# STL Repair & Watertight Checker Paste an STL or OBJ mesh to weld vertices, drop degenerate and duplicate triangles, fix flipped normals, fill holes and check watertightness. ## Run it - **CLI:** `gizza tool stl-repair "solid part facet normal 0 0 0 outer loop vertex 0 0 0 vertex 1 0 0 vertex 0 1 0 endloop endfacet endsolid part"` - **Web:** https://gizza.ai/tools/stl-repair/ - **Agents:** machine-readable descriptor (parameters JSON Schema) at https://gizza.ai/tools/stl-repair/tool.json ## Inputs - `stl` — Mesh text (ASCII STL or OBJ) _(field)_ - `output` — Return _(field)_ - `weld_tolerance` — Weld tolerance (model units) _(field)_ - `remove_degenerate` — Remove degenerate (zero-area) triangles _(field)_ - `remove_duplicates` — Remove duplicate triangles _(field)_ - `fix_winding` — Fix winding and point normals outward _(field)_ - `fill_holes` — Fill open boundary loops _(field)_ - `keep_largest_shell` — Keep only the largest shell _(field)_ - `stl_encoding` — STL encoding (when returning a mesh) _(field)_ ## Output - Repair report (text) ## Query parameters Open the tool pre-filled and auto-run via URL: - `stl` — Mesh text (ASCII STL or OBJ) - `output` — Return - `weld_tolerance` — Weld tolerance (model units) - `remove_degenerate` — Remove degenerate (zero-area) triangles - `remove_duplicates` — Remove duplicate triangles - `fix_winding` — Fix winding and point normals outward - `fill_holes` — Fill open boundary loops - `keep_largest_shell` — Keep only the largest shell - `stl_encoding` — STL encoding (when returning a mesh) Example: `https://gizza.ai/tools/stl-repair/?stl=solid%20part%0Afacet%20normal%200%200%200%0Aouter%20loop%0Avertex%200%200%200%0Avertex%201%200%200%0Avertex%200%201%200%0Aendloop%0Aendfacet%0Aendsolid%20part&output=report&weld_tolerance=0.000001&remove_degenerate=true&remove_duplicates=true&fix_winding=true&fill_holes=true&keep_largest_shell=true&stl_encoding=ascii` --- ## About this tool Slicers are unforgiving: a model that looks perfect in a 3D viewer can still refuse to slice because two triangles overlap, one face points inward, or a hairline crack runs along a seam. This tool reads a mesh as **ASCII STL** or **Wavefront OBJ** text, tells you exactly which of those problems it has, fixes the ones it can, and reports whether the result is finally watertight. The repair pass does five things, each of which you can turn off: - **Weld coincident vertices.** STL stores every triangle's three corners separately, so nothing is connected until matching corners are merged. Corners closer together than the weld tolerance become one vertex — this is what closes hairline cracks from a sloppy export. Set the tolerance to `0` to merge only bit-identical positions. - **Remove degenerate triangles** — zero-area faces whose corners collapse onto the same vertex or sit exactly on a straight line. They carry no surface and confuse slicers. - **Remove duplicate triangles** — faces built from the same three vertices in any rotation or winding. Duplicates show up downstream as non-manifold edges and doubled walls. - **Fix winding and normals.** Every face in a shell is made to agree on winding, then each closed shell is turned so its normals point outward. Facet normals are always recomputed from the geometry, so whatever normals the file claimed are ignored. - **Fill holes** (off by default) and **keep only the largest shell** (off by default), for meshes with open boundaries or stray scanner fragments. Then it measures the result: triangle and vertex counts, non-manifold edges, open boundary edges, shell count, watertight yes/no, surface area, volume, and the bounding box. Everything runs locally in your browser — the mesh is never uploaded anywhere. ### Worked example A tetrahedron whose base face was exported wound the wrong way: ``` solid tetra facet normal 0 0 0 outer loop vertex 0 0 0 vertex 1 0 0 vertex 0 1 0 endloop endfacet facet normal 0 0 0 outer loop vertex 0 0 0 vertex 1 0 0 vertex 0 0 1 endloop endfacet facet normal 0 0 0 outer loop vertex 0 0 0 vertex 0 0 1 vertex 0 1 0 endloop endfacet facet normal 0 0 0 outer loop vertex 1 0 0 vertex 0 1 0 vertex 0 0 1 endloop endfacet endsolid tetra ``` With the default settings the report comes back as: ``` STL repair report ================= Input Format ASCII STL Solid name tetra Triangles 4 Distinct vertices 4 Problems found Degenerate triangles 0 Duplicate triangles 0 Coincident vertices 0 Non-manifold edges 0 Open (boundary) edges 0 Flipped triangles 3 Disconnected shells 1 Watertight no Repairs applied Vertices welded 0 Degenerate removed 0 Duplicates removed 0 Triangles re-wound 1 Shells turned outward 0 Holes filled 0 (0 triangles added) Fragments removed 0 (0 triangles