STL repair.

Maintained by Bake3D · Updated 8 October 2026

What this STL repair fixes

An STL file is a long list of triangles. For a good print, those triangles must close up like a sealed box, with every face pointing out. Small mistakes break that. A slicer may then skip a wall, fill a hole the wrong way or refuse the file.

This tool fixes the most common mistakes, one step at a time:

• Seams that look closed but are not. Two edges sit a hair apart, so the surface has a crack. The tool joins corners that are closer than the weld distance.
• Broken triangles. A triangle with no area, or the same triangle saved twice, confuses slicers. The tool removes them.
• Flipped faces. A face that points into the model instead of out of it. The tool turns it to match the faces next to it.
• Parts that are inside out. Every face of a part points in. The tool turns the whole part around.
• Holes. Missing triangles leave a gap. The tool closes each hole with flat triangles between the corners around it.
• Small loose bits. Specks of stray geometry far from the model. The tool can remove them if you ask.

Everything runs in your browser. The file is never uploaded.

How to repair an STL file

1. Drop a binary or ASCII .stl file on the box above, or try the broken sample.
2. Watch the repair steps. Each one says what it changed, in numbers.
3. Switch between Before and After in the viewer. Problem edges are drawn as colored lines. New patches are orange.
4. Read the before and after table. “Watertight: yes” means every edge now has a face on both sides.
5. Download the repaired STL. You can also save the report as a JSON file.

If you already ran a file through our STL checker and it found problems, press Repair this file there. The file comes straight here, still on your computer.

What the words mean

Open edge. An edge with a face on one side only. Open edges are the rim of a hole or a crack.

Hole. A group of open edges that connect into a loop.

Non-manifold edge. An edge shared by three or more faces, like a fin stuck onto a wall. A real solid never has one.

Flipped edge. Two faces that share an edge but point opposite ways. One of them is flipped.

Watertight. No open edges and no non-manifold edges. Water poured inside could not leak out.

Shell. One connected part. A figure with a separate base has two shells.

Normal. The direction a face points. STL stores one per triangle. This tool works each one out again from the order of the triangle’s corners, the way slicers do.

The settings

Weld distance decides how close two corners must be to join. The default, 0.01 mm, is far below what any printer can show, so the shape does not change. Only corners on open edges are joined. Corners inside closed areas stay as they are, so fine detail is safe. On a very small model, the distance is cut to one thousandth of the model’s size.

Fill holes sets the largest hole to close, counted in edges around it. Holes bigger than the limit stay open, and the report says so. Keep this low if your model is meant to have an opening, like a cup or a vase with a real wall.

Remove small loose bits deletes parts smaller than 1%, 2% or 5% of the model, measured corner to corner. It is off unless you turn it on, because some small parts are real, like a separate eye. The biggest part always stays.

What repair cannot fix

A repair works on the triangles that are there. It cannot guess shapes that are missing. Here is what this tool does not do:

• Overlapping parts stay separate. Two shapes that pass through each other are not merged into one. Most slicers join them when they slice. The volume in the report counts the overlap twice.
• Thin walls stay thin. A wall too thin for your nozzle or resin is not made thicker.
• Holes become flat. A missing ear is closed with a flat patch, not rebuilt as an ear.
• Some edges are only shown. Non-manifold edges, holes whose edge folds over itself and holes over the size limit are drawn in the After view but not rebuilt.
• Files have a size limit. The tool reads files up to 64 MiB. A binary STL that size holds about 1.3 million triangles.

For those cases, repair the model in a 3D program. Blender’s free 3D Print Toolbox can check wall thickness and make a mesh manifold. To make a heavy model lighter first, see our guide on how to reduce polygon count in Blender.

How we tested it

We broke models on purpose, most of them in Blender 4.4. Then we repaired them here and opened every repaired file in Blender again. Blender counted the open, non-manifold and flipped edges for us, so the check does not rely on our own code.

• The Bake3D sample fox, scaled to 120 mm long, with 60,000 triangles. We deleted 25 faces and flipped 300. The tool closed all 25 holes and turned the 300 faces back. In Chrome on a Mac the whole run took about half a second.
• The same fox, subdivided to 360,000 triangles, with 40 faces deleted and 2,000 flipped. The run took about 1 second.
• A test sphere with 1.2 million triangles, written by a script into a 58 MB file, with 56 small holes and 3,654 flipped edges. The run took about 2 seconds.
• Blender’s monkey head, subdivided to 15,704 triangles, with 40 faces deleted and 60 flipped. All 44 holes were closed. Four of them come with the original model, around its eyes.
• A 40 mm rounded box saved inside out with its top missing. The tool closed the top and turned the box right side out. The volume went from minus 55,470 to plus 62,297 mm³.
• Our broken sample tower. It has a seam 0.004 mm wide, a window and a roof panel missing, three flipped faces, a doubled face, a flat triangle and an inside-out block.

Every repaired file came back from Blender with 0 open edges, 0 non-manifold edges and 0 flipped edges. Blender’s volume matched our report to the hundredth of a cubic millimetre.

We also kept one case the tool does not fix: two 20 mm cubes that pass through each other. Both are closed, so nothing changes. The report says 16,000 mm³, but the solid you would print is 13,750 mm³, because the 2,250 mm³ where they overlap counts twice.

Automated tests cover each step with small shapes whose answers are known. They include a cube with a seam 0.0005 mm wide, a T-junction where a corner sits on a neighbour’s edge, a hollow box whose inner wall must stay facing in, two boxes sharing a wall, a doubled face and a hole made of two loops that touch at one corner. The binary layout the tool reads and writes is the one the Library of Congress describes for STL.

Common questions

Is my STL file uploaded?

No. The file is read and repaired inside your browser, in a background worker. Nothing is sent to Bake3D or anyone else. The download is made on your computer.

Why is my repaired file still not watertight?

Some problems are only shown, not fixed. Look at the After view. Red lines are open edges left in holes that were over the size limit or folded over themselves. Yellow lines are non-manifold edges. Try a larger hole limit first. If a seam is still open, try a larger weld distance, such as 0.05 mm.

Will repair change the shape of my model?

Only where it fixes something. Welding moves a corner by no more than the weld distance. Flipping a face does not move it. Hole patches are flat, so they fill the gap but do not rebuild curves. Everything else stays exactly as it was.

What weld distance should I use?

Start with 0.01 mm. If the Before view shows a ring of red open edges along a line that looks closed, the corners there are further apart. Try 0.05 mm, then 0.1 mm. Use “Exact matches only” if your model has real gaps smaller than 0.01 mm that must stay open.

Can I repair a model made by an AI 3D generator?

Yes, for the problems listed above. Generated models often have small holes, flipped faces or loose specks. Check the result in your slicer, because a repair cannot thicken thin parts. To make a full 3D model from a picture first, see image to 3D and the 3D printing workflow.

How do I make an STL in the first place?

Our guide on how to make an STL file covers five ways, from a photo to a 3D program. After you make one, run it through the STL checker to see what needs fixing.