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How to Repair STL Files with 3 Free STL Repair Software Tools

Picture of Dibya Chakravorty
by Dibya Chakravorty
Apr 12, 2018

Struggling with a broken or bloated STL file? Learn to repair STL files with the 3 best free STL repair software tools available right now: Netfabb, Microsoft 3D Tools, and 3D Builder.

Nothing is more frustrating than a 3D model that won’t print. STL files need to be watertight, have the right wall thickness and be properly optimized in order to be printable. Even the most experienced 3D designer will sometimes leave unprintable defects in the model. So you need software to repair your STL files.

If you have a broken STL file, your Slicer software will often warn you about the defects. But in the worst case, the slicer won’t complain at all. You will start printing as usual, spend time and money, and eventually land up with a failed print.

Another common scenario is when you try to upload an STL file in an online 3D printing service like i.materialize or Sculpteo, but get an error message saying that the file size is too big. In this case, you are dealing with a bloated STL file.

So what do you do in these situations? How do you repair STL files to get a printable version?

In this article, we introduce you to the best free software that can check your STL file for errors and also repair STL files automatically to produce a printable version.

How Did we Pick the STL Repair Software?

We tested all the notable free software available to repair STL files – including 3D Builder, Netfabb, Meshmixer, Meshlab, Microsoft 3D Tools, FreeCAD, and Blender.  The focus was on the automatic repair and optimization tool. We found that only three of these packages were able to fix most broken STL files automatically. These repair STL file packages are 3D Builder, Microsoft 3D Tools, and Netfabb. The other packages struggled to produce good results without a great deal of manual intervention.

So in this article, we will focus on Netfabb, Microsoft 3D Tools, and 3D Builder.

Don’t miss: STL Repair - 17 Best Software Tools to Repair STL Files


Netfabb is a powerful tool to repair STL files. It used to be available for Windows, Linux, and MacOS. But then it was acquired by Autodesk in 2015, who decided to stop supporting Linux and MacOS. You can still download an older version of Netfabb (prior to the acquisition) for Linux and Mac from this unofficial GitHub repository.

The official version of Netfabb (Windows only) can be downloaded here. After registration, you’ll get the full-blown version for 30 day, which then degrades into a basic version that still is perfect for this use case.

If you are a Windows user, there is no real reason to prefer Netfabb over 3D Builder. But we are including it here for three reasons:

  1. If 3D Builder is unable to repair an STL file, it is worth checking if Netfabb can do a better job.
  2. Linux and Mac users might be able to install the unofficial version and get the job done (we haven’t tested it, but feel free to give it a go as a last resort).
  3. Netfabb’s optimization tool is more intelligent than 3D Builder’s. It can identify parts with and without fine details, and optimize them accordingly.

Free STL Repair Software: Netfabb Import STL File

Launch Netfabb and from the menu, click File -> Open Part. A settings panel will appear that will give you several options for importing the part. Just use the defaults, and click Add Parts.

If the STL file is defective, you will see a big red exclamation mark appear on the bottom right corner (as you can see in the GIF above). This means you need to repair the STL file. If the exclamation mark did not appear, that means the part is fine and can be printed without problems.


Free STL Repair Software: Netfabb Fix the Files

To repair STL files, locate the repair tool on the top toolbar. The repair tool is represented by a red cross icon. Click on the Repair tool. At this point, Netfabb will create a copy of the model. Applying the repair to a copy ensures that the original model is not altered.

On the left-hand side, you will now see the Repair Panel. The repair panel will give you information about the following important properties:

  1. Is the mesh closed or watertight?
  2. Is the mesh properly oriented?
  3. How many “border edges” are present? These are edges that are not shared by two triangles in the mesh.
  4. How many triangles are not oriented as expected (with respect to the neighboring triangles)?
  5. How many holes does the model have?

On the Repair panel, click Run Repair Script and select Default Repair in the popup that appears. If the Default Repair doesn’t manage to fix the file, you can try out Extended Repair next. Click Execute to run the repair process.

GIF illustraing how to repair STL files in Netfabb

If the Repair went well, then the repair panel will show that the mesh is now closed and properly oriented by displaying two green check marks. You can also check if there are any remaining border edges, wrongly oriented triangles or holes. This information is available in the repair panel. In the best case, there wouldn’t be any of these artifacts left.


Free STL Repair Software: Netfabb Optimize Unwieldy STL Files

Netfabb also allows you to optimize the STL file if it is unwieldy. While in the Repair mode, choose Mesh Edit from the top menu and select Reduce Triangles. This will open the Reduce Triangles dialogue.

Make sure you select All Triangles in the options. For the other settings, the default values usually work out pretty nicely. Netfabb’s optimization routine is actually better than 3D Builder, because it can automatically detect parts with low and high amount of detail and optimize the file accordingly. This ensures that you do not inadvertently lose details in the model because of the optimization.

When you are ready, click Reduce. Now zoom into the model and check if you are satisfied with the final result.

GIF illustrating how to optimize an STL file using Netfabb

Free STL Repair Software: Netfabb Apply Repair and Optimization

Happy with the repair and optimization? Then it’s now time to apply it. The repair can be applied by right-clicking on the copy that was created automatically earlier. You can find this in the Parts menu on the top left. Click on Apply Part Repair.

