What OBJ to STL keeps and what it drops
OBJ was developed by Wavefront Technologies in the 1980s for animation software, and STL by 3D Systems in 1987 for stereolithography. The printer only needs the surface, so the STL keeps every face of the model and leaves its look behind.
| In the OBJ | In the STL |
|---|---|
| Faces with 3 or more corners | Triangles only; a four-sided face becomes two |
| Materials from the .mtl file, texture images, UV coordinates | Dropped: STL has no field for them |
| Several objects or groups | One list of triangles; Info still counts the separate parts |
| Numbers without a unit | The same numbers, shown here as millimeters |
Keep the original .obj next to the STL if you need the colors later. For a print, color is chosen in the slicer anyway.
Convert an OBJ file to STL in five steps
- Drag the .obj onto the page or press Open File. You can drop the .mtl and texture images with it; the viewer accepts several files at once.
- The bar at the top shows
name.stl, the file size and a Download STL button. - Open Info and read the dimensions X × Y × Z, the triangle count and the watertight line of the mesh check.
- Switch Units between mm and in if the part was drawn in inches, to see which reading makes sense.
- Press Download STL. Other formats gives ASCII STL, 3MF or GLB from the same model.
Units and orientation before the slicer
Neither OBJ nor STL records a unit, so the numbers pass through unchanged and the viewer reads them as millimeters. A part drawn in inches arrives 25.4 times too small. Edit → Inches → mm multiplies it by 25.4.
A model exported from a 3D app in meters looks tiny for the same reason. Edit → Size sets one axis to an exact length in mm and scales the rest with it.
Many OBJ exports stand the model Y-up, so it lies on its back on the print bed. Press Other formats and tick Rotate Y-up model to Z-up (for slicers) before you save the STL.
When the OBJ will not convert cleanly
- Gray model, missing texture warning. The .mtl or images were not dropped with the .obj. The STL does not need them, so you can ignore it.
- Info lists dozens of parts. The OBJ was built from separate objects. They print side by side in the slicer; Edit → Combine with union of all parts merges them once each one is watertight.
- Mesh check says not watertight. Render models are often open where nobody looks, such as the base of a statue. Edit → Repair closes simple holes and lists what is still open.
- The tab slows down. A sculpt with millions of faces is heavy. Edit → Reduce triangles brings it to a share you choose before saving.
How the conversion was checked
In the automated test run an OBJ was converted to STL and the STL was opened again. Size, volume and triangle count matched the source. A 20 mm cube went through every output format the same way.
Expect about 1.5 s for a 250,000-triangle model and 5 to 12 s for a million triangles. Above roughly 1 to 2 million triangles the tab may run out of memory. The OBJ is parsed by JavaScript inside this tab, and during the full test run every request went to the site's own address with nothing posted.
After the STL: repair, price, other formats
If the mesh check reports holes or flipped triangles, STL repair closes simple holes and turns the normals outward. The 3D print cost calculator then gives the weight in PLA or PETG and a price.
Going the other way is STL to OBJ. For a web preview there is OBJ to GLB, and CAD users can try OBJ to STEP, which writes a faceted body. To look at the file without converting, open the OBJ viewer.
Questions and answers
How do I convert an OBJ file to STL?
Open the .obj on this page. The STL is made as soon as the model loads, and one click on Download STL saves it.
Will the textures and colors of my OBJ carry over?
No. STL stores triangles only, so materials, texture images and UV coordinates stay behind in the .obj and the .mtl.
Why is the STL 25.4 times too small in my slicer?
The OBJ was drawn in inches and the slicer reads millimeters. Use Edit → Inches → mm, then save the STL again.
Can I get an ASCII STL instead of binary?
Yes. Press Other formats and pick STL ASCII. It holds the same triangles in a text file roughly 5.6 to 6 times larger than binary.
Does the OBJ to STL converter upload my model?
No. The file is read and written by code in this tab, and the test run saw zero requests to other domains.
Which browsers does the OBJ to STL converter run in?
It is tested in Chromium browsers (Chrome 155). Firefox and Safari have not been tested yet.