Convert OBJ to STL — Free Browser Tool for 3D Printing

No — and this is the most common surprise. STL is geometry-only: no colors, no materials, no textures. Your .mtl file and any texture images are completely ignored. The output is a plain gray mesh. If you need color in your print, you have two real options: (1) use a multi-material printer with a slicer that supports non-standard color STL extensions, or (2) switch to a different target format — OBJ to 3MF preserves color data and is supported by PrusaSlicer and Bambu Studio.

Usually yes, but "usually" is doing a lot of work there. The conversion itself is lossless for geometry — what you get out is exactly what went in, triangulated. The problem is if your OBJ already had issues: non-manifold edges, holes, inverted normals, or self-intersecting faces. Those problems transfer directly to the STL. Before sending to your printer, drop the STL into your slicer and look for warnings. Cura shows a mesh error icon; PrusaSlicer highlights problem areas in yellow/red. If you see issues, Meshmixer's Inspector tool (free) fixes most common problems automatically.

Binary STL is what you want. It stores the same geometry data but in a compact binary encoding — typically 50–90% smaller than ASCII STL for the same model. A 10 MB ASCII STL might be 1–2 MB as Binary. This converter outputs Binary STL by default. ASCII STL is human-readable text (you can open it in a text editor and see the coordinates), which is occasionally useful for debugging, but no modern slicer requires it.

Expect the STL to be larger than the OBJ — typically 1.5–2× the size. The reason: STL stores an explicit normal vector for every triangle face, and it doesn't use indexed vertices (each triangle stores all 3 vertex coordinates independently, even if adjacent triangles share vertices). A 10 MB OBJ with ~200K polygons typically becomes a 15–20 MB Binary STL. If file size matters, consider converting to 3MF instead — it's a ZIP-compressed format that's usually smaller than both OBJ and STL.

For OBJ→STL conversion, .mtl files are irrelevant — STL has no concept of materials, so there's nothing to do with them. You only need to upload the .obj file. If you want to convert OBJ with materials preserved, use OBJ→GLB or OBJ→3MF instead — both support materials and textures, and you can upload a ZIP containing the .obj, .mtl, and texture files.

Three quick ways: (1) Cura — import the STL, look for the orange mesh-error icon in the bottom-left of the model card. Click it to see what's wrong. (2) PrusaSlicer — the model turns yellow or red if there are issues; hover for details. (3) Blender — select the mesh, go to Edit Mode, open the 3D Print Toolbox addon (built-in, just enable it in Preferences), click "Check All". It lists every non-manifold edge and open boundary. For automated repair without manual work, Netfabb Online (free tier at netfabb.autodesk.com) fixes most common issues in under a minute. A truly watertight mesh has zero non-manifold edges and zero open boundaries — that's what you're aiming for.