Analyse an STL file for common mesh problems and get a clear report. This checker reports issues only, it does not repair or modify your file.
The file is opened locally in your browser and is never uploaded.
The checker loads an STL file, welds vertices by rounded position, and examines the triangle mesh. It counts degenerate zero-area triangles, builds an edge map to find boundary edges (used by only one triangle, meaning a hole or open shell) and non-manifold edges (shared by a number of triangles other than two), and computes signed volume from the tetrahedra formed with the origin. A negative signed volume usually means the normals point inward. A mesh is reported as watertight when every edge is shared by exactly two triangles. To be explicit: this tool reports issues and does NOT repair or modify the file, use the report to decide what to fix in a mesh editor or CAD tool.
Does this tool fix my STL? No. This checker reports issues and does not repair or modify the file. Nothing is re-saved or changed; use a mesh editor to make fixes.
What does watertight mean? A watertight mesh is a closed solid with no holes: every edge is shared by exactly two triangles. Boundary edges or non-manifold edges mean the mesh is not watertight.
Why is the signed volume negative? Signed volume is negative when triangle winding makes the normals point inward. Many slicers still print such files, but it is a sign the mesh should be flipped in an editor.