Plug in a new 1TB SSD and Windows File Explorer shows about 931GB. The usual follow-up is that the missing 69GB must be bad blocks or space the maker hid. The two numbers are not a "gone" and "left" pair. They are the same storage measured with two different rulers.
There are two gigabytes
Apple's note on how storage capacity is measured sets the two systems side by side. Storage device makers measure capacity with the decimal system, base 10, which is why packaging and specifications state that 1GB is one billion bytes and 1TB is one trillion bytes. Measured in binary, 1GB is 1,073,741,824 bytes and 1TB is 1,099,511,627,776 bytes. The document was published on 21 May 2024.
Seagate's support article describes the same situation: a new 1TB drive reports only about 931GB because two different number systems are used to express units of storage capacity, and the storage industry standard is to display capacity in decimal.
The ratio is the same at every size
Convert one decimal GB to binary units: 1,000,000,000 ÷ 1,073,741,824 = 0.9313. Whatever the labelled capacity, the screen shows about 93.13% of it — 6.87% smaller. On 1TB that is a gap of 68.68GB.
| Labelled capacity | Bytes (decimal) | Converted to binary units (GiB) |
|---|---|---|
| 128GB | 128,000,000,000 | 119.21 |
| 256GB | 256,000,000,000 | 238.42 |
| 512GB | 512,000,000,000 | 476.84 |
| 1TB | 1,000,000,000,000 | 931.32 |
| 2TB | 2,000,000,000,000 | 1,862.65 |
The third column is the second column divided by 1,073,741,824, rounded at the third decimal place. That column is what Windows File Explorer labels "GB".
The names split; the on-screen labels did not follow
According to the NIST summary, the International Electrotechnical Commission approved names and symbols for binary prefixes as an international standard in December 1998. Under it, 1 GiB is 2^30 bytes, or 1,073,741,824 bytes, while 1 GB is 10^9 bytes, or 1,000,000,000 bytes. The labelling never caught up: file browsers still print binary values as "GB".
Units explain part of it; the rest is something else
The Apple document adds that "actual formatted capacity" means the formatting of the storage device uses some of that space, and that additional space is used for other purposes such as the device operating system. Divide the byte figure in the drive properties window by 1,073,741,824. If the result lands near 93% of the labelled capacity, the unit difference has explained it. If it is visibly lower, look at file system metadata, recovery partitions, and space taken by the operating system. You will not get the full labelled capacity either, so when buying, assume the on-screen number starts at about 93% of the label and leave headroom.