has exactly two per edge, one on each side.
• Flipped normals. The order of a triangle's corners sets which side is outside. A flipped triangle faces inward, and a fully flipped shell is inside out.
• Zero-area and duplicate faces. Slivers with no area and exact copies add nothing to the shape, but they break the edge counts every check relies on.
• Floating shells. Small separate pieces, often left over from a scan or a boolean, that print as stray blobs.
• Self-intersections. The surface passes through itself, for example two parts that overlap without being merged.

What you see and what gets fixed

How the repair works

1. Corners closer than the weld tolerance become one vertex. When open edges look like hairline cracks, the tool suggests a higher tolerance that closes them.
2. Slivers are flipped into the face next to them, copies are dropped, and the wall between parts that touch face to face is removed so they join.
3. Loose flaps on an edge are removed. A T-junction, where a corner sits on another triangle's edge, is closed by splitting that triangle.
4. Faces are turned to agree with their neighbours, holes are closed, and each closed shell is pointed outward. A shell inside another one stays inward, so a hollowed resin print keeps its cavity.
5. The result is checked from scratch: every edge must be shared by exactly two triangles, once in each direction.

Each hole gets a flat patch across its edges, so a big hole in a curved surface leaves a flat spot. Reshape it in a 3D editor if that area matters. You can spin the result in the STL viewer, and a 3MF from your slicer goes through the 3MF to STL converter or straight into this tool.

What it does not fix

Self-intersections and overlapping shells need a boolean union, which rebuilds the surface. Most slicers merge overlapping parts as they slice, so it often does not matter. When it does, use one of the tools below.

Other ways to repair an STL

• PrusaSlicer repairs some errors automatically when it loads an STL and marks broken models with a warning icon. On Windows, right-click the model and choose Fix by Windows repair algorithm (older versions say Fix by Netfabb). See Prusa's guide to corrupted models.
• Bambu Studio also repairs to a degree on import. Right-click the object and choose Fix Model, or click the warning symbol in the object list. It is Windows only, according to the Bambu Lab wiki.
• OrcaSlicer has the same Fix Model command. Since version 2.4.0 it runs on macOS and Linux too, per the 2.4.0 alpha release notes.
• Blender with the 3D Print Toolbox extension (Blender 4.2 and later): its 3D Print tab in the sidebar runs Solid, Intersections and Degenerate checks, and Make Manifold fills holes, fixes normals and removes loose geometry.
• Meshmixer: Analysis, Inspector, then Auto Repair All. Edit, Make Solid rebuilds overlapping shells as one solid, as an approximation that loses fine detail. Autodesk no longer develops it and lists it for Windows only, per its Inspector docs and Marketplace page.
• UltiMaker Cura has Mesh Fixes settings such as Union Overlapping Volumes and Remove All Holes that act while slicing, described in UltiMaker's support article.