Free online 3DM viewer

Drop a file and it renders instantly: drag to rotate, scroll to zoom, right-drag to pan, and toggle wireframe in the corner to see the triangle structure. The panel below the viewer reports exact dimensions, volume, surface area, triangle count, and whether the mesh is watertight.

3DM is Rhino's own format. The viewer reads the render meshes Rhino saves alongside the NURBS surfaces, and converts the file's units to millimeters so the measurements below match the rest of the tools. Files saved with "Save Small" have no render meshes and will come up empty.