You’ve bought a new shiny drive for your system, looking forward to the added space on your system. You install your new 1TB drive, but then notice you only have 900 or so GB available – or a 10% or so difference. Why the difference? It all comes down to how manufacturers of hard drives and Windows count – basically, how the storage is calculated versus how it’s sold.
Humans generally count in base 10 (aka the decimal counting system). That’s the number system we all grew up using, has been used for centuries and is what makes sense to us. In this system, the prefixes, kilo, mega, and tera mean something – in this case, 1000, 1,000,000, and 1,000,000,000, respectively. Drive manufacturers follow this universal standard, so 1 kilobyte is 1000 bytes of data. So when you buy a 1TB (terabyte) drive, you’re getting one trillion bytes of storage space.
However, computers don’t count in base 10. They count using Base 2 (aka binary) numbers. Numbers are process in powers of two. So in the early day of computing, the industry saw it convenient to map “Kilo” to 2 to the power of 10 (2^10), which equals 1024 because they figured it was close enough. Multiply that by a trillion and the math gets messy quickly.
As storage capacity got larger, those definitions got more blurred. While drives are manufactured using gigabytes of storage, operating systems like Windows uses gibibytes (1024 megabytes mebibytes) but they’re displayed as one in the same (GB, MB, TB, etc…).
Basically, it’s a confusing mess of math and marketing and Windows and the drive manufacturers use a different definition of what a GB and TB is.
Couple this with operating system overhead and potentially hidden drive partitions used for recovery, you can generally expect to lose about 10% of your drives due to funky math. If you need to plan for drive/storage expansion, plan for that overhead and loss accordingly to make sure you don’t have any surprise “Out of Storage” warnings later.