system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does a 1TB SSD offer only 931GB of space?

I installed windows on a 1TB SSD, and then when I see the local drive in windows explorer, it shows as only 931GB. Why is this?
gpt
Part unit confusion, part reserved space.

The unit confusion is that early on, the computing industry appropriated the metric prefixes like "kilo" and "mega" to mean powers of 2. So in metric, "kilo" is 1000, in "computer tech" it's 1024. A megabyte is then 1024 * 1024 = 1048576 bytes, a gigabyte is 1073741824 bytes, and a terabyte is 1099511627776 bytes. Operating systems tend to stick to this system, so a 1 MB file is 1048576 bytes long.

The hard disk industry on the other hand prefers the metric meanings, because 1000000000000 bytes are cheaper to provide, you get to deliver almost 10% less!

Then there's that disks are specified in their raw size, but to actually store something on them various accounting data needs to be written, and those consume part of the space. So even if a disk technically has a trillion bytes on it, in practice you are using some to keep track of where what is located.