STL Checker: Is Your Model Ready to Print?
Drop an STL, 3MF, OBJ or any 3D file and get a printability report in seconds — closure, face direction, loose parts, overhangs, thin walls and build-plate fit — then repair it for free.
What each check means
- Closed (watertight): every edge must belong to exactly two triangles. Open edges are holes; slicers either guess how to fill them or leave gaps in the walls.
- Faces point outward: flipped triangles make the slicer think inside is outside, which shows up as missing walls or strange infill.
- One part: several shells are fine for an assembly, but stray crumbs or a duplicate inner shell usually come from a bad export or a scan.
- Overhangs over 45°: surfaces facing down more steeply than 45 degrees sag without support. Turn on the Overhangs view to see them in red.
- Thin walls: features thinner than about two extrusion widths may disappear in the slicer preview or print as weak, wobbly lines.
- Build-plate fit: the model's size is compared with common printers in any orientation.
How the thin-wall test works
We turn the model into a fine signed-distance grid, shrink the solid by half the wall limit and grow it back. Anything that vanishes in that round trip is thinner than the limit. The grid size is shown next to the result, so you know how fine the check was; very large models are checked at a coarser step to stay fast in the browser.
The number is a share of the model's volume, which makes it easy to judge: a few tenths of a percent are usually antenna-like details or text edges, while several percent means real parts of the design will not print at this scale. Scaling the model up, thickening those features in CAD or switching to a 0.2 mm nozzle are the usual fixes.
Fixing problems in one click
If the model is not closed, press Repair for printing. MeshShift rebuilds it as a single closed, manifold solid with the original size: holes are capped, flipped faces are corrected, overlapping shells are merged and tiny floating crumbs are removed. The report then runs again on the repaired version, and you can download it as STL or 3MF.
Repair does not change overhangs or thin walls — those are design questions. Use supports, rotate the part so large overhangs face up, or scale it so thin details reach a printable width.
Checker vs. your slicer
Bambu Studio, PrusaSlicer and Cura also detect and silently repair many errors, but they do it at slicing time and do not always tell you what changed. Checking first shows exactly what is wrong, keeps a clean file for print services that reject broken meshes, and avoids surprises like a missing wall in the middle of a long print. It also catches problems slicers don't flag at all, such as features that are simply too thin for your nozzle.
Frequently asked questions
Is the STL checker free and does it upload my file?
It is free with no account, and your file is never uploaded: it is parsed, checked and repaired by your browser. You can confirm this in the browser's network panel while a check runs.
What does "non-manifold" mean in an STL?
An edge shared by more than two triangles, or a vertex where two surfaces only touch at a point. Slicers can't decide what is inside the model there, so walls may be dropped or doubled. Repair rebuilds the surface so every edge has exactly two faces.
Which file types can I check?
STL, 3MF, OBJ, PLY, GLB/glTF, FBX, DAE, STEP/STP, IGES, AMF, 3DS, VRML and OFF. The check always runs on the geometry as a slicer would see it, in millimetres and with Z up.
Why is my model flagged as too thin?
Parts thinner than about 0.8 mm are below two extrusion lines of a standard 0.4 mm nozzle. They may vanish in the slicer or print as fragile single lines. Scale the model up, thicken those details, or print with a smaller nozzle.
Can it fix overhangs automatically?
No — overhangs are part of the shape. The report tells you how much of the surface needs support, and the viewer highlights those areas in red so you can rotate the part or enable supports in your slicer.
