Free online ISO to CSO converter

People convert disc images to CSO mainly to save space on a PSP memory card or in a PSP emulator library. The conversion keeps the image data intact while changing how it is stored, but the target device must support CSO, and highly compressed images can load more slowly.

What the ISO format is

ISO is a single-file image of an optical disc, preserving the disc’s sectors, filesystem, and file layout. CD and DVD imaging tools create ISO files for game dumps, software installation media, bootable recovery discs, and optical-media archives; desktop operating systems can often mount them as virtual discs.

An ISO can also be extracted, written to a physical disc, or opened by compatible emulators. Its size reflects the source image, including sectors that contain little or no useful file data. ISO is a broad disc-image format, not a format limited to games or PSP software.

What the CSO format is

CSO, also called CISO, is a compressed ISO image format associated mainly with Sony PlayStation Portable software and PSP emulators. It divides the source image into blocks, compresses those blocks independently, and stores an index so supported software can read the image without fully expanding it first.

CSO can be substantially smaller than its ISO source, especially when the image contains unused or repetitive sectors. The compression is normally lossless: it changes the storage representation, not the files and sectors represented by the image. CSO is not a general replacement for ISO, and ordinary disc-mounting tools usually do not mount it as an optical disc.

How to convert an ISO to CSO

Using maxcso

maxcso is a current command-line converter with builds for Windows, macOS, and Linux. Keep the original ISO until the resulting CSO has been tested.

1. Download the appropriate maxcso build and place it in a convenient folder, or add it to your system path.
2. Put the ISO in an accessible folder and open Command Prompt, PowerShell, or a terminal there.
3. Run maxcso "Game.iso" -o "Game.cso", replacing the filenames as necessary. Quotation marks preserve spaces in filenames.
4. Wait for the conversion to finish, then confirm that the output file exists and has a plausible size.
5. Copy the CSO to the PSP location or emulator library and test it. If it fails to boot or has read-related problems, retain the ISO and try a less aggressive compression setting supported by your build, using maxcso --help to view available options.

Compression time and the final size depend on the image contents. The output extension normally selects the CSO format; if a particular build requires an explicit format option, follow that build’s help text.

Using 101convert.com

You can convert the file for free on 101convert.com without installing software: open its converter, upload the ISO, select CSO if a format selector appears, start the conversion, and download the result. Large disc images require sufficient upload and download time; do not upload images containing private or sensitive data unless the service’s handling policy is acceptable to you.

Compatibility and quality limits

CSO support depends on the PSP firmware, emulator, or other application. A PSP does not universally require CSO; compatible systems may also use ISO, and ISO generally offers the broadest support and avoids decompression during reads.

Some games or homebrew applications load more slowly from CSO, particularly at high compression levels or from slower storage. If gameplay stutters, loading becomes excessive, read errors appear, or the image fails to boot, test a lower compression level or use the original ISO.

Compression cannot repair a damaged ISO or restore missing sectors. If the source is corrupt, the resulting CSO can also be unusable. Verify that the target software recognizes the output and that the required content works before deleting or archiving the ISO.