STL Units

This page explains the importance of setting the correct units when exporting and importing STL files.

The first thing to note is that STL files do not specify what units their distances are in. When a program opens a STL file, it only knows that the model measures a certain number of units in each dimension. These units might be meters, inches, kilometers, Planck lengths, or any other unit that exists.

The Afinia printing software uses millimeters for geometry, so it interprets STL files as having units of millimeters.

If you are using software, like Blender, that uses arbitrary units, you must scale your model so that 1 unit equals 1 millimeter.

If you are using SolidWorks, SketchUp, or other software that keeps track of its units, it is important to configure the export to use units of millimeters.

 

There are two basic ways that units can go wrong:

1. An object is modeled with arbitrary units (in Blender-like software) and its scale is not considered. It ends up being only a few units across. That scale works for modeling, and it is exported to an STL file a few units across. The Afinia printing software interprets the model as being only a few millimeters across.
2. An object is modeled with specific units and its scale is considered. It ends up being a few centimeters across. During the export process, the dimensions are converted to meters (or some other unit) and saved to the STL file. The STL file says that the object is a few hundredths of a unit across. When the Afinia printing software opens the file, it interprets units as millimeters and thinks that the object is a few hundredths of a millimeter across.

 

Unit problems can cause the STL file to be imported as smaller or larger than expected. Although this can easily be corrected by scaling the model up or down by the conversion factor between millimeters and the exported units, having each STL file already with the correct units makes things faster.