You will be presented with several options. You can keep the original defective part loaded in Netfabb if you would like to compare the repaired part with it. In this case, select Keep Old Part. If you don’t want to keep the old part in the Netfabb workspace, select Remove Old Part.

GIF illustrating how to apply repairs and optimizations in Netfabb

You now have the repaired model on the workspace. Notice that the big exclamation mark no longer appears, meaning that all is good with this part.


Free STL Repair Software: Netfabb Export Repaired and Optimized Model as STL

Let’s save the repaired model. This is easy enough. Simply click on File on the Top Menu and select Export Part. The great thing about Netfabb is that it allows you to save the repaired part in multiple formats. We will  export to STL in this example. As always, remember to name the repaired file properly.

Microsoft 3D Tools

If you are on Linux or Mac and want a free tool to repair STL files, then you are out of luck. The free options available on Linux or Mac are Meshmixer and Meshlab, which are not the “repair STL Files with one click” type of software – they are more powerful and require expertise on part of the user to maneuver correctly.

But there is an easy browser-based alternative to repair STL files that might work for Linux and Mac users. It’s called Microsoft 3D Tools. It’s operated by Microsoft on their Azure cloud platform and the software runs in the cloud. You need to sign in with a Microsoft account in order to use it.

Free STL Repair Software: Microsoft 3D Tools Repair STL Files

Microsoft 3D Tools provides a minimalist and super simple user interface, providing an option to upload an STL file and download the repaired file. Here’s how the process looks like.

There is a downside though. Microsoft, in a bid to promote its own 3MF format, only allows users to download the 3MF version of the repaired file!

If you use a slicer that can handle 3MF files (Cura and Simplify3D can), then you should be alright. Otherwise, you will need to convert the 3MF file to an STL file using another software.

3D Builder

For Windows users, 3D Builder is perhaps the most easy to use tool for automatically detecting defects and to automagically repair STL files.

3D Builder comes pre-installed in every Windows 10 distribution, so you don’t have to install anything. Just find it using Cortana (the search utility) in the taskbar and run it.


Free STL Repair Software: 3D Builder Import the Broken STL Files

Click Open and in the following screen, click Load Object. This will allow you to choose a file from your computer.

In this example, we are using a defective STL file depicting the Pyramid of Kukulcan at Chichen Itza, which is downloadable here.

Once you load the object, it will appear on the workplane. By the way, I really love the look and feel of the 3D Builder interface.

The object has been loaded at this point, but it has not yet been imported. To import it, just click on Import Model from the top Menu bar.

GIF illustrating how to import an STL file in 3D Builder

Free STL Repair Software: 3D Builder Fix the Files

Once the model has been imported, 3D Builder will automatically inform you of any defects in the model using a popup. In case of a defect, the popup will say “One or more objects are invalidly defined. Click here to repair.” In addition, a red rectangle will appear beneath the model.

If you don’t see a popup and the object already has a blue outline, this means that 3D Builder did not detect any problems with your file. You can print it as it is.

If you got the popup with the error message, 3D Builder will automagically repair STL files when you click that popup . Once the model has been repaired, a blue outline will appear around it.

GIF illustrating how to repair STL files with 3D Builder

Free STL Repair Software: 3D Builder Optimize Bloated STL Files

Sometimes, you may want to optimize the STL file so that it has fewer triangles and therefore, smaller file size. Some online 3D printing services like i.materialize or Sculpteo may refuse to take your file because it’s too big. 3D Builder offers a simple and intuitive way to optimize an STL file.

Simply go to Edit on the top menu and select Simplify. The Simplify menu is going to appear at the top. It features a Reduction level slider, which controls the amount of optimization. Be careful with this slider! Too much, and you might lose finer details in the model. Too little, and you may not get sufficient reduction in file size. It is important to play with this slider and zoom in onto the finer parts of the models to ensure that the optimization routine does not destroy the finer details. Once you are happy with a reduction level, simply click Apply.

The optimized part may develop its own problems and might need an additional round of repairing. If a red rectangle appears beneath the optimized part, it has errors. In this case, just repeat the repair process again.


Free STL Repair Software: 3D Builder Save Repaired and Optimized STL File

Now, you can download the repaired and optimized model by clicking on the Hamburger menu and selecting “Save as”. Note that the default output is in the 3MF format! Slicers like Cura can indeed print from a 3MF file. But you can also change the output format to STL if you want.

Make you rename the repaired file appropriately (I usually add a “_repair” to the original file name) to ensure that you don’t confuse it with the original design.


Broken STL files can be really frustrating. But now we at least know how to repair STL files automatically using the best free tools available and make them printable again. It’s good practice to run your STL file through 3D Builder, Microsoft 3D Tools or Netfabb before you start printing them. You will save a lot of time and money that way.

While this article focused on free and automatic solutions to repair STL files, there’s also two other types of STL repair software. There’s software like Meshlab or Meshmixer, which provides very fine-grained control over the entire repair process, but requires extensive experience to use correctly and efficiently. There’s also software like MakePrintable, Materialize Cloud, Limitstate FIX, Cur3D or Emendo, which are expensive enterprise-grade solutions.

If 3D Builder, Microsoft 3D Tools or Netfabb are not cutting it for you,  you can get a complete overview of the STL repair software ecosystem in our article covering 17 different STL repair tools.

License: The text of "How to Repair STL Files with 3 Free STL Repair Software Tools" by All3DP is licensed under a Creative Commons Attribution 4.0 International License.

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