Storage Size Calculator
Tell me what you want to store, and I'll work out how much space you need and the right drive size to fit it.
Advertised vs. usable capacity
Drive makers count in decimal: a "1 TB" drive is 1,000,000,000,000 bytes, which your computer reports as about 931 GiB. The calculator then sets aside a small slice for filesystem overhead, a few percent more on an internal drive than on an external one, and sizes its recommendation around the usable space that is left.