STL Checker

How to check an STL file

1. Select an STL file using the picker above (or drag one in once you're on the page).
2. MeshWrench reads and analyzes the file locally in a background worker — nothing leaves your device.
3. Review the watertightness verdict, the individual check results, and the 3D preview with issue overlays.
4. Download a JSON report, or continue to STL Repair once it's available.

What each check means

Degenerate triangles

A triangle with a repeated vertex, or with zero/near-zero area. These are excluded from every other check below, and reported separately.

Duplicate faces

Two or more triangles that share the exact same three vertices, either with the same winding (an exact stacked copy) or reversed winding.

Boundary / open edges & holes

An edge used by only one triangle is a boundary edge. Boundary edges are grouped into connected components and classified as a closed loop (a likely hole), an open chain, a branched junction, or a non-simple shape.

Non-manifold edges

An edge shared by more than two triangles — ambiguous "inside vs. outside" geometry at that edge.

Inconsistent winding

Two triangles sharing an edge that traverse it in the same direction, rather than opposite directions — a sign the surface isn't consistently oriented there.

Inward-oriented shells

A closed, consistently-wound shell whose normals point inward rather than outward, determined from its own signed volume. Still watertight, but likely inverted.

Disconnected shells

The model is made of more than one separate, unconnected piece of geometry — sometimes intentional (multi-part models), sometimes a sign of a fragmented file.

Self-intersections

Triangles that geometrically cross through each other. Checked with a broad-phase spatial index plus a robust triangle-triangle test — ordinary shared-edge adjacency is never flagged.

What "watertight" means here

This checker's watertight verdict is a strict, disclosed policy: zero boundary edges, zero non-manifold edges, zero winding conflicts, zero duplicate faces, and a completed self-intersection check with none found. A model that fails one of these criteria is reported "not watertight," with the specific reason. A model whose self-intersection check couldn't complete within this checker's safety limits is reported "some checks incomplete" — never silently reported as passing.

Common slicer symptoms this checker can explain

• "STL not watertight" / slicer refuses to slice: check the boundary-edge and non-manifold results.
• "STL has holes": look at the closed-loop boundary component count.
• Non-manifold geometry errors: the non-manifold-edge check reports the exact count.
• Inverted normals / model renders inside-out: the inward-shell and winding-conflict checks.
• Self-intersections / slicer produces strange internal geometry: the self-intersection check.
• Disconnected pieces flying apart: the disconnected-shells check and per-shell bounds.
• Degenerate triangle warnings: the degenerate-triangle count and its two sub-categories.
• "STL will not slice" with no clear reason: download the full JSON report for every count at once.

How the analysis works

Every check starts from an exact-identity topology layer: source STL vertices (which commonly repeat for every triangle) are collapsed to unique positions using the same bit-exact float32 matching every MeshWrench converter already uses — never a distance tolerance, since welding nearby-but-different vertices would change the model's real topology. From there, an edge-incidence map classifies every edge as boundary, manifold or non-manifold and detects winding conflicts; boundary edges are grouped into components; triangles are grouped into connected shells; each shell's orientation is propagated and, for closed shells, its signed volume decides inward vs. outward; duplicate faces are grouped by canonical identity; and self-intersections are found via a spatial-grid broad phase followed by a robust triangle-triangle test, always excluding ordinary shared-edge adjacency.

Limitations

• This is a diagnostic tool only — it never modifies, repairs, welds, reorients or exports a changed mesh.
• Vertex identity is exact-position matching, never tolerance-welded.
• Passing every check does not guarantee successful printing; failing one does not mean a file can't be sliced.
• Wall thickness, physical scale, your printer's limits, support needs and manufacturing tolerances are outside this checker's scope.
• Every check has an explicit safety limit; a check that can't complete safely is reported "not checked," never as passing.

What's next: STL Repair

Problems found? STL Repair — a separate MeshWrench tool — will fix holes, flip inverted normals, weld gaps and resolve non-manifold geometry, locally in your browser. It isn't built yet; this checker never claims to repair anything, and never links to a route that doesn't exist.