Hard drive show less space than stated?

I have just bought a 14 TB external hard drive. yet when I connect it to my pc it says I only have 12.7 TB available free space. I have checked my other hard drives and they are within a few gb of the advertised size.

That’s correct … 12.7329 TB to be precise

Here’s a Real HDD Capacity Calculator: USB Hard Disk Real Capacity Calculator -- EndMemo

Why ? … link below.