STL vs OBJ vs glTF: 3D File Formats Compared
Why the Format You Choose Actually Matters
Choose the wrong 3D file format and you'll spend hours wrestling with missing textures, broken geometry, or a slicer that just won't open your model. STL, OBJ, and glTF are the formats you'll see most often, but they were born in different decades for entirely different reasons. STL was standardized way back in 1987 for early 3D printers. OBJ appeared around 1992 as a flexible interchange format from Wavefront Technologies. And glTF 2.0? It was ratified by the Khronos Group in 2017, built from the ground up for real-time rendering on the web. That 30-year chasm between the oldest and newest format explains nearly every practical difference. A file that prints flawlessly in PrusaSlicer might look like a grey blob in a web viewer, and a game-ready model is often total overkill for a simple desktop FDM printer. This guide breaks down each format's structure, features, and typical file size, helping you make the right choice before you even think about converting—or before you ask a tool like CocoConvert to do it for you.
STL: The Workhorse of 3D Printing
STL stands for Standard Tessellation Language, though you'll also hear it called STereoLithography. The format is brutally simple: it represents a 3D surface as a mesh of triangles. That's it. No color, no textures, no materials, just pure geometry. Each triangle gets defined by its three vertices and a normal vector pointing outwards. In a binary STL file, this information is packed into a tight 50 bytes—12 for the normal, 36 for the vertices, and a 2-byte 'attribute' count that is almost universally ignored. This efficiency means a model with a million triangles comes out to a manageable 50 MB. There's an ASCII version that's human-readable, but it's 5-6 times larger, so binary is the way to go. For 3D printing, STL's simplicity is its greatest strength. Slicers like PrusaSlicer, Cura, Bambu Studio, and Chitubox all open STLs natively, no questions asked. When you export from a program like Fusion 360 (File → Export → .stl), you can dial in the refinement, setting a chord tolerance as low as 0.001 mm. This number dictates how closely the flat triangles hug the ideal curved surface. A lower tolerance creates more triangles and a bigger file, but it's the key to getting smooth curves on your final print. But the format's limitations are just as clear. Because it's geometry-only, it can't handle any visual information. If your model needs multiple colors or different material finishes, STL won't help you. It also has no concept of units. A value of '1.0' could be one millimeter or one inch, a classic source of frustration for anyone who has ever imported a model only to find it's the size of a dust speck or a skyscraper.
OBJ: Geometry Plus Materials, at a Cost
By default, OBJ files are plain-text ASCII that list out vertices (v), texture coordinates (vt), normals (vn), and faces (f) as individual lines. A separate, companion MTL file (Material Template Library) defines the materials and points to external texture images like JPEGs or PNGs. This multi-file structure is the single biggest headache when working with OBJ. If you send someone just the .obj file, they get a model with no color information at all. You have to be diligent about bundling the .obj, the .mtl, and all referenced textures into a Zip file or shared folder, otherwise the recipient is just looking at a flat grey object. OBJ's saving grace is its massive software support. Blender, Maya, Cinema 4D, ZBrush, Rhino—basically every 3D application in existence can read and write it. It also has a few key advantages over STL, like support for multiple named objects within one file, making it better for complex scenes. Plus, it can define faces using quads and n-gons, not just triangles. This is huge for artists using subdivision modeling workflows, as it preserves the clean topology they worked so hard to create before it gets triangulated for the final render or print. The file sizes can get big. A detailed architectural model might have a 200 MB OBJ, a tiny 5 KB MTL, and 800 MB worth of high-resolution textures. The format has no built-in compression, although some tools offer a compressed .objz variant. OBJ also completely lacks support for modern features like animation, skeletal rigging, or complex scene hierarchies. This makes it a poor choice for game assets or any interactive experience. For delivering static, visually rich models across different software, OBJ is still a necessary evil—just remember to keep all its friends in the same folder.
glTF: The JPEG of 3D Formats
The Khronos Group famously calls glTF 2.0 the 'JPEG of 3D,' and the analogy is perfect. It's a format built for efficient delivery and fast loading, not for active editing. A glTF file is JSON-based and can reference external binary data (.bin) and textures. Even better, it can be packed into a single, self-contained binary file with a .glb extension. For any kind of distribution, you should always use GLB. It's one file, no chance of missing assets, and it's often 20–40% smaller than the equivalent OBJ bundle once textures are embedded. glTF 2.0 was designed with modern graphics hardware in mind, which is why it supports physically-based rendering (PBR) materials right out of the box. Its core material model uses metallic-roughness parameters that map directly to what engines like Three.js, Babylon.js, Unity, and Unreal expect. Set a metallic value of 1.0 and a roughness of 0.1, and you get a convincing chrome. Set metallic to 0.0 and roughness to 0.8, and you get a flat matte plastic. These properties are stored right in the file, ensuring the model looks the same everywhere without you having to manually re-assign materials in every application. This is also where glTF pulls far ahead of older formats. It supports morph targets, skeletal animation, multiple cameras, and a full scene hierarchy. Extensions even add support for advanced visual effects like glass (transmission) and wax-like materials (subsurface scattering). A single GLB file can power a product configurator on a website, an AR preview in a mobile app, and a character in a browser game. But glTF is a terrible format for iterative editing. You wouldn't model directly in glTF any more than you'd edit a novel in a PDF. Every time you export to glTF, the geometry is typically triangulated and complex material shaders are baked down into simple PBR textures. Round-tripping a model through this process will degrade it. So, think of glTF as the final-delivery format for web and real-time projects, but always keep your original source files in a native format like `.blend`.
