Free online CISO to ISO converter

Some PSP tools, patchers, archive workflows, and older loaders accept only an uncompressed .iso image, even when the source game is stored as a compressed CISO/CSO file. Decompression restores the disc image so the target software can read its sectors directly.

What CISO and CSO files are

CISO is a block-compressed disc-image format, most commonly associated with Sony PSP UMD game dumps. Its usual filename extension is .cso; in this context, CSO is normally the extension for a CISO-format image, not a separate PSP image format.

A CISO image stores an index plus individually compressed data blocks, usually compressed with zlib/DEFLATE. This reduces storage and transfer size while preserving the original image data losslessly. PSP emulators such as PPSSPP generally open both .cso and .iso files, so conversion is unnecessary unless the intended program specifically rejects CSO.

The term “CISO” and the .ciso extension are not universally unique. If a tool reports an invalid header or unsupported format, confirm the file's source and format instead of assuming every file named .ciso is a PSP CSO image.

What an ISO file provides

An ISO image is an uncompressed disc-image byte stream commonly stored with the .iso extension. PSP ISO images normally contain the UMD filesystem and game data, including entries such as PSP_GAME and UMD_DATA.BIN.

ISO files consume more disk space than their compressed CSO equivalents, but they work with software that cannot decode compressed blocks. They are commonly required by image editors, some patching utilities, checksum workflows, and loaders that expect an uncompressed UMD image.

Convert the image

On 101convert.com, upload the CISO/CSO file, select ISO as the output format, run the conversion, and save the resulting .iso. Keep the original file until the ISO opens successfully in the program that required it.

For local conversion, use a desktop CISO/CSO decompression utility that explicitly supports the source format. In many builds of the classic ciso command-line utility, decompression uses:

ciso 0 game.cso game.iso

The 0 mode conventionally selects decompression, but command syntax differs among utility builds. Run ciso --help or check the program documentation before processing files. A tool that supports only PSP CSO cannot necessarily decode other files called CISO.

Do not merely rename game.cso to game.iso; the file remains compressed and unsupported software will still fail to read it. UMDGen is mainly useful for inspecting or editing PSP images; a dedicated decompressor is preferable for a direct, byte-preserving expansion.

Compatibility and verification

CISO-to-ISO decompression is lossless when the source image is valid: it restores the original uncompressed image data and does not improve graphics, repair corruption, or alter the game. Reserve enough free storage for the ISO, which may be close to the original UMD image size, plus any temporary space used by the converter.

Decompression errors, invalid block offsets, or a game that fails after conversion usually indicate a damaged download, an incomplete source file, or a CISO variant unsupported by the selected tool. If a known checksum exists for the original ISO, compare its SHA-1, SHA-256, or MD5 value with the converted file's checksum. Test the ISO in the exact loader, emulator, patcher, or utility that required the conversion.