STL Repair: Close Holes and Fix Flipped Faces, Free
Make an STL watertight for printing: holes closed, flipped and inside-out faces turned round, near-miss corners welded, duplicate triangles removed. Every fix listed, checked against trimesh. Nothing uploaded.
How it works
Open the model
Drop an STL (or OBJ, PLY, 3MF, GLB). It is read in your browser and shown in 3D, with the number of open edges if it has holes.
It is repaired at once
Near-miss corners are welded, collapsed and duplicate triangles removed, flipped faces turned round, inside-out parts turned outward and holes capped. The page lists every fix.
Download the fixed STL
Or save it as 3MF, OBJ or any other format. Untick repair to compare with the original.
Repair an STL file online, free
STL repair here makes a mesh watertight for 3D printing: every edge shared by exactly two triangles, every face pointing outward. Drop the file and it is repaired online at once, with each fix listed: holes closed, flipped faces turned round, inside-out parts turned outward, corners that nearly met welded, collapsed and duplicate triangles removed.
To make an STL watertight the holes must be found first: an edge used by only one triangle is the rim of a hole, and those edges are chained into loops and capped. Small and flat holes close exactly; a large hole on a curved surface is capped flat and the page says so, so you can check that spot.
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Why it matters
The right way up and the right size
A 20 mm part stays 20 mm and upright: glTF's metres and Y-up axis are converted to and from the millimetres and Z-up of STL, 3MF and slicers.
Checked against a reference
Every format pair is tested against trimesh: the same volume, surface area and size come back out.
Nothing uploaded
Client parts and unreleased designs stay on your computer. The model is read and written in this tab.
Questions
What does the STL repair fix?
Holes (edges used by only one triangle), faces wound the wrong way, parts that are inside out, corners that nearly meet but were never joined, and collapsed or duplicate triangles. Each fix is listed, and the result is checked again for open edges.
What can it not fix?
An edge shared by three or more triangles (non-manifold geometry) and parts that pass through each other. Those are reported; they need the model's author or a CAD tool such as Blender's 3D-Print toolbox or Meshmixer.
How is a hole closed?
A three-sided hole gets one triangle; a larger one a fan of triangles round its centre. That is exact for a flat hole and close for a small curved one. A large hole on a curved surface is capped flat, and the page says so.
Is it checked against anything?
Yes: broken test models (holes, flipped faces, inside-out parts, a sphere with its cap cut off) are repaired and handed to trimesh, which must find them watertight, consistently wound and the right volume.
Is my model uploaded?
No. It is read, repaired and written in your browser tab.