Standard Tessellation Language

STL File (Format for 3D Printing) – Explained in Simple Terms

STL File

What is an STL file? What does .STL stand for? What’s the STL format good for? We explain the STL file format in simple terms.

Here’s a primer on what they are and how they work, the advantages and disadvantages of their use, plus alternative file formats to consider. In this article, we’re talking about the CAD / 3D printing file format, not about the Standard Template Library file format used in C++.

Did we leave any questions unanswered? Let us know in the comments, and we’ll address your queries in a future update.

1. What is an STL File?

STL File Format

In a nutshell, an STL file contains data describing the layout of a three-dimensional object. These files are usually generated by a computer-aided design (CAD) program. “.STL” is the file extension of the STL file format.

The STL file format is the most commonly used file format for 3D printing. When used in conjunction with a 3D slicer, it allows a computer to communicate with 3D printer hardware.

Since its humble beginnings, the STL file format has been adopted and supported by many other CAD software packages, and today is widely used for rapid prototyping, 3D printing, and computer-aided manufacturing. Hobbyists and professionals use it alike.


2. What does the file extension .STL stand for?

The meaning of the file extension .STL has been lost to the mists of time.

It’s widely believed to be an abbreviation of the word STereoLithography, though sometimes it is also referred to as “Standard Triangle Language” or “Standard Tessellation Language”.

3. Why is the STL Format Important for 3D Printing?

In software terms, the STL file is perhaps the single most important item of any 3D printing workflow. It contains the 3D model that is used to make a physical object, and as a standard data format, it has reigned supreme for nearly thirty years.

4. How does an STL File work?

STL File Format

In simple terms, the STL file format uses a series of linked triangles to recreate the surface geometry of a solid model.

For a basic model, its surfaces can be represented using a few triangles. For higher resolution models, more triangles are required to replicate the surface of the model. The more triangles that make up a model, the bigger the file size and the more detailed the object.

The STL file format can define complex shapes (mathematically speaking, polyhedrons with any polygonal facets). In practice, it is mostly used to describe the layout of triangles in a virtual space.

Diving a bit deeper, each triangle facet is described by a perpendicular direction and three points which represent the corners of the triangle. An STL file provides a complete listing of the x, y and z coordinates of these corners and perpendiculars.

5. Who invented the STL File Format?

STL File Format

The STL file format was invented by the Albert Consulting Group for 3D Systems in 1987, in turn, to support the stereolithographic 3D printer invented by Chuck Hull. The STL file format made it possible to transfer three-dimensional CAD models to 3D Systems’ very first commercial 3D printers, the StereoLithography Apparatus.

Since this initial release, the technical specifications of the STL file format have remained virtually unchanged.

6. Is Every STL File 3D Printable?

STL File Format

Unfortunately not. Only a 3D design that’s specifically made for 3D printing is 3D printable. The STL file is just the container for the data, not a guarantee that something is printable.

3D models suitable for 3D printing need to have a minimum wall thickness and a “watertight” surface geometry to be 3D printable. Even if it’s visible on a computer screen, it’s impossible to print something with a wall thickness of zero.

There’s also the consideration of overhanging elements on the model. Look at the ALL3DP logo in the picture above; if the model is printed upright, then overhanging elements with more than a 45-degree angle will require supports (which you can see in green).

When downloading an STL file that you haven’t created yourself, it’s worth taking the time to verify that it is indeed 3D printable. This will save you a lot of time and frustration (and wasted filament).

Further reading: MeshMixer Tutorial for 3D Printing Beginners and 9 Important 3D Printing Concepts Everyone Should Know


7. What are the Benefits of the STL File Format?

STL File Format

In a word: simplicity. Even the most complex design can be reduced to simple geometrical forms.

Open: The STL  file format is non-proprietary, and allows for both (human readable) ASCII and (smaller) binary representations.

Vector-based: As the STL file format describes geometrical figures, it is scalable, which is a huge advantage if you want to change the size of a 3D object.

Universal: STL files can be used with nearly every 3D printer or CAD program.

Readable: Users can code up an STL file very quickly, and can read or write to it very easily. Almost all of today’s CAD systems are capable of producing STL files. For the user the process is often as simple as selecting “File,” “Save As” or “Export,” and then clicking on “STL”.

In turn, a large software and service ecosystem has grown around repairing and manipulating STL files.

Most 3D printable models you can find on the internet are in the STL file format. The existence of this ecosystem, combined with STL-based software investments made by 3D printer manufacturers, has given rise to a large user-base that’s heavily invested in the format.

8. What are the Disadvantages of the STL File Format?

STL File Format

There are a quite few. For starters, STL files are limited to describing only the surface geometry of a three-dimensional object. There is no representation of color, texture, material, substructure, or other attributes typically found in other CAD model formats.

