What is the ARC file format?
.arc is a compressed archive file format created by System Enhancement Associates (SEA) in 1985. Compression reduces the size of the archive - which may contain one or more files or folders - to facilitate sharing of files with other users.
The .arc format uses a lossless compression algorithm to store data: an .arc archive reduces the overall size of the files and folders it contains without any loss of file information. Each member file within the archive is introduced by the byte 0x1A followed by a one-byte compression-method code, which signals how that particular entry was compressed.
The ARC compression method was introduced in 1985 and quickly reached its peak of popularity. It was a time of dynamic growth in dial-up networking, and many modem users welcomed the .arc format as a very convenient way to share data. This format was especially popular among Bulletin Board System (BBS) users. Internet connection speeds at that time were rather slow, and .arc files offered smaller file sizes and thus required less bandwidth when uploading or downloading. Nowadays the ARC algorithm is rarely used, having been superseded by more advanced formats such as ZIP archives, .arj, and .lzh.
It is worth noting that before .arc was introduced, archiving and compression of data were two separate processes. The introduction of the .arc format joined those two operations into a single step - a considerable innovation at the time. This was achieved by combining the functions of two separate programs: SQ for compression and LU for archiving (library management).
Security & safety
RISK: MEDIUMThe ARC format itself is inert data, but like any archive it can hide executables or scripts until extracted, so scan the contents of an .arc from an untrusted source before running anything. One extra caution specific to ARC: it is an ancient format, so a surprising .arc may actually be a game data file or a deliberately obscure container rather than something you need to "open" - identify the source first. Avoid sketchy "ARC opener" downloads and online converters that push installers - stick to PeaZip/IZArc/The Unarchiver from their official sites.
Format details
in a nutshell- FreeArc archive - Unrelated high-ratio archiver by Bulat Ziganshin (circa 2006) that also uses .arc; its file starts with ASCII 'ArC' + a version byte and development has since stopped.
- Internet Archive ARC (web crawl) - Container storing many crawled web resources in one file; the text-header predecessor of WARC, unrelated to the SEA archive.
- Game data archive - Many games (Capcom, Nintendo tools and others) ship a proprietary .arc data container that only their modding tools understand.
Programs that open ARC files
Windows3 apps
IZArc
Free Open the .arc and Extract. Long-standing free archiver (by Ivan Zahariev) that handles legacy ARC well. 
PeaZip
Open-source Right-click the .arc > PeaZip > Extract. Free; has the broadest legacy-ARC support of the mainstream tools. 
macOS2 apps
The Unarchiver
Free Double-click the .arc; The Unarchiver handles many legacy formats including classic ARC. Free on the Mac App Store. 
Linux2 apps
nomarch / arc (CLI)
Open-source Use 'nomarch' or the classic 'arc' package to extract SEA-format .arc archives from the terminal. Technical details
deep spec| Developer | System Enhancement Associates (SEA); created by Thom Henderson |
| Magic bytes | 0x1A at offset 0 followed by a one-byte compression-method code; each member entry repeats this 0x1A+method header pattern |
| MIME type | application/x-arc |
| Compression methods | Multiple lossless methods per entry: uncompressed (methods 1-2), packed/RLE (3), squeezed/Huffman (4), crunched/LZW variants (5-8), squashed (9) |
| Member filename limit | Up to 12 characters; stored as a null-padded 13-byte field - reflects legacy DOS 8.3 filename conventions |
| Integrity check | CRC-16 checksum stored per member entry header |
| File structure | Sequential flat list of member entries; each header contains filename, compressed size, original size, date/time stamp and CRC-16, followed by compressed data |
| Date/time stamp | Packed DOS date/time word per member entry; 2-second modification-time resolution |
| Origin platform | Designed for MS-DOS and CP/M; binary format optimized for low-bandwidth dial-up file transfer |
| Historical significance | First widely adopted tool to combine file compression and archiving into a single operation, replacing the separate SQ+LU two-step workflow |
| Peak usage era | Mid-1980s to early 1990s; dominant archive format on Bulletin Board Systems (BBS) before ZIP became standard |
| Displaced by | ZIP (1989), ARJ, LHA/LZH, and later RAR and 7-Zip; longest retained in BBS preservation and retro-computing communities |
| Modern extraction tools | nomarch and arc CLI on Linux; PeaZip and IZArc on Windows; The Unarchiver on macOS |
| Encoding | Binary; no text-based or ASCII header section |
| Released | 1985 (SEA ARC, the BBS-era archiver) |
| Open standard | Yes · royalty-free |
| Specification | fileformats.archiveteam.org |
ARC conversions
Community Q&A
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