In a working environment you will need a minimum of 25% free space to allow your clients to work from the disk, and to keep the file system healthy. As I am sure you are aware, a 1TB drive contains exactly 1,000,000,000,000 bytes. This is what the "terra" means, 10^12.

However, a computer doesn’t see the world in powers of ten, but in powers of two. 2^10 bytes for many years was called a kilobyte, even if it doesn’t match the 1000 that “kilo” means in other aspects of measuring. Therefore, 1024 has now gotten a new name, kibi, to avoid the confusion - so kibibyte means 1024 bytes. Likewise, 2^20 (1024kib) has been named the mibibyte, and is slightly more than a megabyte (because megabyte literally translates to “million bytes”). This leads us to the gibibytes- 1,024mibibytes, or 1,073,741,824 bytes, this is what the computer uses internally when addressing the drive. However, to add to the confusion again, every OS displays this as GB (not GiB).

In practice, 1TB (1,000,000,000,000 bytes) is approximately 931GiB (999,653,638,144 bytes). Your computer will then show it as 931GB. Because 1TB sounds larger than 931GB and it actually has a trillion bytes, it is sold as 1TB. All of the above said, we suggest leaving roughly 230-250GB free on a 1TB NAS.