STL, OBJ, 3MF, glTF and GLB are triangle meshes. They are not the same format, and they do not share one unit rule. None of them is a precise CAD solid. A mesh cannot be converted back into the original STEP surfaces or feature tree.
Open the mesh you have
| Format | Where to open it | Units on this site | Conversion |
|---|---|---|---|
| STL | STL viewer | Assumed millimetres. No reliable unit field | STL to OBJ |
| OBJ | OBJ viewer | Assumed millimetres | OBJ to STL |
| 3MF | 3MF viewer | Declared supported units converted to millimetres. Core mesh only | No public 3MF-input conversion |
| glTF / GLB | glTF viewer and GLB viewer | glTF metres normalized to millimetres. Local resources must travel with the file | No public glTF-input conversion |
| 3DXML | 3DXML viewer | Unverified; coordinates assumed millimetres | STL, OBJ, GLB, glTF, 3MF, mesh STEP |
| PLY | Workspace | Unverified; coordinates assumed millimetres | STL, OBJ, GLB, glTF, 3MF, mesh STEP |
| VRML97 | Workspace, .wrl, .vrml, .wrz |
Unverified; coordinates assumed millimetres | STL, OBJ, GLB, glTF, 3MF, mesh STEP |
| X3D | Workspace, .x3d, .x3dv |
Verified only from a length UNIT statement; otherwise X3D default metres assumed | STL, OBJ, GLB, glTF, 3MF, mesh STEP |
| UNV | Workspace, .unv |
Verified from dataset 164; otherwise assumed millimetres | STL, OBJ, GLB, glTF, 3MF, mesh STEP (not wireframe-only files) |
| U3D | Workspace, .u3d |
Unverified; coordinates assumed millimetres | STL, OBJ, GLB, glTF, 3MF, mesh STEP |
| PRC | Workspace, .prc |
Verified from the unit declaration; otherwise assumed millimetres | STL, OBJ, GLB, glTF, 3MF, mesh STEP |
| AMF | Workspace | Declared units, default millimetres, normalized to millimetres | STL, OBJ, GLB, glTF, 3MF, mesh STEP |
PLY surfaces, VRML scenes and AMF volumes
PLY accepts ASCII and little- or big-endian binary triangle surfaces with optional colors. A scan stored as a point cloud has no supported surface here; points and textures are not displayed. Confirm the source scale, because PLY coordinates are treated as millimetres without verified physical units.
VRML97 scenes can include IndexedFaceSet meshes, supported primitives, Extrusion and ElevationGrid, with instance transforms and base colors. Gzip-compressed WRZ is accepted. External Inline content and unsupported geometry are omitted and mark the scene incomplete; lights, scripts, animation and textures are not interpreted. Units are unverified and millimetres are assumed.
X3D is the successor of VRML. Static scenes in the XML (.x3d) and ClassicVRML (.x3dv) encodings open with their triangle sets, primitives, CAD part structure, instance transforms and base colors; NURBS patches are tessellated for the preview and trimmed NURBS edges are approximated. Inline files, prototypes, scripts, animation and textures are omitted. The scale is verified only when the file declares a length UNIT; otherwise the X3D default of metres is assumed and physical properties stay unavailable. What an X3D file is and how to open it covers the encodings and the unit check.
UNV is the I-DEAS Universal text format that Salome, Code_Aster, CalculiX and many CFD tools use to exchange finite-element meshes. Solid elements show their outer faces, beams show as lines and named node or element groups become separate parts; quadratic elements use their corner nodes. Results, loads and materials are not displayed. Units are verified from dataset 164 when it is present; otherwise millimetres are assumed and physical properties stay unavailable. Older I-DEAS CAD exports show only a partial straight-line wireframe. What a UNV file is and how to open it lists the datasets that are read.
U3D (Universal 3D, ECMA-363) is the compressed mesh format embedded in 3D PDFs. Standalone .u3d files open with their meshes, transforms and base colors; progressive meshes are refined until the preview is fast enough, and very large ones are marked simplified. Line and point sets show as lines; textures, animation and proprietary compressed meshes are not read. Units are unverified and millimetres are assumed. What a U3D file is and how to open it covers the format.
PRC (ISO 14739-1) is the other 3D format inside 3D PDFs. It can hold exact B-Rep as well as tessellation. Standalone .prc files open with their saved triangles and the supported B-Rep faces rebuilt as a mesh, with transforms, part structure and colors; PMI, textures and wires are not read and no exact STEP is produced. Units come from the model's unit declaration, otherwise millimetres are assumed. What a PRC file is and how to open it covers the format.
AMF reads flat triangle volumes, constant colors and constellation placements from XML or a supported compressed container. It applies the declared unit, defaulting to millimetres when omitted, then normalizes coordinates to millimetres. Curved triangles, textures, formulas and print settings are unsupported. AMF unit declarations do not make the mesh an exact CAD solid.
The 3DXML path reads XML and CATIA V5 binary (UVR) .3DRep tessellations and package-local references. Other binary representations remain unsupported. Each of these inputs can export its loaded triangle mesh after sign-in; mesh STEP contains triangles rather than reconstructed B-Rep. Incomplete scenes have no whole-model physical properties, and open surfaces have no reliable volume or calculated mass.
How to open and measure an STL file explains the binary and ASCII layouts, two-point distances and what a mesh cannot tell you compared with CAD.
STEP can be tessellated outward: STEP to STL, STEP to OBJ, STEP to 3MF, STEP to glTF and STEP to GLB. Those outputs are meshes. Check a known dimension in the receiving program.
Measurement limits
Mesh dimensions and area are approximate. Volume and density-based mass require a closed, consistent mesh; otherwise they stay unavailable rather than becoming zero. Watertight meshes and STL volume explain that status. STL units covers the 25.4 inch/millimetre trap. Tessellation is why a finer mesh is still not B-Rep.
3MF here is core geometry, not a print profile. glTF animations and remote textures are not fetched. A ZIP may carry a glTF scene with its local buffers; the package limit is 50 entries and 200 MiB expanded.
Do not mix these up with solids
STEP vs STL and 3MF vs STL are the format comparisons. For precise volume, start from STEP or another enabled B-Rep input on the STEP hub. Rhino 3DM can display saved meshes or tessellated supported NURBS as an approximate preview, with no exact properties, area, volume, calculated mass or exact STEP export. Use the 3DM viewer and read object and unit warnings. DWG and DXF use the 2D drawing viewer with separate measurement and editing limits; see 3DM and DWG or DXF.
Samples for a known-good mesh are listed in the format matrix.
Original article: CADProps · https://www.cadprops.com/mesh-formats/