With the evolution of additive manufacturing technology from primarily single-material, low-detailed shapes to multi-material geometries in full color (with functionally graded materials AND micro-structures), there’s a growing need for a standard interchange file format that could support these features.

A secondary factor, but no less important, is the improving resolution of additive manufacturing technologies. As the fidelity of printing processes embraces micron-scale resolution, the number of triangles required to describe smooth curved surfaces can result in massive file sizes.

Other things lacking from the STL file format is the inclusion of metadata (such as authorship and copyright information), little or no file security, and the inability to detect or fix errors in the surface mesh.

To put it kindly, the STL file format simply hasn’t kept pace with new developments and features available to modern 3D printers.

9. Can You 3D Print an STL File Directly from your Computer?

STL file format

Nope! You need an additional step.

For 3D printing, the STL file has to be opened in a dedicated slicer. What’s a slicer? It’s a piece of 3D printing software that converts digital 3D models into printing instructions for your 3D printer to create an object.

The slicer chops up your STL file into hundreds (sometimes thousands) of flat horizontal layers based on the settings you choose, and calculates how much material your printer will need to extrude and how long it will take to do it.

All of this information is then bundled up into a GCode file, the native language of your 3D printer. Slicer settings do have an impact the quality of your print so it’s important to have the right software and settings to get you the best quality print possible.

Once the GCode has been uploaded to your 3D printer, the next stage is for those separate two-dimensional layers to be reassembled as a three-dimensional object on your print-bed. This is done by depositing a succession of thin layers of plastics, metals, or composite materials, and building up the model one layer at a time.

More information: 3D Slicer Settings for Beginners – 8 Things You Need to Know

10. How Can You Open an STL File?

STL File Format

Fortunately, opening an STL file is not too complicated. There are several free STL file viewers for this purpose, which you can either use online or as a desktop application. Refer to our dedicated guide here: 20 Free STL File Viewers (Online, Mac, PC, Linux, Android, iOS)

11. How Can You Edit or Convert an STL File?

STL File Format

Yes, it is entirely possible to edit an STL file and convert the STL to another file format. Because the format is open, there is nothing to prevent you from changing the contents of a file. Actually, the process of editing is quite easy. We have a dedicated article on this topic: 5 Free STL Editors + How to Edit STL Files 


12. How Can You Repair a Broken STL File?

STL File Format

There are several programs which can help with repairing a broken STL file. For example, Netfabb Basic is a great tool for repairing the most common STL file problems. You find more information on these programs in our article: 20 Best 3D Printing Software Tools (Most are Free)

13. Where Can You Find Free STL File Downloads?

STL File Format

There are many repositories, marketplaces and search engines on the web containing literally millions of free STL files. You can refer to our regularly updated list — 34 Best Sites for Free STL Files and 3D Printer Files/Models  — or you can choose one of these models to get started: 35 Cool Things to 3D Print Which Are Actually Useful

14. Are there any Alternatives to the STL File Format?

STL File Format

The STL file format is not the only format used in 3D printing. There are over 30 file formats for 3D printing. Most important is the OBJ file format, which can store color and texture profiles. Another option the is Polygon file format (PLY), which was originally used for storing 3D scanned objects.

More recently, there have been efforts to launch a new file type by The 3MF Consortium, which is proposing a new 3D printing file format called 3MF. They claim it will streamline and improve the 3D printing process.

To implement it, Microsoft has partnered up companies like Autodesk, HP, and Shapeways to make their vision a reality. More details on the 3MF Consortium can be read on their website, together with preliminary documentation about the 3MF file type on their GitHub page.

It’s far too early to say whether this will become widely adopted, however, so we’d recommend sticking with the STL file format for the foreseeable future.

15. Can the STL File Format Display Colors?

3d selfies Yes, it can, but then you’re leaving the standard.

The reason the STL file format lacks color information is simple. When rapid prototyping evolved in the 1980’s, no one thought of color printing. Nowadays, 3D printing materials and processes have evolved rapidly. Some allow you to print in full-color – just think of sandstone 3D selfies, as pictured above.

There are just a few format deviations:

  • Some software packages (i.e. VisCAM and SolidView) use the two “attribute byte count” bytes at the end of every triangle to store a 15-bit RGB color information. 
  • Materialise Magics software uses an 80-byte header at the top of the file to represent the overall color of an entire part.

As mentioned, the 3MF Consortium wants to add color information to their new 3D printing file format 3MF. They claim it will streamline and improve the 3D printing process.

16. Can the STL File Format Interpret 3D Printing Materials?

RBX_filament_lineup_fullframe1-e1431954932936
No, it can not. The STL filr format wasn’t invented with materials in mind. Only the 3MF file format to adds material data to the STL file, but it’s not yet commonly used yet.