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To check the running Windows installation, open Command Prompt and run compact.exe /CompactOS:query. Windows reports whether Compact OS is enabled. To check an offline installation, specify its Windows directory, for example: compact.exe /CompactOS:query /WinDir:E:Windows. Replace that path with the actual Windows directory. This checks status; it does not change it.
Check Compact OS on the running installation
- Open Command Prompt.
- Run
compact.exe /CompactOS:query. - Read the result to see whether the current Windows installation is in the Compact OS state.
Microsoft documents /CompactOs as an option that “Sets or queries the system’s compression state.” The query option checks the state; always and never change it, so do not use those options for a status check. See Microsoft’s compact command reference.
Check an offline Windows installation
When examining an installation that is not currently running, such as from Windows PE, give the command the path to that installation’s Windows directory:
compact.exe /CompactOS:query /WinDir:E:Windows
Substitute the correct directory for E:Windows. Microsoft documents /WinDir for querying an offline operating system and provides an offline query example in its Compact OS documentation.
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Measure compression information for a selected path
Compact OS runs operating system files in compressed form. Microsoft describes it as installing “the operating system files as compressed files.” The general compact command can report compression information for a directory or files you specify, but that file-level report is not an official, complete Compact OS savings total for the whole Windows installation.
For example, to report compression information for a chosen directory and its subdirectories, use:
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compact /S:C:PathToFolder
Use the real directory path in place of the example. The /S option includes subdirectories. Add /A when you also want hidden and system files considered:
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These options affect the files included in the report. For syntax and option details, consult the Microsoft command reference.
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How much space does Compact OS save?
There is no universal gigabyte figure or standard whole-system before-and-after measurement procedure established by the cited Microsoft documentation. The space difference depends on the particular installation and on what is counted. Treat a local compression report as an estimate for its stated scope, not as the total space Compact OS saved across Windows.
For a useful, reproducible local figure, record the Windows version, the exact paths included, whether hidden and system files were included, and the reporting method. A before-and-after observation should also identify the same installation and scope at both points; otherwise, the figures are not directly comparable. The status query and a file compression report answer different questions: one checks Compact OS state, while the other reports on selected files.
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What the status result does—and does not—tell you
A query result tells you the compression state of the installation you checked. It does not by itself establish the amount of disk space recovered or predict performance on a particular PC. Microsoft notes that performance depends on the relative compute and storage performance of the device; a status result alone cannot determine the effect for your hardware. See Microsoft’s Compact OS overview.
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