How to convert OBJ to STL

STL is what slicers and CAD packages expect. OBJ carries more than STL can hold, so it is worth knowing what is left behind.

Steps

1. Open the mesh converter.
2. Drop your .obj file on the page. It is read on your machine; nothing is sent anywhere.
3. Pick STL (binary) in the format list.
4. Press Convert. In Chrome or Edge you choose the destination file first and the result is written to it as it is produced, so the size of the output is limited only by your disk.

Choose binary STL unless something specifically demands the ASCII form: it is roughly five times smaller and reads faster.

What is kept and what is lost

• Kept: the shape, exactly. Every triangle corner keeps its position.
• Lost: texture coordinates, vertex colour, materials and the .mtl file. STL has nowhere to put them.
• Changed: quads and larger polygons are split into triangles, because STL has only triangles.
• Recomputed: each facet normal is derived from the winding of its own three corners rather than copied, which is what slicers expect.

Why the file gets bigger

OBJ lists each vertex once and refers to it by number. STL has no index at all, so a vertex shared by six triangles is written six times over. A model that is 10 MB of OBJ is commonly 25–30 MB of binary STL, and several times that as ASCII STL. Nothing is wrong; that is the format.

Large files

The converter streams: it never holds the whole model in memory, so a multi-gigabyte OBJ converts on an ordinary laptop. In Chrome and Edge the result is written straight into the file you choose. Firefox and Safari stage it in browser storage first, which has a size ceiling — for very large models, use a Chromium-based browser.

Related

• The other direction: STL to OBJ
• OBJ vs STL vs PLY vs OFF vs glTF
• What is an STL file?