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

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.