Microsoft and memory RAM manufacturers calculate 1 Gigabyte as 1,073,741,824 bytes.
Hard drive and SSD drive manufacturers calculate 1 Gigabyte as 1,000,000,000 bytes.
The difference between the two methods is 7%. 7% of 750 is 52.
So whenever you buy a HDD or SSD there will always be a 7% between what's advertised by the drive and what's available in Windows.
It says that I have 540GB's free of 698GB's so that means that I have taken up 158GB's of space on the hard drive including windows?