I had 12 GB free on a 256 GB SSD and no idea where the space went. I'd run Disk Cleanup, deleted old downloads, uninstalled games I never played. Nothing made a real dent. Then someone told me about hiberfil.sys — a hidden file at the root of my C: drive that was taking up 9 GB. I had never seen it, because Windows marks it as a protected system file.

That file is Windows hibernation. Every PC with hibernation enabled reserves space equal to roughly 40% of your installed RAM, on disk, at all times. On a 16 GB machine that's about 6 GB. On a 32 GB machine it's closer to 13 GB. It's there whether you use hibernation or not — and most people don't.

Here's how to get it back.

Check how much you'd reclaim first

Before you delete anything, see what's actually being used. Open Command Prompt and run:

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powercfg /a

This lists every sleep state your PC supports. Look for a line that says Hibernate — if it's not listed, hibernation is already off and this article won't help you. If it is listed, keep going.

To see the actual file size, open File Explorer, go to C:\, and enable "Hidden items" and "Protected operating system files" in the View tab. You'll see hiberfil.sys sitting there with a size next to it. That's your potential savings.

The command

Open Command Prompt as Administrator and run:

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powercfg -h off

That's it. The file is deleted immediately and the space is freed. No restart required, no prompt asking you to confirm. On my machine it reclaimed 9 GB in under a second.

This is a Windows feature, not a hack. Microsoft built it. It's safe.

What you lose

Hibernation and Fast Startup. If you're not sure what those are:

Hibernation writes your open apps and RAM contents to disk, then fully powers off. When you boot, everything is restored exactly as you left it. Useful on laptops that sit in a bag for days.

Fast Startup is a hybrid version of this that makes "shut down" and "start up" faster on some machines. It's the reason some PCs boot in 8 seconds. Disabling it may add a few seconds to boot time — usually 2–5 seconds, sometimes nothing at all, depending on your SSD.

If you have a fast SSD (which most people do now), you won't notice the difference. If you're on an older spinning hard drive and rely on hibernation to keep your work between sessions, think twice before running this.

How to turn it back on

If you decide you want hibernation back, it's the same command with a different flag:

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powercfg -h on

The file gets recreated, using the same amount of space, and everything works as before. There's no permanent change — you can toggle this as often as you like.

The trade-off nobody mentions

For laptops that stay plugged in all day, this is a clear win. You get 4–16 GB back, boot time goes up by maybe three seconds, and you never notice.

For laptops that genuinely hibernate — closed in a bag for a day, reopened, expecting your tabs to still be there — this changes how you use the machine. Your laptop will sleep instead, and sleep drains battery slowly. If you rely on hibernation, keep it.

A reasonable middle ground some people use: leave hibernation off during normal use, turn it back on before a long trip. It's a single command either way.

Why this is worth knowing

This is the kind of thing that never appears in "how to free up disk space" guides because it isn't a folder or a program — it's a system file Windows hides from you by default. Disk Cleanup won't show it. The Storage settings page in Windows 11 doesn't list it clearly. It just sits there, eating space, until you know to look.

If you've ever wondered why a fresh Windows install already shows 30+ GB used before you've installed anything, this is part of the answer. There's more to it than just hibernation, but this is the single largest chunk most people can reclaim with one command.