What is the PGM file format?
.pgm (short for Portable Gray Map) is a file format associated with Netpbm, a cross-platform bitmap graphics toolkit.
What do PGM files contain?
PGM files store 8- or 16-bit grayscale images. A single .pgm file can store multiple such images. Each pixel can take a value from 0 (black) to 65 535 (white).
How is data saved in PGM files?
PGM files can save data in the following variants:
- plain text (file header with magic number
P2), human-readable ASCII encoding - binary (file header with magic number
P5), storing raw binary samples per pixel
Both variants support multiple concatenated images in a single file. The file header defines width, height, and the maximum gray value (Maxval). The body of the file contains sample values for each image pixel.
What are PGM files used for?
A .pgm file is an example of the .pnm (Portable Anymap) standard implementation. PGM files are chiefly used as an image interchange format in image processing, computer vision pipelines, and graphics toolchains. Besides PGM, other implementations in the same family are available:
Security & safety
RISK: LOWPGM is a passive, fully-documented raster image - header plus pixel data, no scripts or executable content, so opening one is safe. The only practical cautions are non-malware: files are large because PGM is uncompressed (convert to PNG to save space), and a malformed or oversized header could make a naive parser allocate a huge buffer, so use a maintained viewer. There is no metadata/EXIF privacy concern as the format carries none.
Format details
in a nutshellFULL NAMEPortable Graymap (Netpbm grayscale image)aka Portable Gray Map, Netpbm PGM
DEVELOPERJef Poskanzer (Pbmplus / Netpbm project)since 1988 (Pbmplus toolkit, later Netpbm)
CATEGORYRaster Image Files
MIME TYPEimage/x-portable-graymap
TYPEUncompressed Netpbm raster grayscale bitmap; ASCII (P2) or raw binary (P5) variant
STANDARDOpen · royalty-free
MAGIC BYTES · FILE SIGNATURE
OFFSET
0001
HEX
5032
ASCII
P2
The file begins with a two-byte magic number: "P2" (50 32) for the human-readable ASCII variant or "P5" (50 35) for the raw binary variant. After it come whitespace-separated ASCII fields - width, height and the maximum gray value (Maxval, e.g. 255) - then the pixel data. Siblings: P1/P4 = PBM (bitmap), P3/P6 = PPM (color RGB).
Programs that open PGM files
Windows4 apps
GIMP
Open-source File > Open the .pgm directly; GIMP reads P2 and P5, then export to PNG/JPG via File > Export As. 
ImageMagick
Open-source Command line: 'magick input.pgm output.png' to convert; 'magick identify input.pgm' to inspect. 
XnView MP
Free Open or browse PGM files; XnConvert (same maker) batch-converts folders of PGM to PNG/JPG. 
macOS3 apps
Preview
Built-in macOS Preview opens many PGM files directly by double-clicking; export via File > Export. 
Linux3 apps
Netpbm
Open-source The reference toolkit: 'pnmtopng', 'pgmtoppm', 'pnmtojpeg' convert PGM to other formats; 'display' shows it.
ImageMagick / GraphicsMagick
Open-source 'display in.pgm' to view; 'convert in.pgm out.png' to transcode. Technical details
deep spec| Format family | Netpbm PNM family, alongside .pbm (1-bit bitmap) and .ppm (RGB color) |
| Color model | Grayscale only - no color channels, no alpha channel |
| Color depth | 8-bit (Maxval 255, 256 shades of gray) or 16-bit (Maxval up to 65 535) |
| Magic number | "P2" (0x50 0x32) for the ASCII variant; "P5" (0x50 0x35) for the binary variant - located at offset 0 |
| MIME type | image/x-portable-graymap |
| Encoding variants | P2: human-readable plain ASCII pixel values; P5: raw binary pixel data - the header is always ASCII text in both variants |
| Compression | None - pixel samples are stored raw and uncompressed |
| Byte order | Big-endian for 16-bit samples (when Maxval > 255); 8-bit samples occupy a single byte each |
| Bytes per sample | 1 byte per pixel when Maxval ≤ 255; 2 big-endian bytes per pixel when Maxval > 255 |
| Header structure | Whitespace-separated ASCII fields in order: magic number, image width, image height, Maxval - followed by a single whitespace delimiter, then the pixel raster |
| Multi-image support | Multiple images may be concatenated sequentially within one .pgm file (both P2 and P5 variants) |
| Dimensions limit | No format-imposed maximum; bounded only by the ASCII integer range in the header and available memory |
| Typical file size | Uncompressed; scales as width × height × bytes-per-sample - ranging from kilobytes for small images to tens of megabytes for large ones |
| Developer | Jef Poskanzer, originating from the Pbmplus toolkit (later continued as the Netpbm project) |
| Platform support | Windows, macOS, Linux, Unix - natively handled by Netpbm tools and supported by most image processing libraries (OpenCV, ImageMagick, GIMP) |
| Released | 1988 (Pbmplus toolkit, later Netpbm) |
| Latest version | Stable since the 1990s; current Netpbm 10.86+ implements it (no format version bump) |
| Open standard | Yes · royalty-free |
| Specification | netpbm.sourceforge.net |
PGM conversions
Possible
.PGM .JPEGOnline converter
Yes -- open the PGM in IrfanView or GIMP and save as JPEG.
How to convert
.PGM .PNGOnline converter
Yes - open the PGM in IrfanView (Windows) or GIMP and save it as PNG; it takes under a minute.
How to convert
.PGM .PPMOnline converter
Yes - the fastest way is a single Netpbm command: pgmtoppm on Linux/macOS, or ImageMagick on any OS.
How to convert
.PGM .BMPOnline converter
Yes - open the PGM in IrfanView and use File > Save As > BMP; that takes about ten seconds.
How to convert
.PGM .PDFOnline converter
Yes - the fastest single-step method is ImageMagick: magick input.pgm output.pdf wraps the image in a…
How to convert
Community Q&A
asked by usersNo questions yet - be the first to ask about PGM files.
Frequently asked questions
What program opens a PGM file?
The free GIMP, IrfanView and XnView all open PGM (both the P2 ASCII and P5 binary variants) on Windows; GIMP and Preview work on macOS; GIMP, Netpbm and ImageMagick on Linux.
How do I convert a PGM to PNG or JPG?
Open it in IrfanView, XnConvert, GIMP or ImageMagick and Save As / Export to PNG or JPG. For many files at once, IrfanView's and XnConvert's batch mode, or ImageMagick's 'mogrify -format png *.pgm', convert a whole folder.
What's the difference between PGM, PPM and PBM?
They're the same Netpbm family: PBM is 1-bit black-and-white, PGM is grayscale, and PPM is full RGB color. PNM/PAM are umbrella names covering all three.
What's the difference between P2 and P5 PGM?
P2 stores pixels as human-readable ASCII numbers (you can open it in a text editor) - larger but inspectable. P5 stores the same pixels as raw bytes - smaller and faster. Both are valid PGM.
Why is my PGM file so large?
PGM uses no compression at all - it stores every pixel in full. Converting to PNG (lossless) or JPG (lossy) shrinks it dramatically with no or little quality change.
Can I open a PGM in a text editor?
Only the P2 (ASCII) variant meaningfully - you'll see the header and gray-value numbers, not an image. P5 (binary) PGMs are raw bytes and look like gibberish in a text editor; use an image viewer instead.
References
1Netpbm - PGM format specificationnetpbm.sourceforge.net
2Netpbm project homenetpbm.sourceforge.net
