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Archive format

ISO converter

An .iso file is a sector-by-sector image of an optical disc's filesystem, almost always ISO 9660, often augmented with UDF on DVDs and Blu-rays. It is less an archive than a byte-exact snapshot: mount it or burn it and the original disc is back.

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Batch files can each use a different output. Nothing uploads for local conversions.

Working inputs include camera RAW, browser-local audio/video, PDF, CBZ/CBR comics, office documents, ebooks, markup, 3D models, structured text, images, and archives.

Where ISO comes from

ISO 9660 was standardized in 1988, formalizing the High Sierra format the CD-ROM industry had drafted in 1986. Its restrictive 8.3 uppercase filenames were later relaxed by Microsoft's Joliet extension, which added Unicode long names, and by Rock Ridge, which layered in POSIX permissions and deep directory trees.

How ISO works

  • Data is organized in 2,048-byte logical sectors mirroring the CD-ROM Mode 1 layout.
  • Base Level 1 allows only 8.3 uppercase names; Joliet stores a parallel UCS-2 directory tree while Rock Ridge tucks POSIX metadata into extension records.
  • The El Torito extension embeds boot images, which is how installer ISOs boot; modern OS images are often isohybrid, valid as both optical and USB layouts.
  • There is no compression whatsoever — an image is as large as its data plus filesystem overhead.

When to use ISO

  • Operating system installation media for Windows and Linux.
  • Bit-exact archiving of CDs and DVDs before the discs degrade.
  • Distributing bootable rescue and diagnostic tools.
  • Attaching installation media to virtual machines.

Strengths and limitations

Strengths

  • Mounts as a virtual drive natively on Windows 8+, macOS, and Linux.
  • Preserves bootability and exact on-disc layout.
  • The universal standard for disc imaging across three decades.
  • Easy to verify end to end with a single checksum.

Limitations

  • Zero compression, so images are large.
  • The layered naming extensions can show different filenames on different operating systems.
  • Read-only by design — changing contents means rebuilding the image.

Compatibility

Windows 8 and later mount ISOs straight from File Explorer, macOS uses DiskImageMounter, and Linux loop-mounts them with one command. Tools built on libarchive can also list and extract the contents without mounting, which is how browser-side extraction works here.

Per-file timestamps survive, Rock Ridge adds POSIX ownership and permissions, and the volume descriptor carries a label plus publisher and preparer fields.

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