dropped) Result Triangles 4 Distinct vertices 4 Non-manifold edges 0 Open (boundary) edges 0 Disconnected shells 1 Watertight yes Surface area 2.366025 Volume 0.166667 Bounding box 1 x 1 x 1 Bounds min 0 0 0 / max 1 1 1 ``` Three edges disagreed about which way round the surface went, one face was to blame, and after re-winding it the solid is watertight with a volume of 1/6. Switch **Return** to *Repaired mesh (STL)* to get the corrected file instead of the report, or to *Report as JSON* to feed the same numbers into a script. ### What it does not do - **Self-intersections** are neither detected nor repaired — that needs exact geometric predicates and a CSG-grade remesher. - **Hole filling is a flat fan**, not a curvature-aware patch: each closed boundary loop is capped with triangles fanned from the loop's own centroid. Simple, roughly flat holes come out fine; a large or twisted hole gets a visibly flat cap. Boundary chains that never close are left alone rather than patched wrongly. - **No remeshing, decimation or smoothing**, and no 3D preview. - **Binary STL, PLY and OFF cannot be pasted** — this is a text-in, text-out tool. Re-export binary STL as ASCII STL first. (Binary STL *output* is available, as a data URL.) - Input is capped at **100,000 triangles**. ## FAQ
What does "watertight" actually mean here? A mesh is reported as watertight when every edge is shared by exactly two triangles that traverse it in opposite directions. That means no open boundary edges, no edges shared by three or more faces, and no neighbouring faces that disagree about which side is out. It is the property a slicer needs in order to decide what is inside the solid and what is outside.
My mesh looks closed but reports thousands of open edges. Why? Almost always the weld tolerance is too small for the file's precision. STL writes each triangle's corners independently, often rounded to six or seven digits, so two faces that should share a corner end up a fraction of a unit apart and the edge between them never pairs up. Raise the weld tolerance — try `0.001` for a model measured in millimetres — and run it again. The "Vertices welded" line tells you how many corners the tolerance merged.
Why is the volume shown as "n/a"? Volume is only reported when the repaired mesh is closed. An open surface does not enclose a region, so any number would be meaningless — the same triangles could be read as enclosing almost anything. Close the mesh (enable hole filling, or raise the weld tolerance) and the volume appears. Surface area and the bounding box are always reported, closed or not.
What is the difference between a flipped triangle and an inside-out model? A flipped triangle disagrees with its immediate neighbours, so it shows up as a count under "Flipped triangles" and is re-wound individually. An inside-out model is internally consistent — every face agrees with every neighbour — but the whole shell faces the wrong way. That is detected from the enclosed volume's sign and fixed by turning the entire shell, which is what the "Shells turned outward" line counts.
Can I feed it an OBJ file? Yes. The format is auto-detected from the text: `solid`/`facet`/`vertex` lines are read as ASCII STL, and `v`/`f` lines as Wavefront OBJ. OBJ polygons are fan-triangulated, and OBJ materials, UVs and vertex normals are dropped — STL stores raw triangles only. Output is always STL.
Does my model get uploaded anywhere? No. The whole repair runs as WebAssembly inside this page, so the mesh text never leaves your device and no network request is made with it. The same code is available offline through the command-line tool.
## Related tools - [OBJ ↔ STL Mesh Converter](https://gizza.ai/tools/mesh-convert/): Convert pasted OBJ or ASCII STL meshes to OBJ, ASCII STL, or binary STL data URLs. Supports scaling, Y-up/Z-up axis swaps, and triangulation. - [STL inspector](https://gizza.ai/tools/stl-inspector/): Inspect an ASCII or binary STL in your browser: triangles, vertices, bounding box, surface area, volume, watertight and manifold status, weight estimate. - [Before / after image slider generator](https://gizza.ai/tools/before-after-slider/): Turn two images into an interactive before/after comparison slider: one self-contained HTML file with inline CSS + JS, no libraries. - [Color Contrast Checker](https://gizza.ai/tools/color-contrast-checker/): WCAG colour contrast checker — get the contrast ratio and AA / AAA pass-fail for any text and background colour, plus an accessible suggestion. Free, in-browser. - [Color converter](https://gizza.ai/tools/color-format-convert/): Convert a color between HEX, RGB(A), HSL, HSV and CMYK, in your browser. Paste any notation and get them all. Nothing is uploaded.