What is the FCSTD file format?
.fcstd is a document file associated with FreeCAD, a parametric 2D/3D modeling software package. Internally, an .fcstd file is a ZIP archive containing Document.xml (the parametric object tree), GuiDocument.xml (view/camera state), one or more .brp OpenCASCADE BREP geometry files, and a thumbnail image. You can rename .fcstd to .zip and inspect the contents in any archive tool. FreeCAD also offers various simulation functions, and the .fcstd format can be converted to other popular formats including STEP and STL.
FreeCAD
FreeCAD is available for Windows, Linux, and macOS. Its GUI is based on Qt technology; a CLI is also available. FreeCAD is based on the concept of a workbench - a set of tools and functions grouped for a certain task that needs to be performed during a stage of project development. Workbenches provide users with the best selection of tools for a given task. FreeCAD also supports many other file formats used by various CAD software programs.
Each project save also produces an .fcstd1 backup file containing the previous revision in the same ZIP-based format.
Security & safety
RISK: MEDIUMAn FCStd is data (a ZIP of XML + geometry) and does not itself run code on opening. The real caution is FreeCAD's macro system: FreeCAD documents and add-on workbenches can carry Python (.FCMacro) automation, and the Addon Manager installs community code - only run macros/workbenches from sources you trust. The FCStd file alone is safe to inspect; rename to .zip to look inside before opening if you're unsure of the source. Download FreeCAD only from freecad.org or its official GitHub to avoid bundled-adware repackages.
Format details
in a nutshellPrograms that open FCSTD files
Windows2 apps
FreeCAD
Open-source Install FreeCAD, then File → Open (or drag the FCStd onto the window). The full editable model loads. 
Ondsel ES / FreeCAD-based distributions
Open-source Any FreeCAD build (including community spins) opens FCStd identically; download only from freecad.org or the official GitHub releases. 
macOS1 app
Linux1 app
FreeCAD
Open-source Install from your distro, Flatpak, AppImage or Snap; File → Open the FCStd. The AppImage is the most portable option.
FreeCAD-derived web viewers (limited)
Open-source No official full web viewer exists; for a quick look, export to STEP/STL in FreeCAD and view that in an online STEP/STL viewer. The FCStd itself needs FreeCAD. Technical details
deep spec| Container format | ZIP archive (PKZIP / DEFLATE); an `.fcstd` file is a valid ZIP and can be opened with any archive tool |
| Internal files | `Document.xml` (parametric object tree), `GuiDocument.xml` (view and camera state), one or more `.brp` BREP geometry files, and a PNG thumbnail |
| Compression | DEFLATE per ZIP entry; geometry `.brp` entries are stored compressed by default |
| Magic bytes | `50 4B 03 04` ("PK") at offset 0 - standard ZIP local file header signature |
| MIME type | `application/x-extension-fcstd`; also recognized as `application/zip` by generic tools |
| Geometry kernel | OpenCASCADE BREP boundary-representation data stored in `.brp` files - same kernel used for STEP/IGES import-export |
| Schema versioning | `SchemaVersion`, `ProgramVersion`, and `FileVersion` attributes in the `Document.xml` root element record the writing FreeCAD version |
| Parametric history | Full parametric model history stored as an object dependency tree in `Document.xml`; re-computed on open |
| Backup file | Each save produces an `.fcstd1` sibling file holding the previous revision in the identical ZIP-based format |
| Integrity check | ZIP per-entry CRC-32 checksum only; no whole-file digital signature and no encryption |
| Typical file size | 50 KB - 50 MB; grows with shape complexity, mesh density, and embedded TechDraw templates |
| Platform support | Windows, macOS, and Linux - FreeCAD (the primary application) is cross-platform and open-source |
| GUI state storage | Active workbench, viewport settings, and visible/hidden states are saved in `GuiDocument.xml` alongside the geometry |
| ZIP entry limit | Standard ZIP allows up to 65,535 entries; ZIP64 extension lifts this; real `.fcstd` documents use far fewer |
| Released | 2002 (FreeCAD project start); .FCStd container in current form by FreeCAD 0.13+ |
| Latest version | FreeCAD 1.0 (stable, November 2024); FCStd schema versioned inside Document.xml (SchemaVersion attribute) |
| Open standard | Yes · royalty-free |
| Specification | wiki.freecad.org |
FCSTD conversions
Community Q&A
asked by usersNo questions yet - be the first to ask about FCSTD files.
