UV unwrap: texture coordinates for a mesh that has none
Drop a model without UVs and get it back with an automatic layout: Blender's Smart UV Project, which splits the surface at a 66° angle into flat islands and packs them with a 2% margin. A mesh that already has UVs is returned as it is unless you tick replace. GLB, FBX or OBJ out.
Off: meshes that already have a UV map are left alone.
STL has no UVs and is not offered. Defaults to the format you dropped.
What Smart UV Project makes
The surface is cut wherever neighbouring faces turn more than 66 degrees, each piece is flattened by projecting it onto its average plane, and the pieces are packed into the unit square with a 2 percent gap. The result is a layout with no stretching inside an island and a seam at every sharp turn: right for baking, for vertex paint to texture, for a tiling material, for a scan or a CAD export that arrived without any coordinates at all.
It is not an artist's unwrap. A character's face comes out as several islands rather than one continuous map, and a texture painted for it has to follow those islands. For a model that will be hand-painted, unwrap it in your 3D tool with seams you choose; for everything else this is the layout the tool would have made for you.
What this tool cannot do
Islands, not a continuous map
Every sharp turn is a seam, and a curved surface becomes many flat pieces. Fine for baking and tiling; a painted texture will show the seams.
Only meshes without UVs, by default
A mesh with a UV map is returned unchanged and the report says hadUv. Tick replace to throw the existing maps away and make one new one named UVMap.
No STL out
STL stores triangles only; writing it would throw the new coordinates away. The output is GLB, FBX or OBJ.
One job at a time on a shared machine
Jobs run one after another on one converter. A dense model can take a few minutes; a job that needs more than five minutes of Blender time is stopped and reported, not left hanging.
Three days, then gone
The upload and the result are deleted three days after the job, downloaded or not. The page keeps no history: bookmark the download or take the file while it is there.
Frequently asked questions
Does this make a texture?
No. It makes texture coordinates, the map from the surface to an image. The image is yours to paint, bake or assign; the model will now accept one.
My model already has UVs, what happens?
Nothing, unless replace is ticked. The file is re-exported in the format you chose with its UVs as they were, and the report says hadUv so the no-change is not a mystery.
Will the UVs line up with a texture I already have?
No. A new automatic layout bears no relation to the old one. Unwrapping is for models that have no texture yet, or whose texture will be baked onto the new layout.
Can I unwrap a rigged model or one with shape keys?
Yes to both. The unwrap adds coordinates and changes no vertex, so skin weights and shape keys are untouched.