How to Use This Tool
Enter the size printed on the box. You get the exact byte count, what each operating system will display, and the separate, much smaller amount that formatting really costs.
The whole answer in one line
A terabyte is a trillion bytes. Windows divides by 1024 three times, which gives 931.32, and then writes GB next to it — but 1024-based units are called GiB (gibibytes). The number is right and the label is wrong, and that mislabelling is the entire mystery.
The gap is always 9.05% at terabyte scale, whatever the size, because it is a fixed ratio between two counting systems rather than anything being consumed. A 4 TB drive loses the same 9.05% and shows 3,725 "GB".
Who is right
Both, in their own terms — but only one is consistent with everything else. Every other SI prefix is decimal: a kilometre is 1,000 metres, not 1,024. Drive manufacturers use the same convention, which is why they have won every lawsuit brought over this. The 1024-based prefixes were standardised as kibi, mebi, gibi in 1998 precisely to end the ambiguity.
macOS switched to decimal units in 2009 and Linux tools mostly followed, so both show a 1 TB drive as 1 TB. Windows still shows 931 and still calls it GB. If your drive "shrank" when you moved it between machines, nothing happened to it.
Formatting really does cost something, but not much
This is the part that is genuinely consumed, and it is far smaller than people assume. A filesystem needs space for its own bookkeeping — the file table, journal, allocation bitmaps, and space reserved so the disk does not become unusable when full.
- NTFS — roughly 0.5%, mostly the Master File Table and journal.
- exFAT — roughly 0.3%, since it keeps less metadata.
- ext4 — roughly 1%, including 5% reserved for root by default on older setups, which is often reduced.
So on a 1 TB drive formatting costs a few gigabytes, and the unit mismatch accounts for around sixty-nine. Blaming formatting for the whole gap is the common misdiagnosis; the chart above keeps them separate.
SSDs and over-provisioning
Solid-state drives set aside additional capacity you never see, used for wear levelling and to replace failed cells. On consumer drives this is usually already excluded from the advertised figure, so it does not add to the gap. It is why some enterprise SSDs are sold in odd sizes like 960 GB rather than 1 TB — that one is visible in the number on the box.