Head-to-Head Comparison: A Practical Reference
Here’s how the three formats stack up across the criteria that actually matter in a production workflow. **Geometry support:** All three can handle a basic triangle mesh. OBJ is more flexible, adding support for quads and n-gons, which is useful for modeling. glTF focuses on render-ready data, supporting triangles and line sets (for wireframes). **Color and materials:** STL is colorblind; it has no native support. OBJ uses external MTL files to define basic materials with Phong shading. glTF is the clear winner here, with a full PBR material model baked directly into the format. **Textures:** Again, STL has none. OBJ relies on external image files that are referenced from its MTL companion file. glTF can either reference external textures or, in the GLB format, embed them directly. It has dedicated slots for base color, normal, metallic-roughness, occlusion, and emissive maps. **Animation:** This one is easy. STL and OBJ have no animation support. glTF supports both skeletal and morph-target animation, making it the only choice of the three for animated content. **Typical file size:** For a mid-complexity model (around 500k triangles, one material), a binary STL will be about 25 MB. The equivalent OBJ with its MTL and textures could be anywhere from 35–120 MB depending on texture size. A GLB, thanks to modern compression, can shrink that down to just 8–25 MB. **Software compatibility:** STL is the universal language of 3D printing and manufacturing tools. OBJ is the classic interchange format, supported by nearly every digital content creation (DCC) tool. glTF is now the dominant force for all things real-time and web, with support in DCC apps improving every day. **Primary use case:** Send an STL to your FDM, SLA, or SLS 3D printer. Use OBJ to move a static model between different modeling and rendering programs. Choose glTF (specifically GLB) for web viewers, AR/VR, game engines, and e-commerce. If you're printing a physical part, export an STL. If you're sending a detailed architectural model to a rendering artist, OBJ is a safe bet (though FBX might be better). If you're putting a product on a website or in an AR app, GLB is the only answer.
Converting Between Formats: What Works and What Doesn't
CocoConvert can handle the most common conversion jobs: STL to OBJ, OBJ to STL, OBJ to glTF/GLB, and GLB to OBJ. When you're just converting geometry—especially anything involving STL—the process is rock solid. STL only contains triangle data, and that information maps cleanly into the geometry sections of both OBJ and glTF. The real challenge is going from a rich format down to a simpler one. If you convert a fully textured OBJ into an STL, all the material and texture data gets stripped away. Why? Because the STL format literally has no place to put it. CocoConvert will warn you about this, but it can't magically create features in a format that doesn't support them. For most people, getting the raw geometry for a 3D printer is exactly what they need anyway. Converting from OBJ to GLB is a popular task, but it's not a simple 1:1 mapping. CocoConvert works to translate the old Phong-based MTL parameters into modern PBR equivalents. The diffuse color becomes the base color, and the specular exponent (the 'Ns' value in an MTL file) is used to approximate the roughness. This is an approximation, not a perfect translation. A glossy plastic that looked perfect in a legacy renderer might look slightly different in a PBR viewer. For mission-critical work, you'll get the best results by exporting directly to glTF from your original authoring application (like Blender or Maya), which gives you full control over the PBR material setup. It's also important to know that CocoConvert does not currently support animated GLB files. If you upload a GLB containing skeletal animation and convert it to OBJ, that animation data will be lost, leaving you with a static mesh in its default pose. We're actively working on animation-aware conversions, but for now, Blender's built-in glTF exporter (File → Export → glTF 2.0) is the most reliable tool for handling animated assets.
Choosing the Right Format for Your Project
The right choice is actually simpler than it seems once you know where the file is headed. Is your model going to a 3D printer? Use STL. It's that simple. Every slicer on the planet prefers it, the files are reasonably small, and its lack of features doesn't matter because printers don't render materials. Just make sure to export a binary STL, not ASCII, and set your chord tolerance tight enough for smooth curves—0.01 mm is a great starting point for a 200 mm tall model. Is your model moving between different 3D modeling programs or going to a rendering artist? OBJ is the classic, safe interchange format. Just be disciplined: keep the MTL and all texture files in the same folder with clear names. And please, avoid spaces in filenames; you'd be surprised how many older tools still choke on them. If your scene is complex, you might also consider FBX, which is better at preserving scene hierarchies and animation, though it has its own set of proprietary quirks. Is your model destined for the web, an AR app, a game, or any real-time viewer? Use GLB. There is no better choice. The single-file package prevents missing assets, the PBR material model ensures visual consistency, and modern compression like Draco and KTX2 can shrink a 50 MB asset down to 5 MB with barely any quality loss. It's the native language of the modern 3D web. If you're ever unsure what a client or collaborator needs, just ask what software they use. That one question will give you your answer. A Cura user needs an STL. A Blender artist can work with OBJ or GLB. A web developer building a product viewer is definitely going to ask for a GLB. Matching the format to the destination tool saves everyone a headache.