Non-manifold geometry
Edges shared by more than two faces or surfaces that intersect themselves. Duplicate and invalid faces are removed so every edge is shared by exactly two faces where possible.
01 Free browser-based repair
Free Online
STL Repair Tool
Fix broken STL files, repair mesh errors, and prepare your model for 3D printing. Analyze the mesh, fix non-manifold geometry, holes and normals, then download a repaired STL.
Free · No sign-up Private – your STL never leaves your device Binary & ASCII STL
or drop your STL hereBinary & ASCII · Max 100 MB · processed locally
DRAG TO ROTATE · RIGHT-DRAG TO PAN · SCROLL TO ZOOM02 Online repair
Repair STL Files Online
Repair STL files online without installing CAD software. Upload a broken or non-watertight model, and the analyzer reports exactly what is wrong: open edges, holes, non-manifold edges, flipped normals, degenerate faces and duplicate vertices. One click runs the automatic fix, and a before/after report shows what changed.
Because the whole repair runs in your browser, your model is never uploaded. That makes this STL repair tool safe for unreleased designs, licensed models and client work, and fast enough for large meshes — most repairs finish in seconds, entirely online.
A typical scanner-exported cube arrives looking fine but fails in the slicer. The analysis reveals the damage, and the repair report reads like this:
| Error | Before | After |
|---|---|---|
| Open edges | 18 | 0 |
| Holes | 3 | 0 |
| Non-manifold edges | 7 | 0 |
| Bad normals | 21 | 0 |
| Watertight | No | Yes |
Every number is measured from your mesh, not estimated — before the fix runs, you already know what it will change.
03 Root causes
Why STL Files Break
Broken STL geometry rarely comes from nowhere. Most mesh errors trace back to a handful of predictable causes, and knowing which one damaged your file tells you how deep the repair needs to go.
- Boolean and modeling operations
- Union and cut operations leave internal walls, coincident faces and zero-thickness slivers where two solids overlapped.
- 3D scanning
- Noisy point clouds triangulate into overlapping patches that share edges with three or more faces.
- Export tolerances
- Rounding vertices on export opens microscopic seams between faces that should share corners.
- File conversion
- Converting from surface-based formats loses the closure a printable solid needs, leaving open shells.
- Manual mesh editing
- Deleting a face or dragging a vertex breaks the two-faces-per-edge rule the rest of the mesh follows.
Cracks and duplicate vertices from export tolerances are the easiest case — vertex welding closes them invisibly. Holes and flipped normals repair just as cleanly. Meshes torn into separate fragments are the hard case described below.
04 Simple workflow
How to Fix an STL File
- 01
Upload STL
Drop in a binary or ASCII STL file. It stays on your device.
- 02
Analyze mesh errors
See watertight status, open edges, holes and normals at a glance.
- 03
Fix STL
Run the automatic repair: weld, clean, fill and reorient.
- 04
Preview and download
Compare before/after, then download the repaired STL.
05 Automatic fixes
What Does the STL Fixer Repair?
Each repair targets a specific mesh error that blocks slicing or corrupts prints.
Holes
Missing triangles leave openings in the surface. Boundary loops are detected and filled with new triangles centered on the hole, closing the mesh.
Open edges
Boundary edges that belong to only one face are the technical signature of holes and cracks. After vertex welding and hole filling, open boundaries are closed.
Bad normals
Faces whose winding conflicts with their neighbors confuse slicers and shading. The fixer propagates a consistent orientation across the mesh and points it outward.
Degenerate faces
Zero-area triangles caused by modeling errors or bad exports break slicing and booleans. They are detected and removed automatically.
Duplicate vertices
Cracked meshes store the same corner twice with microscopic offsets. Nearby vertices are welded within a small tolerance so faces share vertices correctly.
Non-watertight mesh
A printable model must enclose a volume. Welding, hole filling and normal repair combine to close the surface so slicers see a solid.
