Free Online STL Repair and Print Check
Check any STL, 3MF or OBJ for holes, non-manifold edges, flipped normals and loose parts, then rebuild it as a closed solid at the same size.
What the print check looks for
- Holes: open edges where faces are missing, common in scans and game assets.
- Non-manifold edges: an edge shared by more than two faces, or surfaces touching at a single line. Slicers cannot decide what is inside.
- Flipped normals: faces pointing inward, which can make walls invisible or inverted in the slicer.
- Loose parts: separate shells. Sometimes intended (multi-part models), sometimes stray fragments that print as blobs.
The stats panel also shows triangle count, volume and surface area, so you can compare before and after repair.
How to fix an STL file, step by step
- Drop the file in. Large meshes are fine; processing happens on your device.
- Read the report. If it says watertight yes and one part, the file is probably fine and the problem lies elsewhere, such as scale or orientation.
- Click repair. The mesh is converted to a voxel volume, rebuilt as a closed surface and cleaned with manifold-3d. The bounding box stays the same size.
- Compare in the viewer. Use the section slider to look inside and confirm walls are solid, and check small details at full zoom.
- Export STL or 3MF and slice.
When repair is not the right fix
Some print problems look like broken files but are not. A model 1000 times too small has a unit problem; convert it with the correct unit setting. A model lying on its side has a Y-up axis problem; use the Up-axis option in the converter. Walls that the slicer ignores may simply be thinner than your nozzle; check thickness with the section slider and measure tool.
Also, repair closes the shape you have. It does not rebuild missing geometry from a damaged scan, such as half an ear. For that you need to rescan or model the missing piece. Intentional separate parts in a multi-part model may be merged by repair, so export those as 3MF instead if you want them separate.
What slicers do on their own
Bambu Studio and OrcaSlicer offer a fix model option, and PrusaSlicer repairs some errors on import, while Cura has mesh fixes such as union overlapping volumes and remove all holes. These handle small defects well. Larger problems, such as open scans, heavily intersecting shells or game models built from separate pieces, often slice with missing walls even after those fixes.
The MeshShift print check is useful before the slicer: you see what is wrong, the repair rebuilds the surface rather than patching it, and you can verify the result in the viewer with the overhang highlight and section view.
Frequently asked questions
What does watertight mean for an STL?
A watertight mesh has no holes or gaps: every edge is shared by exactly two faces, so the surface fully encloses a volume. Slicers need that to know what is inside the model and where to put walls and infill.
Will repair change the size of my model?
No. The rebuilt solid keeps the same size and position. Very fine details smaller than the voxel resolution may be smoothed, so compare small features in the viewer before printing.
Is my file uploaded for repair?
No. Detection and repair both run in your browser using WebAssembly, so the STL stays on your device. There is no account and no file storage.
Can I repair 3MF or OBJ files too?
Yes. The print check reads every format the converter supports, including 3MF, OBJ, GLB, PLY and STEP. After repair, export to whichever format your slicer prefers.
Why does my repaired model still fail to slice?
Check scale first: a model a few microns tall or several metres tall can fail even when watertight. Then check that walls are thicker than your nozzle width. If both are fine, look at the section view for internal walls left from overlapping parts, and confirm the slicer is not running out of memory on a very dense mesh.
