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compact /compactos:always tells Windows to store operating-system files in compressed form, which can reduce their storage footprint. It does not guarantee a particular amount of free space or a speed increase: reading compressed files adds decompression work for the CPU, and the net effect depends on your storage, processor and workload. Check the current state first, and measure the tasks you care about before deciding whether to keep it enabled.
What CompactOS changes
CompactOS compresses Windows operating-system files in place to reduce the space they occupy. When Windows reads a compressed file, it must decompress the data, adding CPU work. In return, the system may need to read fewer bytes from storage. Whether that trade-off helps depends on the balance between storage and compute performance, as well as the workload.
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Microsoft’s guidance is specific: a system with a fast CPU and slower storage may benefit from reduced reads, while another system may behave differently. Microsoft does not give a universal performance percentage or promise that every recent PC will avoid a downside. The relevant question is how your own system performs with the tasks you use.
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Check, enable or disable CompactOS
Open an elevated Command Prompt or Windows Terminal session: search for the app, choose Run as administrator, and approve the User Account Control prompt. Then use the relevant command:
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| Action | Command | What it does |
|---|---|---|
| Check status | compact /compactos:query |
Reports whether CompactOS is enabled or disabled. |
| Enable | compact /compactos:always |
Enables CompactOS. |
| Disable | compact /compactos:never |
Disables CompactOS. |
Microsoft’s runtime how-to describes these instructions for Windows IoT Enterprise. Its command reference lists Windows 10 and Windows 11 as applicable systems. That scope matters: the commands are documented for those systems, but the runtime how-to is not a blanket instruction for every Windows edition or configuration. The commands above control CompactOS; they are distinct from the compact utility’s other file-compression options, including /EXE.
How to benchmark the trade-off on your PC
A useful comparison tests the same PC, Windows installation and workload with CompactOS disabled and enabled. Record the Windows edition and build, processor, memory and storage device, and note whether the test is before-and-after or enabled-versus-disabled. Keep other conditions as consistent as practical, repeat measurements, and report the spread as well as any average. A single run can be affected by background activity and is not enough to establish a reliable difference.
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- Record the starting state. Run
compact /compactos:queryin an elevated command window. Note the result and available storage before changing anything. - Choose workloads that matter to you. Measure boot and shutdown time, app launch time, disk I/O throughput and CPU use during reads. If update installation time is a concern, measure it as its own workload rather than assuming that a result for one task applies to updates.
- Capture a baseline. Run the selected workloads while CompactOS is in its current state. Use the same apps, files and measurement method in each state.
- Change state and repeat. Use the enable or disable command, verify the reported state with
compact /compactos:query, then repeat the same workloads and measurements. - Compare like with like. Look for consistent changes across repeated runs, including CPU use and disk activity—not only elapsed time. If results vary substantially, the test does not support a confident claim of a meaningful difference.
Microsoft identifies the Windows Assessment and Deployment Kit and DiskSpd as tools to consider when evaluating performance. Tool choice and test design should match the question: a disk-throughput result alone does not establish how an app launch or update will behave.
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There is no reliable fixed amount to promise for a current installation. The result depends on the device and its Windows installation. Microsoft’s Windows Storage and Deployment Teams reported approximately 1.5 GB returned on 32-bit Windows 10 and 2.6 GB on 64-bit Windows 10 in a 2015 post. Those are historical Windows 10 figures, not a forecast or guarantee for a current PC.
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The same 2015 post discussed recovery-image size separately, giving a typical range of 4 GB to 12 GB depending on make and model. That figure describes recovery-image storage, not space saved by CompactOS; system-file compression and recovery-image changes are separate footprint-reduction approaches.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the results can—and cannot—show
If your measurements show faster reads or launches with CompactOS enabled, that is evidence about those workloads on that particular system, not proof that every task will improve. If CPU use rises or a workload slows, that reflects the added decompression work in those conditions; it does not establish a universal penalty. Keep the Windows build and hardware details with any reported result so readers can judge whether it applies to their own setup.
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