STL Check: is my STL watertight?

No. The check runs inside this page, in your browser. There is no server to send it to. Load the page, switch off your wifi and it still works.

Every edge of every triangle is shared with exactly one other triangle, so the surface closes all the way round with no holes or gaps. CAM software and slicers need that to tell the inside of a model from the outside.

Holes and open edges, non-manifold edges (three or more triangles on one edge), faces pointing the wrong way, broken and repeated triangles, separate pieces and loose fragments, faces crossing each other, volume, size in millimetres and inches, and the triangle count. Pick CNC or a printer to add the checks that machine needs.

It does not say whether a design will carve well with your bits (Will it cut? does that), whether a print needs supports, or whether the model fits your machine. Faces crossing each other are not checked above 700,000 triangles, and wall thickness is checked only for prints, by sampling. Your file is never changed.

The general check looks at the mesh only. CNC or laser relief adds a flat back facing down. FDM and resin prints add wall thickness (0.8 mm and 0.3 mm), how the model sits on the bed, and the best side to print on.

Press Repair it. The same file opens in Broken STL Auto-Repair in this browser, without choosing it again, and comes back repaired for the machine you pick, checked again before it says ready.

Files of about two million triangles work on an ordinary computer. The check for faces crossing each other is skipped above 700,000 triangles, and the results say so instead of passing the file.