.STL File Extension

File Type 1

Stereolithography File

Developer 3D Systems
3.8  |  63 Votes
Category CAD Files
Format Text and Binary

What is an STL file?

An STL file is a 3D model saved in the stereolithography file format developed by 3D Systems. It contains plain text or binary data that describes a set of triangular facets, which comprise a model. STL files are quite common, and they can be opened in many CAD and 3D modeling programs.

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Screenshot of a .stl file in Microsoft 3D Viewer
STL file open in Microsoft 3D Viewer

Stereolithography is a form of 3D printing primarily used for creating product prototypes. In the late 1980s, 3D Systems co-founder Chuck Hull patented stereolithography technology, and 3D Systems created the STL file format. Primarily, STL files contain 3D models meant to be printed using stereolithography.

Because the STL format was one of the first formats used to store models for 3D printing, a large number of 3D model viewers and CAD programs support it. STL files may be saved as plain text files, which contain human-readable triangular tessellation data, or as binary files, which contain the same data saved in a binary format. When opened in a text editor, plain text STL files appear as a list of facets and vertexes, complete with coordinates. These files always begin with the string solid ModelName.

NOTE: STL files are also sometimes referred to as Standard Triangle Language files and Standard Tessellation Language files.

How to open an STL file

Plain text STL file open in Apple TextEdit
Plain text STL file open in Apple TextEdit

You can open an STL file in many image viewers, 3D modeling, and CAD programs, including:

You can also open plain text STL files in text editors, such as Microsoft Notepad and Apple TextEdit, to view the plain text data they contain. (If you open a binary STL file in a text editor, its data will appear as a series of unintelligible characters.)

How to convert an STL file

Several CAD and 3D model editing programs, including Autodesk Fusion 360 and Microsoft 3D Builder, can convert STL files to other 3D model formats. For example, Fusion 360 can export STL files to:

  • .F3D - Fusion 360 Archive
  • .3MF - 3D Manufacturing File
  • .IPT - Inventor Part
  • .FBX - Autodesk FBX Interchange File
  • .OBJ - Wavefront 3D Object
  • .SKP - SketchUp Document
  • .SMT - Autodesk Shape Manager Text File
  • .STP, .STEP - STEP 3D Model
  • .USDZ - Universal Scene Description Zipped

Apple Preview and Microsoft 3D Viewer can convert STL files to raster images. For example, Microsoft 3D Viewer can convert an STL file to the following formats:

  • .PNG - Portable Network Graphic
  • .JPEG -JPEG Image
  • .BMP - Bitmap Image
  • .TIFF - Taged Image File Format

Apple Preview can also convert STL files to .PDF documents.

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Programs that open STL files

Updated May 27, 2022
File Type 2

Certificate Trust List

Developer Microsoft
3.0  |  12 Votes
Category Web Files
Format N/A

What is an STL file?

List of predefined items that have been signed by a trusted entity; may consist of a list of filenames or a list of certificates; each item in the list has been approved by the signing entity.

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Certificate trust lists (CTLs) are used by Microsoft IIS to store trusted websites and other addresses that require a secure connection.

Programs that open or reference STL files

Updated June 7, 2007
File Type 3

Fireworks Style Library

Developer Adobe Systems
2.5  |  10 Votes
Category Data Files
Format N/A

What is an STL file?

Library of styles referenced by Macromedia (now Adobe) Fireworks; may be used to apply Stroke, Fill, Effect, and Text attributes to an object within an open Fireworks project; can be helpful for creating a uniform look across multiple objects.

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Some styles are included with Fireworks, though additional styles may be added to the program. To add extra styles, click the small icon in the top right corner of the Styles panel in Fireworks and select "Import Styles." Then browse to an STL file and click OK to add the style library to your current collection.

Programs that open or reference STL files

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Updated May 2, 2014
File Type 4

Spruce Technologies Subtitle File

Developer Spruce Technologies
2.4  |  10 Votes
Category Video Files
Format Text

What is an STL file?

Movie subtitles file used by video playback and authoring applications; stores the time each subtitle should be displayed; also specifies instructions for how to display subtitles; used for broadcasts, annotating video files, or for storing movie translations.

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STL files contain commands, comments, and entries. Commands begin with the dollar sign ($) character and can specify fonts, colors, alignment, and other subtitle properties. Comments are delimited by "//" characters. Entries contain the actual subtitle timestamp and text. Here is an example entry:

00:00:11:13 , 00:00:11:42 , This is an example subtitle.

NOTE: STL subtitles are sometimes referred to as EBU subtitles, since the standard comes from the European Broadcasting Union (EBU).

Programs that open or reference STL files

Updated February 18, 2014
File Type 5

SuperTux Level File

Developer SuperTux
2.4  |  9 Votes
Category Game Files
Format Text

What is an STL file?

Game file used by SuperTux, a 2D jump and run side scrolling game similar to Super Mario Bros.; saved in a plain text format and specifies the features of a level, such as enemy unit locations, positions of blocks, and the time limit; also includes a reference to the music and background image for the level.

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STL files are stored in the /​data/​levels/​ directory of the installation.

Programs that open or reference STL files

Updated October 21, 2011

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