15% is the value that can be found on some Microsoft articles, this value is probably from the Windows XP era.
In a search found this, more realistic in my opinion...
Free hard disk space
Drives without any free disk space cannot be defragmented. For the best possible results, at least 5% of the whole hard disk capacity should be free. Files larger than the maximum free and connected storage space, may not be possible to defragment, and if so only with several runs.
To accelerate defragmentation, we recommend reserving at least 10-15% free hard disk space, however, this percentage decreases the higher the hard disk size is. What really matters is the size of the largest file that needs to be defragmented or moved around. For an optimal defragmentation, the file should have enough connected storage space available.
Edited by SleepyDude, 31 May 2017 - 09:42 AM.
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