A 3D model converter that shows its arithmetic

Any of the nine readable formats into any of the seven writable ones — 60-odd combinations, and each of them is checked rather than assumed. A test box of known size is written through every pair and measured on the way back; a conversion that came back off by so much as a hundredth of a millimetre fails the build. The one gap is FBX, which can be read and cannot be written, because no open FBX writer exists.

Because it is the failure that looks like something else. STL, OBJ and PLY declare no unit and are millimetres by convention; glTF, GLB and USDZ are metres by specification; FBX is centimetres times a factor in its own header; 3MF and STEP state theirs outright. Copy coordinates across those boundaries without converting and the part is a thousand times wrong, which reads as a corrupt file rather than a wrong assumption. Every conversion here prints the factor it applied.

It depends on the pair, which is why the answer is computed from your file rather than written into the page. Writing STL drops colour, materials, texture coordinates and the split into parts. Writing OBJ leaves materials in a .mtl a single download cannot carry. Writing 3MF drops textures but gains a stated unit. The panel lists what your model in particular carried and lost.

As large as this tab can hold. There is no fixed limit, because there is no server whose capacity would impose one — but the whole file is held in memory along with the parsed mesh and the written output, so past 100 MB you are told what that means before anything is read. A server-side converter can promise a bigger number because the memory is somebody else's; the trade is that the file is somebody else's too.

Because FBX is Autodesk's format with no public specification, and the only sanctioned way to write one is their SDK under their licence. Every open tool that reads FBX does so through reverse engineering. Offering an FBX export here would mean writing a guess at a proprietary container and calling it a conversion. Reading it is fine; writing it is not something to promise.

No. Every parse, every measurement and every write happens in this tab. There is no upload endpoint, no queue and no temporary copy on a disk you cannot see.