The formula
GiB = bytes ÷ 1,073,741,824, while GB = bytes ÷ 1,000,000,000
Where it comes from
Nothing is missing and nobody is lying: these are two different units with similar names. The manufacturer uses the decimal prefix of the international system, where a gigabyte is exactly a thousand million bytes. The operating system, by contrast, counts in powers of two — 2³⁰ bytes — but has spent decades calling that a "GB" instead of a GiB, which is its proper name. The gap is not fixed: it grows one step per unit, from 2.4 % in kilobytes to 9.3 % in terabytes.
How to work it out by hand
- Multiply the stated capacity by a thousand raised to the step of the unit
- Those are the real bytes on the drive
- Divide by 1,024 raised to the same step
- That is the figure the system displays
What is worth knowing
On top of the units gap there is a smaller one, and this one really is occupied space: the file system reserves a share for its own structures — allocation table, indexes, spare sectors — and on a modern drive that is between 0.5 % and 3 %. Adding both, a 4 TB drive leaves around 3.6 TiB usable, and that is normal. Worth knowing too: macOS switched to decimal units years ago, so the same drive shows different figures on macOS and Windows without either being wrong.