Repair STL files & make them watertight
Detect holes, non-manifold edges, flipped normals and duplicate faces — then auto-weld, re-orient and hole-fill your mesh into a watertight, 3D-print-ready STL. Free, private, and nothing is uploaded.
Capabilities
What the repair tool does
- Watertight check: instantly see holes, boundary edges, non-manifold edges, degenerate and duplicate faces — a full mesh health report.
- Auto-weld the STL’s duplicated corners into a connected mesh, with an adjustable merge distance to close hairline cracks.
- Fix inconsistent and inside-out normals by re-orienting every face to point outward — the winding STL and slicers expect.
- Fill open holes by triangulating each boundary loop, with a size limit so a genuinely missing chunk isn’t bridged with a flat cap.
- Remove zero-area (degenerate) and duplicate faces that break slicers and boolean operations.
- See remaining open edges highlighted in red on the live 3D preview, then download a compact binary STL. 100% in your browser.
Process
How it works
- Drop your STLDrag a binary or ASCII .stl file onto the tool. It loads and is analyzed entirely in your browser.
- Review the health reportSee exactly what’s wrong — holes, non-manifold edges, flipped normals, junk faces — before and after repair.
- Repair & downloadWeld, re-orient and hole-fill in one click, then download the watertight, print-ready STL.
Notes
Good to know
- Hole filling caps each boundary loop with a triangle fan; very large holes are intentionally left open (shown in red) rather than sealed with a flat lid.
- Non-manifold edges (where three or more faces meet) are reported but not force-merged — resolving them safely needs manual editing.
- Meshes with internal void shells (a hollow part modeled as two surfaces) keep their outer orientation; the inner shell may need a manual flip.
- Repair works on geometry only — STL has no color, materials or units, so the output is unitless like the input.
- Everything is processed on your device, so very large meshes depend on your machine’s speed and memory.
FAQ
Questions, answered
Is the STL repair tool free?
Yes. Checking, repairing and downloading STL files is completely free — no account or payment required.
Are my files uploaded to a server?
No. Everything runs locally in your browser using WebAssembly-speed JavaScript. Your model never leaves your device.
How do I make an STL watertight for 3D printing?
Drop it in, keep “Fill holes” on, and repair. The tool welds duplicated vertices, re-orients flipped normals, removes junk faces and triangulates open holes, then tells you whether the result is watertight. Anything it can’t safely auto-fix is highlighted in red.
What is a non-manifold edge and can you fix it?
A non-manifold edge is shared by three or more faces — often from overlapping or internal geometry. We detect and report them, but merging them automatically can destroy valid geometry, so they’re flagged for manual editing in the full editor instead of being force-fixed.
Why does my STL have holes when it looks solid?
STL stores unconnected triangles (a “triangle soup”). If exported corners don’t match exactly, the mesh has invisible cracks that slicers read as holes. Welding vertices — with a looser merge distance if needed — reconnects them so the surface becomes closed.
Does it handle both binary and ASCII STL?
Yes. Both formats are detected and parsed automatically. The repaired file is always written as compact binary STL for a smaller download.
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