STL Repair & Watertight Checker

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