Why a 1 TB drive shows 931 GB
You bought a terabyte and the computer says 931 GB. Nothing has been taken from you, and nothing is broken. The maker counted in thousands and the operating system counted in 1024s, and the two are counting exactly the same bytes.
Two ways of counting the same bytes
The maker sells 1 000 000 000 000 bytes and calls it one terabyte, which follows the international system where tera means a thousand billion, as kilo means a thousand in every other unit on this site.
The operating system divides by 1024 three times, because memory is addressed in powers of two, and still writes GB on the result. One thousand billion divided by 1024 three times gives 931.32. The 69 gigabytes of difference are not missing space: they are the same bytes under a second name.
Which convention rules where
Powers of ten rule on the box and on the wire: drive makers, the international system, and network speeds, where a 100 Mbit/s line really does carry a hundred million bits per second.
Powers of two rule inside the machine: Windows, and memory sticks, where 16 GB of RAM really is 16 times 2^30 bytes because that is how addresses work. macOS switched to counting in thousands in 2009, so the same drive reads 1 TB there. The IEC named the binary units in 1998, kibibyte, mebibyte, gibibyte, and the whole confusion survives because Windows computes gibibytes and writes GB.
The figures, and the same question backwards
Counted in 1024s, 256 GB gives 238, 500 GB gives 466, 1 TB gives 931, 2 TB gives 1863, 4 TB gives 3725 and 8 TB gives 7451. The gap widens with the size because it is a ratio, not a fixed subtraction: it is always the same 7.4 per cent.
Read it the other way and it answers the question people actually type: a drive showing 930 GB is a one terabyte drive. Formatting then takes its own slice, a few gigabytes for the file system, and that one really is occupied space rather than a second way of counting.
Sold capacity, and the same bytes counted in 1024s
| Sold as | Counted in 1024s |
|---|---|
| 500 GB | 465.66 GiB |
| 1 TB | 931.32 GiB |
| 2 TB | 1,862.65 GiB |
| 4 TB | 3,725.29 GiB |
| 8 TB | 7,450.58 GiB |
The system truncates instead of rounding, and a drive holds slightly more than its nominal capacity, so the last digit moves by one from one model to the next.
Frequently asked questions
How much is 930 GB in TB?
0.9986 TB, which is one terabyte to within a seventh of a per cent. A drive reading 930 GB is a 1 TB drive: the figure comes from counting in 1024s, not from anything missing.
Is my drive faulty?
No. Every byte the maker sold is there and readable. Two conventions are counting them, one in thousands and one in 1024s, and only the name of the result differs.
Does formatting eat space?
Yes, and that part is real. The file system keeps its own tables, a few gigabytes on a terabyte drive, and that space is genuinely occupied. It is far smaller than the 69 gigabytes the two ways of counting explain.