· Honest limits
What the STL Fixer Cannot Repair
Automatic repair has boundaries, and stating them plainly is part of the job. A mesh shattered into dozens of disconnected fragments cannot be reassembled reliably, and a large missing region has no shape information to reconstruct — the hole filler closes boundaries flat rather than inventing geometry.
Deeply self-intersecting surfaces are only untangled where removing duplicate faces solves them; the rest would need full remeshing, which this tool deliberately avoids so your existing surface is never reshaped. If the after-report still lists non-manifold edges, the fix has to happen at the source in your CAD or scan software — the report tells you exactly how many problems remain.
06 No cost, no signup
Free Online STL Repair Tool
Free stl repair with no account and no watermark, for STL files up to 100 MB. The tool works with both binary and ASCII STL files in every modern desktop browser.
The analysis is transparent rather than a black box: before any fix runs, you see the exact counts of open edges, holes, non-manifold edges, normal conflicts and degenerate faces. After the repair, the before/after table confirms what was resolved — you download knowing exactly what changed.
07 Print readiness
Repair STL Files for 3D Printing
Most failed prints trace back to broken surfaces: slicers cannot slice invalid geometry, holes produce open shells that leak infill, and flipped normals make walls print inside-out. Repairing the mesh before slicing removes these errors at the source.
Run the fix when a downloaded model shows slicing errors, when a scan or export comes out non-watertight, or when a mesh repair tool in your slicer keeps failing. The repaired STL downloads as a standard binary file in millimetres, ready for Cura, Bambu Studio, PrusaSlicer or any other slicer.
Cura reports non-manifold edges — will this fix it?
In most cases yes. The fixer removes the duplicate and degenerate faces that cause non-manifold edges, welds cracked vertices, and the before/after report shows the non-manifold count reaching zero. Edges that survive are listed honestly so you know what is left.
Don't slicers already repair meshes?
Cura, Bambu Studio and PrusaSlicer heal small gaps automatically at slice time, but they do not fix the file and do not tell you what they changed. Repairing at the source gives you a clean mesh that slices the same way every time, in every tool.
How is this different from 3D Builder's repair?
Windows 3D Builder rebuilds the surface, which can alter fine detail. This tool keeps every surviving vertex exactly where it was and only welds, removes or adds triangles where the mesh is broken — and shows you the counts before you download.
Does a watertight result guarantee a successful print?
No. Watertight geometry is the entry ticket: wall thickness, overhangs, clearances and printer tolerances still decide printability. A repaired mesh removes slicing errors so you can focus on those decisions.
08 Related workflows
More STL Tools
Inspect, convert and edit STL files — all locally in your browser.
09 Common questions
Frequently Asked Questions
How do I repair an STL file online?+
Upload your STL file, run the analysis to see exactly which mesh errors it contains, then click Fix STL. The tool repairs the mesh in your browser and gives you a before/after report before you download the repaired file.
Can I fix an STL file for free?+
Yes. The STL repair tool is completely free, with no signup and no watermark. Files are processed locally in your browser, so nothing is uploaded to a server.
What does an STL fixer repair?+
This STL fixer merges duplicate vertices, removes degenerate and duplicate faces, fills holes, fixes inconsistent normals, and closes open boundaries so the mesh becomes watertight and ready for 3D printing.
What is a non-manifold STL?+
A non-manifold STL contains geometry that cannot exist in a real solid, such as an edge shared by more than two faces or a vertex where surfaces meet incorrectly. Slicers often fail or produce errors on non-manifold meshes, which is why repairing them before printing matters.
Can this tool make an STL watertight?+
Yes. After merging duplicate vertices and removing invalid faces, the tool fills open boundaries with new triangles, which closes holes and makes the mesh watertight. The before/after report shows the watertight result for your model.
Does STL repair change the original geometry?+
Only where it must. Repairing removes broken faces, welds coincident vertices within a tiny tolerance, and adds triangles to close holes. Your model keeps its size and shape — the tool never rescales or deforms the existing surface.