How Much Free Space Should You Leave on Your Windows PC?

The 15% Rule of Thumb for Mechanical Hard Drives

You’ll commonly see a recommendation that you should leave 15% to 20% of a drive empty. That’s because, traditionally, you needed at least 15% free space on a drive so Windows could defragment it.
If you don’t have 15% free space, Windows won’t be able to properly defragment the drive. Windows will only partially defragment the drive, and it will grow increasingly fragmented over time. However, this just applies to mechanical hard drives that need defragmentation, and not the solid-state drives generally found in more modern PCs.

The 25% Rule of Thumb for SSDs Is Probably Too Conservative

Solid-state drives traditionally needed a large chunk of available free space, too. They slow down over time as they’re filled up. In 2012, Anandtech recommended leaving 25% of a solid state drive empty to avoid a decrease in performance based on their testing.
However, modern solid state drives are “overprovisioned”. This overprovisioning actually means the solid state drive has more memory than it exposes to you. So, even if you fill a solid state drive near full, there’s still a bunch of spare memory on the drive to help maintain performance. That 25% figure is likely too conservative on a modern solid-state drive, although it depends on how overprisioned the drive is. You can afford to use more of the drive and fill it up with more data.

The Answer: It Depends

There’s no specific number or percentage that fits every Windows PC. All Microsoft will tell you is that you need 20 GB of space before you install a 64-bit Windows 10 system on a modern PC. After that, you’re on your own.
The rules of thumb can help. If you have a mechanical hard drive, leaving at least 15% of it empty can reduce fragmentation in newly created files and make it easier for Windows to properly defragment the drive, which is something modern versions of Windows do automatically in the background. If you don’t leave enough empty space, Windows won’t be able to move files around to defragment them and the contents of the drive will become fragmented and slower to access over time. If you have an SSD, this doesn’t apply.
If you have an SSD, leaving at least 25% of the SSD empty will ensure you have excellent performance. On modern SSDs with overprovisioning, this is probably much too conservative, and even 10% could be an okay number. It really depends on the SSD.
If you need to temporarily fill your drives up and only have 5% of disk space to spare, that’s not a problem. Things will just slow down over time, so you’ll probably want to free up some space when you can.

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