Windows has a compression mode built in that Microsoft never mentions, and it gave me 38 GB back on my C: drive

Published Oct 2, 2026, 9:00 AM EDT Abhinav pivoted from a career in banking to pursue his first love in writing. Even while working full-time, he continued contributing as an editor-at-large, a role he has held for more than 7 years. A lifelong tech enthusiast who has built three gaming and productivity powerhouse PCs since 2018, his passion for technology keeps him closely following the semiconductor industry, from NVIDIA and AMD to ARM. His MSc dissertation explored how artificial intelligence will reshape the future of work, reflecting his curiosity about the wider social impact of emerging technologies. Everyone wants just a little bit of more room on their drive, no matter whether it's a multi-terabyte Gen 5 NVMe or a 500GB SATA SSD. And when buying more isn't an option, the next best thing is keeping the drive you have as lean as possible by clearing space for apps and services that you use. A lot of third-party utilities offer to deep-clean your PC and reclaim gigabytes of storage, and sure enough, maybe some of them will do just that after they have your credit card details and your email address. But, if you're on Windows 11, there's a tool that can do it that's already on your machine, and Microsoft has barely mentioned it. Here's how the OS's built-in compression mode works, and what it gave me back. Windows has a compression mode it never puts in front of you It can run the whole OS from compressed files and save you some storage space The first thing you need to know about Compact OS is that it isn't a cleanup trick or a cache clearing method that you must perform once a month. It is a mode that stores Windows' own system files as compressed files and runs the OS directly from them, so the footprint of Windows itself shrinks on your drive. Microsoft has created this for devices with smaller SSDs where every gigabyte saved on the C: drive counts, but anyone with any SSD can stand to save some storage space with this method. The way this works is quite simple to understand. After compression is applied through a simple command line script, Windows decompresses each file as it is read, so all your apps, games, and services behave exactly as they did before. One thing worth knowing is that, as with all forms of compression and decompression, it costs CPU cycles. Every time a compressed file is loaded, your CPU must decompress it, which is why Microsoft reserves this feature for machines that need space rather than turning it on as a default. On anything with a modern processor and a fast NVMe SSD though, you'll never feel it. What's perhaps even more interesting is that the same tool isn't limited to Windows itself. Compact OS is one mode of the built-in compact command, and you can point that same command at a folder of your own (such as a game installation or app directory) to compress it the same way. How to use Windows 11 compression mode? Turning it on takes one command, and about 5 minutes Everything regarding this approach happens in an elevated terminal, so you can open PowerShell or Command Prompt as an administrator to get started. Before you drop in the commands, it's always worth checking what state your OS is in, which can be done with the following command: compact /compactos:query On my two machines (including my desktop with a PCIe Gen 4 NVMe SSD and my Lenovo Legion 5 Pro), this came back stating the system wasn't in the Compact state because Windows had decided it wasn't beneficial, which is the default on any rig with room to spare. That's just how the feature works when Windows looks at your drive and decides you don't need the additional space. You can override that judgment though. I'd recommend checking your free space before and after, so you can easily tell how much space the process freed up for you. You can do so through the following: Get-PSDrive -Name C This should give you a screenshot of your C: drive, detailing the free and used storage. With that figure in mind you're ready to apply compression through the following command: compact /compactos:always Now, all you have to do is wait. There won't be a progress bar, and the window will sit on "Compressing OS binaries" for a while. On my Legion 5 Pro laptop, the process was completed in five minutes, and when it finished, it printed a summary with compression ratio and the number of files it had compressed. You can run the Get-PSDrive -Name C command a second time, and the difference is the space you just reclaimed. I tried this method twice, once on the Legion 5 Pro and once on my desktop. The space freed up through Compact OS alone was nearly 7GB on the laptop and 5GB on the desktop. But you can go further than that, and that's exactly how I got nearly 40GB back on the laptop. How I reclaimed 38GB through compression All it took was pointing at the right folder Compact OS by itself only touches Windows' own files, so once that was done, out of sheer curiosity, I pointed the same compact command at a folder of my choosing instead, which was the Users folder on the laptop. compact /c /s:"C:\Users" /exe:lzx /i LZX is the strongest of the available compression algorithms, /s applies it to every file in the folder, and /i tells it to carry on past anything it can't touch rather than stopping. This approach is nowhere near as quick as Compact OS, and with thousands of files to check, the whole thing took almost thirty minutes. When it finished, the summary stated it had gone through 460,000 files and bought 190GB of data down to 158GB, which is a saving of around 31.5GB. Cumulatively with the 7GB that Compact OS had already reclaimed, my laptop had 38 GB more free storage than when I started earlier that evening. I've experimented enough to know where it doesn't work as well, though. I also tried pointing it at my Xbox Games folder, hoping to free up some space, but almost nothing came back. This is likely because games ship their assets inside archives that are already compressed. Are there any trade-offs? And is the process reversible? These were the two questions I found myself immediately asking before attempting to do this on my main desktop as well. The answer to both questions is yes. Every compressed file costs CPU cycles to open, which is why it's turned off by default on most machines. On a Ryzen 7000 series processor and a Gen 4 NVMe, it wouldn't impact me, and if you're on similar hardware, it won't impact you either. As for reversing it, it's just as simple and straightforward. You can run the following command to undo Compact OS: compact /compactos:never And the following command to do it for any folder you compressed yourself: compact /u /s: "C:\[FOLDER NAME]" /exe /i For all of these reasons, it's worth a try if you're looking to save space on a storage-constrained device. Since the way it works is such that you never have to delete anything and get to free up a decent chunk of storage, it's one of my favorite methods to clean up a drive.