Is your STL watertight?
Check an STL, OBJ, PLY, 3MF or GLB for holes, non-manifold edges, flipped normals and loose parts — with every problem highlighted in 3D. It runs in your browser: the file is never uploaded.
Drop a mesh file here, or click to choose
STL · OBJ · PLY · 3MF · GLB 🔒 Checked locally — nothing leaves your computerWhat “watertight” means
A mesh is watertight (also called closed or manifold) when it has no holes: every edge is shared by exactly two triangles, and neighbouring triangles face the same way. Only then does the surface enclose a volume.
It matters because slicers need a closed surface to decide what is inside the part, and CAD software can only turn a closed surface into a solid. Scans, sculpts and files exported from games or 3D-model sites often are not watertight.
Holes (open edges)
Edges used by only one triangle — the rims of gaps in the surface. Common in scans. Fix: Meshmixer Inspector, PrusaSlicer / Bambu Studio Fix model, Blender Fill.
Non-manifold edges
Edges shared by three or more triangles — usually overlapping or touching parts. Fix: separate or merge the parts (Boolean union), then re-check.
Flipped normals
Neighbouring triangles facing opposite ways, or the whole mesh inside-out. Fix: Recalculate normals in Blender or Meshmixer; most slicers fix this automatically.
Several shells
Pieces that don't touch each other. Fine for multi-part prints; for a single solid, merge them or delete stray fragments.
Degenerate triangles
Triangles with no area. Usually harmless, but many of them hint at export problems. Fix: Remove degenerate / cleanup tools.
Units
STL has no units. If the size looks 1000× off, the file was probably saved in metres (or inches) — scale it on import.
Watertight? Turn it into a real CAD solid
A watertight mesh can be rebuilt as a STEP solid made of smooth NURBS surfaces — something you can section, measure and design around in SolidWorks, Fusion 360 or Onshape. An open mesh, like an intraoral scan, becomes a surface model instead. Mesh to STEP → · Intraoral scan to STEP →
FAQ
Is my file uploaded?
No. The check runs entirely in your browser; the file never leaves your computer. Very large files (hundreds of MB) may be slow on older computers.
Does the checker repair the mesh?
Not yet — it tells you exactly what is wrong and where, and how to fix it in free tools. Repair is on our list.
Why does my slicer print it fine even though it's not watertight?
Most slicers quietly repair small defects. Bigger holes or overlapping parts can still cause missing layers, gaps or wrong infill — and CAD software is much less forgiving.
Can a mesh that is not watertight become a STEP file?
Yes, as an open surface model rather than a solid. Vertessia's reconstruction gives a solid for watertight meshes and a surface model for open ones.