An OBJ file is a list of triangles, not a STEP solid. It is useful for viewing and for mesh-to-mesh exchange. It is the wrong file when someone needs exact CAD volume, edges or a feature tree.
Open it in the OBJ viewer. You can also convert STL to OBJ or convert OBJ to STL. Neither conversion creates a precise solid.
OBJ does not say which unit the numbers use
The coordinates are bare numbers. A value of 25 may be 25 millimetres or 25 inches, and the file will not tell you. CADProps measures OBJ geometry as millimetres. Textures and MTL material libraries are not applied.
That assumption matches the STL unit problem. If the mesh was exported in inches, every length is 25.4 times too small or too large until you compare it with a known dimension. Changing the display unit in the viewer does not rescale the file.
Volume needs a closed, consistent mesh
Surface area and overall dimensions can be reported for an open mesh. Volume cannot. Gaps, overlapping shells and inconsistent face direction make a volume missing or misleading. The same limit is described for STL volume.
When a volume is shown, call it a mesh approximation. It is not the B-Rep volume of the CAD model the mesh was made from. A coarser triangulation changes the number even when the design did not change.
What conversion does not fix
Saving OBJ as STL, or STL as OBJ, copies triangles. It does not:
- add a reliable unit declaration
- fill holes
- recover analytic cylinders, planes or edges
- recreate a STEP or native CAD model
If the next person needs those, ask for the STEP or the original CAD file. STEP and STL are not substitutes.
A short check
| Check | What to record |
|---|---|
| Known dimension | The size you trust, and whether it matches the mesh |
| Unit assumption | Millimetres, unless the sender documented another unit |
| Closure | Closed, open, or not checked |
| Volume | Approximate mesh value, or unavailable |
| Next format | Mesh handoff, or a request for STEP |
Keep the note with the file. A clean shaded view is not a record of scale.
Separate appearance problems from property problems
An OBJ may look gray because its MTL or textures are unavailable or unsupported, while its positions and triangles remain readable. Conversely, a textured preview can look convincing even when the boundary is open. Appearance and geometric enclosure need different checks. See missing OBJ materials when the issue is color or texture rather than dimensions.
For a concrete scale check, take a sender-confirmed 50 mm dimension. If the imported extent is approximately 1.9685 in a workflow assuming millimetres, compare the ratio with 25.4 and ask which unit was used at export. Do not apply a factor solely because the result “looks small”; agree which dimension and direction are being compared first.
When receiving a package, keep the OBJ and its local companion files in their supplied relative folders. Record the geometry filename separately from the material filenames. Re-exporting the triangles as STL removes the material association; it neither fixes a scale mistake nor makes an open shell closed.
If precise mass is required, request the CAD source or an appropriate solid exchange file and a confirmed density. Use the OBJ for the task it can support: inspecting a mesh whose scale, closure and approximation have been made explicit.
Original article: CADProps · https://www.cadprops.com/guides/obj-mesh-units-and-closure/