# Watertight Mesh: When an STL Can Have a Volume

A watertight mesh is a closed, consistent triangle boundary. Open or inconsistent STL and OBJ meshes have no reliable volume, and CADProps does not replace that with zero.

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.

A mesh is watertight when its triangles form one closed boundary: every edge that should be shared is shared, the inside and outside are consistent, and the surface encloses a volume. Rendering does not need that. Volume does.

CADProps estimates STL and OBJ properties from the triangle mesh. Dimensions and area can come from readable triangles. Reliable volume, and any mass based on that volume, requires a suitable closed mesh. If the mesh is open or inconsistent, volume stays **unavailable**. It is not stored as zero.

## Watertight is not the same as "looks solid"

| What you see | What it implies |
|---|---|
| A shaded part with no obvious hole | The camera can draw both sides of an open surface |
| A tiny gap or a missing triangle | The mathematical enclosure can already be broken |
| Overlapping shells | Volume may count geometry twice or become ambiguous |
| Flipped faces | The inside and outside are inconsistent |
| A closed mesh with a ridiculous volume | The enclosure may be valid while the unit assumption is wrong |

[Why STL volume is zero or missing](https://www.cadprops.com/guides/why-stl-volume-is-zero/index.md) walks through those cases. A true zero is only for geometry that really encloses nothing, such as a flat sheet. A damaged solid that fails the check is a different status. Putting both down as `0` hides the reason.

## Units still matter after the mesh is closed

Closure does not declare millimetres or inches. STL and OBJ on this site assume millimetres. A closed mesh interpreted in the wrong unit still has a volume; the volume is the wrong size. Because volume scales with the cube of length, a 25.4 linear factor becomes a very large volume factor. [STL units](https://www.cadprops.com/guides/stl-files-have-units/index.md) is the check. Changing the display unit does not repair a hole, and closing a hole does not fix a 25.4 scale error.

3MF can declare a unit that is converted to millimetres. glTF uses metres, normalized to millimetres. Those rules are format-specific. They do not make an open mesh watertight.

## Repair is a new model

Automatic hole closing can join the gap in more than one way. If the missing surface is a port, a parting line or a thin wall, the repaired volume can be plausible and wrong. Compare a repaired mesh with the source CAD or the drawing before you use it for a quote.

When the original STEP or native solid still exists, re-export from that solid. [Tessellation](https://www.cadprops.com/guides/cad-tessellation/index.md) changes the approximation; it is not a substitute for a closed B-Rep. [STEP vs STL](https://www.cadprops.com/guides/step-vs-stl/index.md) is the reason to keep the solid if the next person needs precise volume.

## A short test

1. Open the original STL or OBJ without renaming it. Use the [STL viewer](https://www.cadprops.com/tools/stl-viewer/index.md) or the [OBJ viewer](https://www.cadprops.com/tools/obj-viewer/index.md).
2. Read whether volume is available or unavailable.
3. Look for gaps, extra shells and inconsistent orientation if the volume is missing.
4. Confirm one known dimension and the intended unit.
5. Re-export or repair only after that, and label mesh results as approximate.
6. Apply a density only to a volume you trust. If the volume is unavailable, the mass is unavailable too.

## Related reading

- [3MF vs STL: Units, Multiple Parts and Mesh Handoff](https://www.cadprops.com/guides/3mf-vs-stl/index.md)
- [CAD Tessellation: When a Solid Becomes a Triangle Mesh](https://www.cadprops.com/guides/cad-tessellation/index.md)
- [OBJ Mesh Units and Volume: Check Scale and a Closed Shell](https://www.cadprops.com/guides/obj-mesh-units-and-closure/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)
- [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/watertight-cad-mesh/

HTML page: https://www.cadprops.com/guides/watertight-cad-mesh/