Runs on your device · Your file never leaves your device.
STL is the printing format: shape and nothing else, and almost nothing on the web will display it. GLB is the web format: one self-contained file that a browser, a model viewer or an AR view can open directly. This converts the printable file into the showable one.
An STL almost never carries colour, so the GLB usually arrives with one plain surface — there was nothing else to bring (the rare binary STL with a colour per triangle keeps it). What you gain is a file you can put on a page, send to a client, or drop into a viewer without them needing any 3D software at all.
The conversion runs on your own machine, which for a 3D file is the whole point: a model is usually a client’s product, a scanned property or something not yet announced. Nothing is sent to a server, so there is nothing to delete afterwards and nothing to take anyone’s word for.
Not for: Adding colour or texture the STL never had — no converter can recover what the source format cannot store.
An STL stores no unit. Almost every STL made for 3D printing counts in millimetres, while a GLB counts in metres, so the page reads an STL as millimetres by default and shows the size the GLB will have: a 40 mm bracket becomes 40 mm, not 40 metres. An STL from a program set to inches takes An inch, and one already in metres takes Leave as it is. The triangles keep their original numbers, and the scale is written on the model's top node, which glTF viewers apply.
Slicers build upwards along Z, so STL files made for printing usually stand with Z as up, while a GLB uses Y. The tool gives the model a quarter turn, so a part that stood upright on the print bed also stands upright in a web viewer or in AR; you can choose to keep the axes instead. The turn is written on the same top node as the scale.
Every part gets the same plain surface: light grey, fully rough and not metallic. An STL stores one facing direction for each triangle, and those directions become the GLB's normals, so curved areas show the flat facets they were printed from instead of looking smooth. Corners that are exactly identical, including their facing direction, are merged when the GLB is tidied.
Text (ASCII) and binary STL are both read. A text STL that holds several named solids arrives as one mesh with one surface. A binary STL that stores a colour for each triangle, as some printing software writes, keeps those colours in the GLB as colours on the corners, slightly darkened by the grey surface they are combined with.
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