# OBJ Mesh Units and Volume: Check Scale and a Closed Shell

OBJ does not declare reliable units. See how millimetres are assumed, when mesh volume is only an approximation, and why OBJ is not a CAD solid.

This guide covers general CAD workflows. CADProps supports STEP, IGES, SolidWorks parts and ZIP assemblies, STL, OBJ, 3MF and static GLB/GLTF. Validated conversions are downloadable. Partial assemblies carry warnings; properties cover loaded geometry only.

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](https://www.cadprops.com/tools/obj-viewer/index.md). You can also [convert STL to OBJ](https://www.cadprops.com/tools/stl-to-obj/index.md) or [convert OBJ to STL](https://www.cadprops.com/tools/obj-to-stl/index.md). 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](https://www.cadprops.com/guides/stl-files-have-units/index.md). 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](https://www.cadprops.com/guides/why-stl-volume-is-zero/index.md).

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](https://www.cadprops.com/guides/step-vs-stl/index.md) 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.

## Related reading

- [CAD File Size vs Part Weight: Why MB Does Not Mean Mass](https://www.cadprops.com/guides/cad-file-size-vs-part-weight/index.md)
- [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)
- [STEP vs STL: CAD Measurement, Geometry and Accuracy](https://www.cadprops.com/guides/step-vs-stl/index.md)
- [Do STL Files Have Units? How to Fix mm vs Inch Scale](https://www.cadprops.com/guides/stl-files-have-units/index.md)
- [Watertight Mesh: When an STL Can Have a Volume](https://www.cadprops.com/guides/watertight-cad-mesh/index.md)
- [STL Volume Zero or Missing? Check the Mesh Is Closed](https://www.cadprops.com/guides/why-stl-volume-is-zero/index.md)

Original article: https://www.cadprops.com/guides/obj-mesh-units-and-closure/

HTML page: https://www.cadprops.com/guides/obj-mesh-units-and-closure/