LinuxWhat is the PRJ file format?
.prj is a file extension primarily used by Esri's Shapefile format to store a coordinate-reference-system (CRS) definition in plain-text Well-Known Text (WKT) syntax. Various GIS applications use this companion file to correctly position and display spatial data.
What type of data is saved in .prj files?
.prj files contain a single WKT coordinate-system definition - entirely human-readable plain text. A projected CRS opens with PROJCS[ and a geographic CRS with GEOGCS[; the newer WKT2 standard uses PROJCRS[ and GEOGCRS[. The definition encodes the datum, ellipsoid, units, and projection parameters needed to align the .shp geometry with other spatial layers.
What program can be used to open .prj files?
.prj files are unique to the dataset with which they were created; GIS tools such as ArcGIS Pro, QGIS, and GDAL/OGR read them automatically when loading a Shapefile. Because .prj is plain UTF-8 text, any text editor - including Notepad - can open one. Copying a Shapefile without its .prj companion causes GIS software to treat the geometry as having an unknown CRS.
Security & safety
RISK: LOWA GIS .prj is a tiny passive text file (a coordinate-system string) with no executable content, so it is safe to open and edit. The practical 'danger' is data misplacement, not malware: a missing or wrong .prj makes a Shapefile's features appear in the wrong location or fail to overlay other layers - correct the WKT or re-define the projection to fix it. For non-GIS .prj project files, normal caution applies to whatever the host application loads (e.g. a project that references external scripts). When a .prj's type is unknown, open it in a text editor first: GIS .prj is short readable WKT, while app project files are usually longer or binary.
Format details
in a nutshell- Generic application project file - Many programs save their workspace/project as .prj - e.g. Microsoft Visual Studio (older), Tracktion/WaveLab audio projects, AIMMS, Monarch, device programmers. Open with the app that created it.
- 3ds Max / CAD scene project (PRJ) - Autodesk 3ds Max and some CAD tools use .prj for a scene/project; this is the source of the db's 3ds/obj/dae/dxf conversion pairings.
- HEC-RAS project file - The US Army Corps HEC-RAS hydraulic-modeling project uses .prj - confusingly the same extension as the GIS projection file, opened in HEC-RAS.
Programs that open PRJ files
Windows4 apps
QGIS
Open-source Add the Shapefile (Layer > Add Vector Layer); QGIS reads the .prj automatically and sets the layer CRS. To assign/fix a CRS, use Layer Properties > Source, or save a new .prj via Export. 
ArcGIS Pro
Paid Add the data; the .prj defines the coordinate system. Use 'Define Projection' to write/fix a .prj. Esri's native environment for this file.
Notepad / any text editor
Open-source Open the .prj as text to read or edit the WKT coordinate-system string (e.g. paste the correct definition from spatialreference.org / EPSG).
Application that created it (non-GIS .prj)
Varies If the .prj is a Visual Studio / 3ds Max / audio (Tracktion, WaveLab) / AIMMS / Monarch project, open it in that specific program - there is no universal .prj opener. 
macOS1 app
QGIS
Open-source Load the Shapefile to apply its .prj, or edit the WKT in a text editor (TextEdit in plain-text mode). Technical details
deep spec| Format type | Plain-text Well-Known Text (WKT) coordinate-reference-system definition; one CRS definition per file, no binary content |
| File encoding | UTF-8 or ASCII plain text; no binary header, no BOM required |
| MIME type | text/plain |
| File signature | No binary magic bytes; identified by WKT keyword at byte offset 0: PROJCS[ or GEOGCS[ (WKT1), PROJCRS[ or GEOGCRS[ (WKT2) |
| WKT dialects | WKT1 (Esri variant and OGC 01-009) and WKT2 (OGC 18-010r11 / ISO 19162:2019); keywords differ between dialects |
| Syntax structure | Nested bracket notation - KEYWORD["name",parameter,...] - forming a tree of datum, ellipsoid, prime meridian, projection method, and unit nodes |
| CRS types encoded | Geographic (GEOGCS/GEOGCRS), Projected (PROJCS/PROJCRS), Vertical, and Compound coordinate-reference systems |
| Parameters recorded | Datum and spheroid/ellipsoid, prime meridian, angular and linear units, projection method, central meridian, standard parallels, false easting/northing, scale factor |
| Shapefile bundle role | Mandatory companion to .shp, .shx, and .dbf; must share the same base filename; absence causes GIS software to mark the layer CRS-unknown |
| Typical file size | 200 bytes to ~2 KB; the complete CRS definition for most coordinate systems fits within a few hundred characters |
| Effect of absence | GIS applications fall back to an undefined or assumed CRS (often WGS 84), which can misalign layers when combining datasets from different sources |
| Developer / origin | Originated with Esri's Shapefile format (early 1990s); WKT CRS syntax subsequently formalized as an OGC standard |
| Software support | ArcGIS Pro, QGIS, GDAL/OGR, MapInfo Pro, FME, Global Mapper; also written by pyproj, PROJ, and most GIS scripting libraries |
| Human readability | Fully human-readable; can be inspected or hand-edited in any plain-text editor without GIS software |
| Released | With the Esri Shapefile (early 1990s); WKT CRS standardized by the OGC |
| Open standard | Yes · royalty-free |
| Specification | desktop.arcgis.com |
PRJ conversions
Community Q&A
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