technology · 1 min read · United States
Your 1 TB drive is not missing 69 GB
A 1 TB drive holds exactly 1,000,000,000,000 bytes. Windows divides by 1,073,741,824 to get a "GB", which gives 931.32. The manufacturer counts in powers of ten and the operating system counts in powers of two. Nothing is lost; the two are using the same word for different quantities.
Buy a 1 TB drive, plug it in, and the computer says 931 GB. The obvious conclusion is that something has taken 69 GB before you touched it, and the obvious conclusion is wrong. The bytes are all there. What differs is the size of the unit each side is dividing by.
| Counted by | Unit | Bytes in the unit | Reported size |
|---|---|---|---|
| The manufacturer | Terabyte (TB) | 1,000,000,000,000 | 1 TB |
| The operating system | Gibibyte (GiB), labelled GB | 1,073,741,824 | 931.32 |
Storage is sold in base ten, where a kilobyte is a thousand bytes, because that is what the prefixes kilo, mega, giga and tera mean everywhere else in measurement. Memory addressing is a power of two, so operating systems have historically divided by 1,024 instead and called the result a kilobyte anyway. The International Electrotechnical Commission fixed the naming in 1998 with kibi, mebi, gibi and tebi, and Windows has still not adopted it.
The gap grows with every prefix
| Prefix | Base 10 | Base 2 | Difference |
|---|---|---|---|
| Kilo | 1,000 | 1,024 | 2.4% |
| Mega | 1,000,000 | 1,048,576 | 4.9% |
| Giga | 1,000,000,000 | 1,073,741,824 | 7.4% |
| Tera | 1,000,000,000,000 | 1,099,511,627,776 | 10.0% |
That is why the missing space looks worse on bigger drives. A 4 TB drive reports 3.64 TB, and the "loss" is 366 GB, which is more than the entire capacity of a drive people were buying fifteen years ago. It is the same 10% at tera every time.
macOS switched to base ten in 2009, which is why the same drive reads 1 TB on a Mac and 931 GB on a Windows machine. Neither is lying and the drive has not changed.
What actually does take space
- The filesystem itself, which reserves a few percent for metadata and journalling.
- A recovery partition, on a laptop that shipped with the operating system installed.
- Reserved free space, which some filesystems hold back to avoid fragmentation.
Those are real and they are small. If you are trying to reconcile a number, subtract the base conversion first, because it accounts for the overwhelming majority of the difference and it is exact.