Convert OBJ to STL

Why convert OBJ to STL?

OBJ is a common export from Blender, ZBrush, scanning apps and asset stores, while slicers are happiest with STL. Converting is quick, but a few things deserve attention. OBJ keeps materials in a separate MTL file and textures in image files; STL has nowhere to put them, so only the triangles are kept. OBJ may also contain quads or larger polygons, which are split into triangles. Many OBJ files exported from Blender are Y-up, so the model can appear lying on its back in a slicer; the up-axis option rotates it upright. OBJ has no unit field either, so you should confirm the size before printing.

How to convert OBJ to STL

1. Drop your OBJ file (with its textures, if separate) onto the converter — it opens in your browser, nothing is uploaded.
2. Check the preview and the dimensions in mm; fix units or the up axis in “Units & orientation” if the size or orientation looks wrong.
3. Choose STL as the output (binary is smaller; ASCII is human-readable) and click Convert. Several files download as one ZIP.

What carries over from OBJ to STL

What changes

• Materials (MTL) and textures are dropped; only geometry is written.
• Quads and n-gons are triangulated, which does not change the shape.
• Named objects and groups are merged into a single mesh.
• No unit is stored in OBJ, so the numbers are treated as millimetres unless you change the scale.
• Y-up files can be rotated to Z-up so they sit flat on the build plate.

Tips

• If your OBJ came with an MTL and textures, you do not need to upload them for an STL; the geometry is all that is used.
• Check the dimension labels; Blender exports are often in metres or arbitrary units.
• Sculpts with millions of triangles produce very large STLs; most slicers handle them, but decimating first helps.
• Use the free print check to find holes and flipped normals before slicing.

About OBJ files

OBJ comes from Wavefront Technologies' Advanced Visualizer software in the 1980s, and it has outlived the product. A file is plain text: lines starting with v define vertices, vt texture coordinates, vn normals, and f faces that reference those lists by index. Faces may be triangles, quads or larger polygons, and the g and o keywords group faces into named objects. Appearance is not in the OBJ at all; a line called mtllib points to a separate .mtl file that lists materials with diffuse colours and texture image names. The format is verbose, has no official unit and no scene hierarchy or animation, but it is trivial to read and write. That is why scanners, photogrammetry tools, sculpting programs and game pipelines still use it to move static meshes around.

About STL files

STL was created by 3D Systems in the late 1980s for its stereolithography machines, and it became the default exchange format for 3D printing. A file describes a surface as a list of triangles, each with three vertices and a facet normal. That is all it contains. There is no header field for units, no colour, no texture and no notion of separate parts. Two flavours exist: ASCII, which is readable text but large, and binary, which spends exactly 50 bytes per triangle after an 80-byte header and a 4-byte triangle count. Binary is what almost every modern tool writes. Because the format is so small and so old, every slicer, CAD package and mesh editor can read it, which is why hobbyists still download most printable models as STL from sites like Printables and Thingiverse.

3D printing notes

STL is still the safest file to hand to a slicer. Bambu Studio, PrusaSlicer, OrcaSlicer, Cura and the resin tools Chitubox and Lychee all import it, and they all treat the numbers as millimetres. The usual problems are scale (a model exported from a metre-based tool arrives 1000 times too small) and meshes that are not watertight, which makes slicers fill or skip regions. MeshShift shows the real dimensions before you convert and can scale from metres automatically. Its print check lists holes, non-manifold edges and flipped normals, and one click rebuilds a closed solid at the same size. If you need colours or several plates in one file, use 3MF instead.

More conversions