Is my STL watertight?
Check an STL print file before the slicer: watertight or not, open edges and holes, non-manifold edges, flipped normals, inside-out shells, degenerate and duplicate faces, separate shells and faces that cross, with its size, volume and area, and a 3D view of where each problem is. Checked on this device: the file is never uploaded.
open edgesnon-manifold edgesflipped facescrossing facesinside of a face (seen through a hole, or a part inside out)drag to turn · scroll or pinch to zoom
The numbers
What each line means
- Watertight
- Every edge is shared by exactly two faces, so the surface closes with no gap. A slicer needs this to know what is inside the part.
- Open edges and holes
- An edge used by one face only: the rim of a gap. The edges that join end to end make one hole; the 3D view draws them in red.
- Non-manifold edges
- An edge shared by three faces or more, as where two cubes touch along an edge. A slicer cannot tell which side is inside.
- Flipped faces
- A face wound the other way from the faces around it, so its normal points in while theirs point out. Inside-out shells are whole parts wound inward.
- Degenerate and duplicate faces
- Faces with no area, or the same three corners twice. Most slicers drop them; the mended copy does too.
- Shells and crossing faces
- Separate closed parts in one file, and faces that pass through each other (often two parts overlapping without being joined).
- Vertices and the weld
- An STL repeats each corner for every face that uses it. Corners within the weld distance are counted once, the way slicers read them.