What is the CAL file format?
A .cal file is a CALS Raster Image (Type 1), a monochrome bitmap format developed under the U.S. Department of Defense CALS (Continuous Acquisition and Life-cycle Support) initiative in the late 1980s. It was designed for the digital exchange of technical documentation - primarily engineering drawings and scanned technical manuals - between defense contractors and government agencies, governed by standards MIL-STD-1840 and MIL-R-28002.
The format stores a single 1-bit (black-and-white) raster image compressed with CCITT Group 4 (ITU-T T.6) encoding - the same lossless bitonal algorithm used in fax machines and bitonal TIFF images. Every .cal file opens with a fixed 2,048-byte ASCII header containing MIL-STD-1840 keyword/value records - fields such as srcdocid:, rorient:, rpelcnt:, and rdensty: - padded to exactly 2,048 bytes with spaces. The compressed image data begins immediately after that header block.
Because the compression codec is identical to Group 4 TIFF, most CAD and document-management tools can convert .cal to TIFF losslessly and back. The format also appears under the alternative extensions .cals, .gp4, and .c4.
Typical files represent scanned B-, C-, D-, and E-size engineering drawings at 200-400 DPI. The governing standard (MIL-PRF-28002) has since been cancelled, and .cal has largely been superseded by PDF and multi-page TIFF for technical document interchange. Many imaging tools still read and write the format, including IrfanView, XnView MP, ImageMagick, AutoCAD raster tools, and GDAL (gdal_translate).
Security & safety
RISK: LOWA CALS raster is passive bilevel image data with no scripting or executable content, so the file itself is harmless. The only practical cautions: confirm the .cal is actually a CALS image (the extension is reused by Windows Calendar and SuperCalc), and avoid dubious 'CAL converter' download sites that bundle adware - the legitimate free tools (ImageMagick, GDAL, XnView, IrfanView) handle it cleanly.
Format details
in a nutshell- Windows Calendar / SuperCalc data (.cal) - The .cal extension is also used by the old Windows 3.x Calendar applet and by SuperCalc spreadsheets - unrelated, non-image formats.
- CADAM / drawing exchange (.cal) - Some legacy CAD systems used .cal for related raster/drawing data; identify by the originating application.
Programs that open CAL files
Windows4 apps
IrfanView
Freemium Open the .cal to view; File > Save As to convert to TIFF/PNG/PDF. Free for personal use. 
ImageMagick
Open-source Command line: 'magick drawing.cal drawing.tif' (or .pdf/.png). Reads CALS Type 1 and batch-converts losslessly. 
XnView MP / XnView Classic
Freemium Open the .cal directly to view; use Tools > Batch Convert or File > Export to save as TIFF/PNG/PDF. Free for personal use.
AutoCAD / Autodesk raster tools
Paid Attach/insert the CALS raster as an underlay in a drawing (CAD raster image support). Use when working with the drawing in CAD. 
macOS2 apps
ImageMagick
Open-source 'magick drawing.cal drawing.pdf' to view/convert from the terminal (install via Homebrew). 
Linux2 apps
ImageMagick
Open-source 'magick drawing.cal drawing.tif' to convert; 'display drawing.cal' to view. Reads CALS Type 1.
GDAL (gdal_translate)
Open-source 'gdal_translate drawing.cal drawing.tif' - GDAL has a dedicated CALS Type 1 read/write driver. Technical details
deep spec| Compression | CCITT Group 4 (ITU-T T.6) two-dimensional run-length encoding - lossless, bitonal only |
| Color depth | 1 bit per pixel (bilevel black-and-white; no grayscale or color support) |
| Header structure | Fixed 2,048-byte ASCII block of MIL-STD-1840 keyword:value records, space-padded to exactly 2,048 bytes; compressed image data follows immediately |
| Key header fields | srcdocid: (document ID), rtype: (raster type), rorient: (image orientation), rpelcnt: (pixel width and height), rdensty: (resolution in DPI) |
| MIME type | image/cals; also recognized as image/x-cals and application/x-cals |
| File identification | No binary magic number; identified by ASCII keyword lines (e.g. srcdocid:, rorient:, version: MIL-STD-1840) within the opening 2,048 bytes |
| Type variants | Type I - single monochrome image (most common); Type II - one or more tiled monochrome images; both variants use CCITT Group 4 |
| Developer / origin | U.S. Department of Defense, under the CALS (Continuous Acquisition and Life-cycle Support) program |
| Governing standard | MIL-R-28002 / MIL-PRF-28002C; standard has been cancelled but format remains readable by current imaging tools |
| Alternative extensions | .cals, .gp4, .c4, .mil - identical binary structure, different naming conventions used across systems |
| Typical scan resolution | 200-400 DPI; optimized for B-, C-, D-, and E-size engineering drawings and scanned technical manuals |
| Typical file size | Tens of kilobytes to a few megabytes for a full-size engineering drawing |
| Platform compatibility | Cross-platform; no byte-order dependency since Group 4 is a bit-stream; supported on Windows, Unix/Linux, and macOS |
| Superseded by | Multi-page TIFF (Group 4) and PDF/A, which replaced .cal for long-term technical-document archiving and inter-agency exchange |
| Released | Late 1980s (U.S. DoD CALS initiative; MIL-STD-1840, MIL-R-28002 / MIL-PRF-28002) |
| Latest version | MIL-PRF-28002C (Type I/II); standard now deprecated/cancelled but format still read by imaging tools |
| Open standard | Yes · royalty-free |
| Specification | en.wikipedia.org |
CAL conversions
Community Q&A
asked by usersNo questions yet - be the first to ask about CAL files.
