STL repair & converter

How to repair an STL file online

1. Drop your STL, OBJ or 3MF file into the box above. It is read locally in your browser.
2. Check the report: open edges (holes), non-manifold edges, flipped triangles, inside-out parts and bad triangles are counted, and problem edges are drawn in red and orange.
3. Click Repair. Bad and duplicate triangles are removed, neighbouring faces are made consistent, holes are filled and inside-out parts are turned the right way.
4. Need a lighter file? Use Simplify to reduce polygons — keep 10–100% of the triangles and see the new triangle count and STL size before you export.
5. Fix the size if needed (inch → mm, cm → mm or a percentage), rotate it and put it on the bed.
6. Export as STL, 3MF or OBJ — or open it in the PrintCAD modeler to keep editing.

Reduce polygons and merge STL files

Scans and sculpts often have millions of triangles, which makes STL files huge and slicing slow. Simplify uses quadric error edge collapse (the Garland–Heckbert method): it removes the triangles that change the shape least first, never flips faces, keeps open edges where they are and keeps closed models watertight. It runs in a background worker, so even a 500,000-triangle mesh only takes a few seconds.

To merge STL files, drop several files at once or click Add another file. The parts are placed side by side along X with a 5 mm gap — or tick Keep original positions for parts that were exported in place — and exported together as one STL, 3MF or OBJ.

Why slicers complain about STL files

Slicers need a watertight (closed, manifold) mesh to know what is inside the part. Models exported from games, scans, sculpting tools or some CAD programs often have small gaps, triangles facing the wrong way or overlapping pieces. These cause missing walls, strange infill or “non-manifold” warnings in Bambu Studio, PrusaSlicer, OrcaSlicer and Cura.

FAQ

How do I repair an STL file for free?

Drop the file above, review the checks and click Repair, then export it again. No sign-up and no upload.

What does “not watertight” or “non-manifold” mean?

A watertight mesh fully encloses a volume with no gaps. Open edges (holes) or edges shared by more than two triangles (non-manifold) confuse slicers.

My model is tiny or huge in the slicer. How do I fix the scale?

It was probably saved in inches or centimeters. Use inch → mm or cm → mm and export again.

Can I convert STL to 3MF or OBJ?

Yes — load STL, OBJ or 3MF and export to binary STL, ASCII STL, 3MF or OBJ.

How do I reduce the polygon count of an STL file?

Load it, move the Simplify slider to the share of triangles you want to keep and click Simplify. The triangle count and the STL file size are shown before and after.

How do I merge multiple STL files into one?

Drop or choose several files at once (or use Add another file). They are combined side by side with a 5 mm gap, or at their original positions, and exported as one file.

Are my files uploaded?

No. Everything runs inside your browser.