STL, OBJ, and 3MF viewer and analyser

Drop an .stl (binary or ASCII), .obj, or .3mf file. Choose the units the file was saved in if it was not millimetres.

Volume is the sum of signed tetrahedra from the origin to each triangle — exact for a closed (watertight) mesh. Area is the sum of triangle areas.
Watertight check: vertices are welded at 0.1 µm, then every edge should be shared by exactly two triangles. Edges used once are open (drawn red); edges used three or more times are non-manifold; a negative signed volume means the mesh is inside-out, and edges walked the same way by both neighbours mean inconsistent triangle winding.
Weight: shell = surface area × wall thickness (perimeters × line width), capped at the part volume; the remaining volume is filled at the infill percentage. Resin is treated as solid. Slicers typically land within ±15 %.
Transform: scale, rotate 90° about an axis, mirror (with winding fixed), flip normals, and centre the part on the bed (X/Y centred, Z from 0), then download binary or ASCII STL.
3MF: all meshes in the package are combined; build-item transforms are not applied.

⛶ Full screen (or F) gives the 3-D view the whole screen — the browser’s full screen where it is allowed, otherwise a full-window layer (iPhone) — and ◫ Panels hides the settings column so the model goes edge to edge. Escape leaves.