What is an STL File? All You Need to Know

What is an STL File?

STL (Standard Triangle Language or Standard Tessellation Language) is a file format that stores information about 3D models. It was first developed in 1987 to document 3-dimensional surface geometry for stereolithography – the oldest additive manufacturing technique.

This format uses small triangles to describe the surface of a 3D model. STL file format is widely used in various fields including 3D printing, CAD software, and rapid prototyping industries.

You can recognize an STL file (with .stl file extension) by its lack of color and texture.

In addition to STL files, there are other 3D printing file types such as OBJ and PLY (Polygon file) formats. However, the STL file format is the most popular because it is widely used and supported by almost all 3D printers.

In this article, we will discuss what an STL file is, how it works, and go over its benefits.

Information within the STL File Format

An STL file reproduces the surface geometry of a 3D object by using a series of linked triangles. It doesn’t contain other design specifications such as color and texture.

The triangular geometries store lots of detailed information about the features which is used by the 3D printer to build the physical object. Larger the number of triangles, the higher the resolution of the 3D model, resulting in a larger file size.


Tessellation (Backronym of Standard Tessellation Language) is an arrangement of triangular tiles that completely covers a geometric surface without overlaps or gaps. STL programs use tessellation to encode the surface geometry of a 3D object.

Types of STL File Formats

ASCII and Binary encoding are two of the most common ways STL files store information. They both are considered universal i.e. supported by almost all 3D printers. ASCII files are read with most text editors, whereas binary files are smaller in size and easier to share.


The ASCII version of the STL file format uses keywords to send instructions to the 3D printer about what object it needs to print. Its simple text structure makes it easier to read, identify bugs in the file, and execute commands.

Here’s a look at ASCII STL file syntax:

Solid <Solidname>
    facet Normal <Xₙ> <Yₙ> <Zₙ>   
        outer Loop
            vertex <X₁> <Y₁> <Z₁>
            vertex <X₂> <Y₂> <Z₂>
            vertex <X₃> <Y₃> <Z₃>
        end loop


  • Solidname is the name of the model which can be omitted
  • n is a floating-point number

Note that ASCII files consume a lot of storage space and take more time to process.

Binary STL Files

STL files using binary format are faster to read and more compact than ASCII files. They are smaller in size, offering a quicker option to transfer over a network.

Here’s a look at the syntax of a Binary STL file:

Byte Data Type Description
80 ASCII Header. No data’s significance
4 unsigned long integer Total number of facets
4 float i for normal
4 float j
4 float k
4 float x for vertex 1
4 float y
4 float z
4 float x for vertex 2
4 float y
4 float z
4 float x for vertex 3
4 float y
4 float z
2 unsigned integer Attribute byte count

3D Printing STL Files

Once you design a 3D model using standard CAD programs (for example, Autodesk, SolidWorks), you’ll need to convert it into a language that your 3D printer can read.

First, the CAD model is turned into the STL file format. This means, what you design in CAD is translated into a polygon mesh in a 3D space (triangular geometries with edges, surfaces, and faces).

Before printing your 3D file, make sure that:

  • All faces must be closed.
  • There must be two shared vertices between adjacent triangles.
  • Orient the vertices with the normal vector using the right-hand rule.

The slicer then determines the print head’s path during the 3D printing process. This is called “slicing” the STL file which is done to obtain G-code.

Most 3D printers use G-Code as the standard language to turn 3D designs (containing information about the geometrical surface) into a set of machine-readable instructions about temperature, extrusion, and movement.

The 3D printer executes the G-code commands (stop/start extrusion, raise the build plate, move the print head, s, etc.) to create a 3D object.

Benefits of the STL File Type

You can use almost any CAD software (AutoCAD, Autodesk, etc.) to create STL files for 3D printing. The STL format is simple, plain text structure and organizes the parameters (like chord height and chordal tolerance) in .txt file table layout.

The STL file is used universally. It is the main file type for 3D printers everywhere.

STL file type is compatible with almost all 3D systems and printing platforms. It has a large user base that depends on the file format for creating a wide range of objects – from jewelry and toys to drones and skateboards. You can produce accurate models and functional parts, regardless of the shape or complexity.

3D printing with STL files is faster because it doesn’t process color and texture specifications. It is the ideal file format if you’re looking to print objects of the same color.

Alternatives to STL File Format

One of the drawbacks to using the STL file format is that it can only read the external shape and surface of a 3D model. This means it doesn’t recognize other CAD file attributes such as color and internal structure.

Secondly, faulty STL files can result in issues such as overlapping or gaps among linked triangles, poor resolution, and other geometric inaccuracies.

While STL is the most popular 3D printing file format, there are other options as well. These include Microsoft’s 3MF, VRML, AMF, OBJ, and PLY. You can compare different file types to identify the one that meets your printing needs.

Streamline Your 3D Printing With Carbon

STL file has a crucial role in 3D printing as it brings together the 3D printer and 3D model. They contain information about the surface geometry of a 3D object.

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