# U3D Files: What They Are and How to Open Them Online

U3D (Universal 3D) is the ECMA-363 mesh format used inside 3D PDFs. Learn what a .u3d file contains, how to open one online, what the preview simplifies and why its units need checking.

U3D (Universal 3D) is a compressed triangle-mesh format standardised as ECMA-363. Its main use is inside 3D PDFs: CAD and scientific tools write the model as a U3D stream and embed it in a PDF page, so a reader can rotate it without the original software. You meet a standalone `.u3d` file when someone exports the stream directly, or when a publishing tool such as a LaTeX package keeps it next to the PDF it builds.

You can open a `.u3d` file in [the CADProps workspace](https://www.cadprops.com/workspace/index.md). It decodes the meshes, applies the model and group transforms and shows the result with its base colours. You can rotate the model, hide parts, check the bounding box, measure point-to-point distances and export STL, OBJ, GLB, glTF, 3MF or mesh STEP after sign-in. CADProps reads standalone U3D files only; it does not open the 3D content of a PDF.

## What is inside a U3D file

A U3D file is a sequence of blocks: a node hierarchy of groups and models, resources that hold the mesh data, and shaders and materials that colour them. Meshes are stored with continuous level of detail (CLOD): a coarse base mesh followed by a stream of vertex splits that refine it step by step. That lets a 3D PDF show something quickly and sharpen it as more data is read.

CADProps follows the same idea. It decodes the base mesh and then the refinements. For very large models it can stop refining early or merge nearby vertices to keep the first view fast; the preview is then marked simplified, and the bounding box describes the geometry actually shown.

## What is shown and what is left out

- Triangle meshes with their transforms, part structure and diffuse material colours. Normals are recalculated for shading.
- Line and point sets are drawn as lines and small markers and marked incomplete.
- Textures, vertex colours, animation, lights and saved views are ignored. A file whose meshes are animated shows them in their rest position, with a warning.
- Some 3D PDF tools write meshes in a proprietary compressed extension rather than the standard CLOD encoding. Those meshes cannot be decoded and the file fails with a message saying so.

## Units and measurements

ECMA-363 does not fix a length unit, and U3D files do not state one. CADProps assumes millimetres and marks the units as assumed, so area, volume and mass stay unavailable. Exports from engineering tools are often in millimetres, but scientific models can be in metres, micrometres or arbitrary units. Measure a feature whose real size you know before quoting a dimension; [the scale factor guide](https://www.cadprops.com/guides/cad-model-wrong-scale-25-4/index.md) shows how to recognise a wrong factor.

A U3D mesh is a display model. It cannot be turned back into exact CAD surfaces, and a mesh STEP made from it contains triangles; see [tessellation](https://www.cadprops.com/guides/cad-tessellation/index.md) for why. For manufacturing work, ask for the source CAD file or a STEP export. The [format matrix](https://www.cadprops.com/supported-formats/index.md) lists every input and its limits.

## Related reading

- [STEP File Scale Wrong by 25.4? Fix Inch and mm Units](https://www.cadprops.com/guides/cad-model-wrong-scale-25-4/index.md)
- [CAD Tessellation: When a Solid Becomes a Triangle Mesh](https://www.cadprops.com/guides/cad-tessellation/index.md)
- [GLB vs glTF for CAD: 3D Model Files, Assets and Units](https://www.cadprops.com/guides/glb-vs-gltf-for-cad/index.md)
- [IGES File (.igs, .iges): What It Is and How to Open It](https://www.cadprops.com/guides/iges-file-what-it-is-how-to-open/index.md)
- [Open a Rhino 3DM File Without Rhino](https://www.cadprops.com/guides/open-3dm-without-rhino/index.md)
- [Open CAD Files on Mac: STEP Viewers and Format Options](https://www.cadprops.com/guides/open-cad-files-on-mac/index.md)
- [PRC Files: What They Are and How to Open Them Online](https://www.cadprops.com/guides/open-prc-file-online/index.md)
- [X3D and X3DV Files: What They Are and How to Open Them Online](https://www.cadprops.com/guides/open-x3d-file-online/index.md)
- [Watertight Mesh: When an STL Can Have a Volume](https://www.cadprops.com/guides/watertight-cad-mesh/index.md)

Original article: https://www.cadprops.com/guides/open-u3d-file-online/

HTML page: https://www.cadprops.com/guides/open-u3d-file-online/