What is a .obj file?

What an OBJ file looks like

OBJ is readable text, and unusually pleasant to read:

v 0.0 0.0 0.0
v 10.0 0.0 0.0
v 0.0 10.0 0.0
vn 0.0 0.0 1.0
o bracket
f 1//1 2//1 3//1

v is a vertex, vn a normal, vt a texture coordinate, f a face and o an object name. Indices are 1-based, which trips up every programmer who writes an OBJ parser for the first time.

Faces can have any number of sides, not just three. A lot of OBJ files in the wild are quad meshes, which is why an OBJ can arrive with a different triangle count than you expected: the triangulation happens on import.

The .mtl problem

OBJ stores colours and materials in a separate file with an .mtl extension, referenced by an mtllib line. Textures are separate files again.

This is the most common OBJ frustration. You download or receive a .obj, open it, and everything is grey, because the .mtl was not sent with it. It is also why OBJ is awkward for any single-file upload, including this site's. If your OBJ references a material library, the colours are simply not in the file you uploaded, and the conversion report says so instead of silently rendering grey.

If you need one self-contained file with colour, use GLB or 3MF.

Units

OBJ has no unit. Like STL, the numbers are just numbers, and millimetres is the usual assumption for anything mechanical. Content pipelines for games and film often treat OBJ as metres or as arbitrary "world units" instead, which is why an OBJ from an artist and an OBJ from a CAD package can be a thousand times apart in scale.

Structure

OBJ does keep object and group names, the o and g statements, so a multi-part model arrives with its parts distinguishable. What it does not have is a transform hierarchy: there is no way to say "this part is positioned here relative to that one". Positions are baked into the coordinates.

That is a real but smaller loss than STL's, and this site's conversion reports distinguish the two.

What OBJ is good for

• Universal interchange for meshes. Almost every 3D tool ever written reads

OBJ.

• Human inspection and scripting. It is text, and a few lines of code can

read or write it.

• Photogrammetry and scanning output, where OBJ plus a texture is a common

delivery format.

What it is not good for

• Manufacturing. There is no unit and no exact geometry. Use STEP.
• Single-file colour delivery. Use GLB.
• Very large meshes. Text is bulky, and a 10-million-triangle OBJ is enormous

and slow. Use PLY binary or GLB.

OBJ vs STL: which file should you send?

For a plain print mesh, STL may be all the recipient needs. OBJ is useful when separate object names or a material workflow matter. It can reference material definitions in another file, so sending an OBJ may mean sending a folder rather than one attachment.

Neither format provides a standard unit declaration for the workflows covered here. Include the intended unit and one reference dimension when handing over a model. A recipient can then catch a scale mistake before using it.

On this site, OBJ to STL exports the supported triangle geometry. It does not repair holes or turn texture detail into physical relief. The single-file OBJ reader also does not load external materials. If you only need to inspect the shape, use the OBJ viewer and keep the original files together.

How to open a .obj file

Drop the file on this page and the viewer reads it in your browser. Blender and MeshLab open it for free and can write it back; FreeCAD, CloudCompare and Meshmixer read it; PrusaSlicer imports it for printing. Fusion and Rhino open it, and SketchUp exports it with the Pro licence.

Keep the folder together. An OBJ names its materials in a separate .mtl file, which in turn names texture images. Send the .obj alone and the shape arrives grey. This site's reader takes one file, so colour will not show here even when the sidecar exists.

OBJ has no unit. A model that opens at a tenth or a thousandth of its intended size was written in different units than the importer assumed; scale it once, on purpose, in the application that will use it.