An FCSTD1 file is a backup of an .FCSTD CAD file created by FreeCAD, a free, cross-platform CAD design application. It contains the save state of an FCSTD file, which includes a 2D or 3D design and the geometry for the components. FCSTD1 files are helpful for saving a backup of a CAD document in case the program crashes or the user needs to revert to a previous save state of the document.

The FCSTD file is the native file format used to save designs made in FreeCAD. You can set the number of backup files that are automatically generated by FreeCAD in the program preferences. The first backup is stored in the FCSTD1 file, the second backup is saved in the FCSTD2 file, and so on. FCSTD1 files are often found accompanying the FCSTD file for which they contain a backup.