The file format consists of an 8 byte header, followed by any number of icons.

HeaderEdit

Icon dataEdit

Icon typesEdit

• ^1. The value inside the parenthesis is the uncompressed length for ARGB and 24-bit RGB icons.
• ^2. it32 data always starts with a header of four zero-bytes (tested all icns files in macOS 10.15.7 and macOS 11). Usage unknown, the four zero-bytes can be any value and are quietly ignored.
• ^†. These formats are supported in standalone icns files but do not display properly if used as application icon inside a .app package.

Image data formatEdit

• Mono icons with alpha mask can display three colors: white, black, and transparent.
• The 4-bit an 8-bit icons use a fixed color palette with 128 colors and 256 colors respectively.
• The 24-bit RGB format consists of the three compressed channels tightly packed (see Compression). The it32 icon must start with a four-byte header, see footnote above.
• The ARGB format consists of the ascii values for 'ARGB' and the four compressed channels tightly packed (see Compression).

CompatibilityEdit

• the ARGB fields also accept files in PNG format – but not vice versa, you can not put ARGB images in any of the PNG-only fields (tested on macOS 11).
• ARGB images are only supported in macOS 11 and newer – macOS 10.15.7 does not display ARGB images. Yet, even the ARGB keys can be displayed on macOS 10.15 if you set a JPEG 2000 or PNG image (see footnote on usage in app packages above).
• The 24-bit RGB icons (is32, il32, ih32, it32) also allow images in JPEG 2000 and PNG format (tested on macOS 10.15.7 and macOS 11).
• The support for newer image types seems to be introduced later than the key field (see previous two points). Therefore, the supported OS version may not be accurate or adjusted based on file format.

Other typesEdit

• The table of contents is a list of all contained types (4 byte type-name + 4 byte length).
• The data for all nested icns files does not contain the icns file-header. So, if you want to save the data to a file you have to prepend the icns header.

Over time the format has been improved and there is support for compression of some parts of the pixel data. The 24-bit RGB (is32, il32, ih32, it32, icp4, icp5) and ARGB (ic04, ic05, icsb) pixel data are compressed (per channel) with a format similar to PackBits.^[5] Some sources mention that the OS supports both compressed or uncompressed data chunks.^{[citation needed]} However, manually crafting icns files with uncompressed 24-bit RGB or ARGB images will not display properly – at least on newer macOS releases (tested on macOS 11).

The following pseudocode decompresses the data:

While there's compressed data:
    Read one byte as an unsigned number N
    If N < 0x80:
        Output the next (N + 1) bytes
    Else:
        Output the next byte (N - 0x80 + 3) times

Example: 02 01 02 02 80 03 81 04 82 05 should decompress to 01 02 02 03 03 03 04 04 04 04 05 05 05 05 05

As of macOS 11, there are certain issues / bugs with the file format:

1. Setting is32+ics8 or ih32+ich8 will display a proper icon. But setting il32+icl8 ignores the transparency mask and displays an icon without transparency.
2. Compressed ARGB data is not interpreted correctly. The last value of the blue channel (aka. the very last value) is ignored and treated as if it were all zero-bytes. Usually this is no issue since most icons will have transparency at the bottom right corner anyway. However, it can become an issue if the last value is a repeating byte (see Compression). Potentially, up to 130 pixels can lack the blue channel value.
   A workaround is to append an additional byte at the end which is interpreted as a control character without following data. You can compare the difference with these two examples:
   • 69636E73 00000024 69633034 0000001C 41524742 FFFFFBFF FF00FB00 FF00FB00 FFFFFBFF
   • 69636E73 00000025 69633034 0000001D 41524742 FFFFFBFF FF00FB00 FF00FB00 FFFFFBFF 00
3. macOS 10.15.7 (likely earlier) and later versions have an issue displaying PNG and JPEG 2000 icons for the keys icp4 (16x16), icp5 (32x32), and icp6 (64x64). The keys work fine in a standalone icns file but if used in an application, the icons are displayed completely scrambled. Either use the new ARGB format ic04 and ic05 (macOS 11+) or the old 24-bit RGB + alpha mask format. Use the latter with the old keys is32+s8mk and il32+l8mk, or with the newer keys icp4+s8mk and icp5+l8mk (writing RGB data into PNG fields^[2]). If using ARGB image data, make sure to provide alternative formats for macOS 10.15 and earlier. This issue is especially tricky to detect if you provide both, 16x16 and 16x16@2x icons, because if you connect your Mac to a non-retina monitor, the non-retina 16x16 icon will be used and thus the icon will be displayed scrambled. The icp6 field does not seem to be used in application icons and can safely be ignored. Additionally, if you don't provide the smaller icon sizes at all the bug will also manifest when the OS scales down your larger PNG/JPEG 2000 icons, so make sure to render smaller sizes and include them.

Various image viewers can load *.icns files, and free and open source converters from or to PNG also exist.^[6][7] GTK+ can load *.icns resources since 2007.^[8] Other tools supporting the format include the Apple Icon Composer and icns Browser, The Iconfactory, and IconBuilder.
MacOS^{[clarification needed]} offers the built-in iconutil command line tool to pack and unpack *.icns files.

• ICO format on Windows
• X PixMap format for X11

• IconFamily (last update 2013) – Open source Objective C class to read and write Apple icns files
• osxiconutils (not maintained) – Command line tools to work with Apple icns files
• icnsutil – Python library to read and write icns files