STLFixer

STLFixer is a free web tool for inspecting STL meshes and applying a limited set of repairs. It accepts ASCII and binary STL files up to 50 MB and processes the model locally in the browser.

You can inspect the mesh in 3D, review supported changes, and download the checked result. Supported work includes cleanup of exact duplicate or degenerate faces, compatible orientation correction, and selected small local repairs. Large holes, general non-manifold rebuilding, and widespread intersections need another tool. There is no account requirement. Check dimensions and the layer preview in your slicer before printing; repair does not guarantee printability.