What does watertight mean for an STL?

Every edge is shared by exactly two triangles, so there is no doubt about what is inside the model. An open edge is a hole, and different slicers guess differently about how to fill it.

How thin can a wall be in a 3D print?

About two extrusion widths: 0.8 mm on a standard 0.4 mm nozzle. Thinner walls are dropped by the slicer or print as a single weak line.

What overhang angle needs support?

Most FDM printers manage 45 degrees from vertical without support. Set your own limit from an overhang test.

Can it fix an STL with holes?

It fixes flipped, duplicate and overlapping faces, but no tool can know the shape of a face that is missing. If a real hole remains, it tells you rather than handing back a hollow shell.

Is my model uploaded?

No. The checks and repairs run on your own computer, in the browser.

STL Checker & Repair — Watertight, Wall Thickness, Overhangs

1 Choose an STL

Drag an STL here, or

Nothing leaves this machine.

2 Set your printer's limits

Minimum wall thickness Two extrusion widths is the usual floor — 0.8 mm on a standard 0.4 mm nozzle. Anything thinner is dropped by the slicer or printed as a single unbonded line. mm Overhang limit Measured from vertical: a wall is 0°, a flat ceiling is 90°. Most FDM printers manage 45° without support. 45° Detail How finely the model is sampled to measure thickness. Higher is more accurate and slower; it also sets the quality of a Rebuild. 180

3 Diagnosis

drag to rotate · scroll to zoom

4 A better way up

Orientation decides how much support you print, how strong the part is along its layers, and which surfaces come out rough. This tries a few hundred ways up and reports the three best.

5 Repair

Quick fix Drops degenerate and duplicate triangles, makes every face agree which way is out, and turns inside-out shells the right way round. Moves no vertex, so sharp edges stay sharp — but it cannot close real holes.

Rebuild Re-creates the surface from a distance field, which resolves non-manifold edges, overlapping bodies, self-intersections and inside-out pockets in one pass — the things the quick fix cannot reach. It re-meshes everything, so edges soften slightly and the triangle count changes; raise Detail in step 2 for a closer match. It cannot close a real hole — nothing can know what shape the missing face had — and it will say so rather than hand back a hollow shell.

What the checks mean

Watertight means every edge is shared by exactly two triangles, so there is no ambiguity about what is inside. An open edge is a hole in the surface; a slicer has to guess, and different slicers guess differently.
Wall thickness is measured with the largest sphere that fits inside the wall, so it is honest about thin diagonal ribs that a straight-down ray would read as thick. Accuracy is about one voxel of the Detail setting, and the error leans towards reading a thin wall as slightly thick — so anything that lands within that margin of your limit is reported as borderline rather than as a pass.
Overhang angle is measured from vertical, matching how slicers phrase their support threshold. Surfaces already lying on the plate are excluded; they need no support.
Rebuild is lossy by nature. Re-surfacing from a voxel field cannot recover an edge that falls between two samples, so a rebuilt cube has very slightly rounded corners. Try the quick fix first; rebuild when the fault is tangled geometry rather than a missing face.
No tool can close a real hole. Deciding what is solid means filling the model from the outside in, and a hole lets that fill straight through — there is no inside left to find. Software can guess a patch, but it is a guess about a shape you know and it does not. This page tells you instead of guessing.
It will not catch everything. Self-intersecting surfaces and inside-out pockets deep in a model are not reported individually — but a Rebuild resolves both, because a distance field has no way to represent them.

Print Doctor