Check whether an STL is ready to print

The one question worth asking

Is the surface closed? Everything else follows from that. A slicer fills a volume, and a volume only exists if there is an unambiguous inside. A model that fails this test can still look perfect in every viewer you own, because a renderer draws surfaces and does not care whether they join up.

The report gives you the answer first and the reasons underneath. Each reason is named twice: once in the term your slicer or your colleague will use, and once in a sentence explaining what it does to a print.

What each finding means

**Holes.** Rings of edges with a face on one side only. Counted as rings rather than as edges, because four hundred open edges usually means one open tube end, and "one hole" is the useful way to know that. This is the finding that stops a print outright.

**Non-manifold edges.** Lines where three or more faces meet. Physically impossible — no real object has an edge that three surfaces share — and slicers disagree about what to do with them. Often the trace of a model built by merging two objects without a proper boolean.

**Pinched points.** Places where two separate pieces of surface touch at a single vertex. The classic bowtie. It renders as a solid join and encloses nothing at all, and it is the only defect on this list that can exist in a model where every edge is otherwise perfect.

**Separate pieces.** Not a fault. A printed assembly is a normal thing to hand a slicer. It is worth knowing about only because a model you expected to be one object arriving as forty usually means thirty-nine stray scraps of surface, which the repair tool can sweep away.

Things the report deliberately does not say

It does not tell you the model has no self-intersections, because it has not looked. Two walls passing through each other leave every edge with exactly two faces, so nothing measured here can see it; finding them means testing every triangle against every other, which is not something a browser can do on a model of any size. If a model passes every check here and still slices strangely, that is the first thing to suspect.

It also does not report the duplicate points and the backwards triangles that every file arrives with, because by the time you see the report those have already been dealt with. Every model read on this site is welded and has its faces turned the right way as a matter of course. Reporting a file as broken for something already fixed would be theatre.

And it says nothing about whether the model is a good print — wall thickness, overhangs, how it should be oriented on the plate. Those are real questions and a slicer is the right place to ask them.

Reading the measurements underneath

The volume figure is the one to look at. It is computed by adding up the signed volume between each triangle and the origin, which cancels exactly for a closed surface and means nothing at all for an open one — so it is shown as a dash whenever the model is not watertight. A number appearing there is independent confirmation of the verdict above.

If a volume is shown and it is not what you expect, check the unit. An STL does not record its own units, so a model exported from a program working in centimetres arrives here assumed to be in millimetres and measures ten times too small in every direction — and a thousand times too small by volume. The unit control is under the model.