Data Storage Converter
Convert between bytes, kilobytes, megabytes, gigabytes and terabytes in decimal and binary units — the difference that makes a 1 TB drive show as 931 GB.
Where the “missing” space on a hard drive goes
Nowhere. There are two counting systems and they do not agree. A manufacturer counts in powers of ten, because that is what “tera” means everywhere else in science. An operating system divides by powers of two, because that is how memory is addressed. Both are defensible, and the disagreement compounds at every prefix:
| Prefix | Decimal | Binary | Binary is larger by |
|---|---|---|---|
| kilo / kibi | 103 | 210 | 2.40% |
| mega / mebi | 106 | 220 | 4.86% |
| giga / gibi | 109 | 230 | 7.37% |
| tera / tebi | 1012 | 240 | 9.95% |
| peta / pebi | 1015 | 250 | 12.59% |
| exa / exbi | 1018 | 260 | 15.29% |
So a drive sold as 1 TB holds 1012 bytes, which is 931.3 GiB, and the machine reports 931 GB because it is using GB to mean GiB. Nothing was lost, nothing was taken by formatting — converting to binary units and back returns exactly the same byte count.
| Sold as | Actually | Reported as | Looks “missing” |
|---|---|---|---|
| 500 GB | 5.0e+11 bytes | 465.7 GiB | 6.9% |
| 1 TB | 1.0e+12 bytes | 931.3 GiB | 6.9% |
| 2 TB | 2.0e+12 bytes | 1.82 TiB | 6.9% |
| 4 TB | 4.0e+12 bytes | 3.64 TiB | 6.9% |
| 16 TB | 1.6e+13 bytes | 14.55 TiB | 6.9% |
Note that the apparent shortfall is identical at every size — 6.9%, whether the drive is 500 GB or 16 TB. Bigger drives do not lose more, because it is always the same single prefix step. The IEC introduced kibi, mebi and gibi in 1998 to end exactly this, and almost nobody adopted them. macOS started counting in powers of ten in 2009, so the same drive reads 1 TB there and 931 GB on Windows.
Which meaning is right depends on what is being measured
Memory really is binary: a 16 GB stick is 16 × 230 bytes, because address lines come in powers of two and there is no way to build it otherwise. Storage is not — a disk has no architectural reason to hold a power of two, so the decimal figure is the honest one. Network speed is decimal too and quoted in bits, so a “100 Mbps” line moves at most 12.5 MB/s before any overhead, which is the same confusion wearing different units.
One rule resolves most of it: if it is addressed, it is binary; if it is merely counted, it is decimal.
How to use
- Enter a size and pick its unit.
- Read the equivalent across every other unit at once.
- Switch between decimal (1000) and binary (1024) as needed.
- Use binary units when comparing against what your operating system reports.
Frequently asked questions
Why does my 1 TB drive show as 931 GB?
Because the manufacturer counts in decimal and your operating system counts in binary. A terabyte sold is 1,000,000,000,000 bytes; Windows divides by 1024 three times and calls the result 931 GB. No space is missing — the same bytes are being described in two different systems.
What is the difference between GB and GiB?
A gigabyte is 10^9 bytes; a gibibyte is 2^30, or 1,073,741,824 bytes — about 7 per cent more. The binary prefixes kibi, mebi and gibi were introduced in 1998 to end the ambiguity, and are correct, precise and almost universally ignored outside technical documentation.
Which does my operating system use?
macOS and most Linux distributions report decimal gigabytes, matching the drive label. Windows reports binary values but labels them GB, which is why the same drive appears smaller on Windows than on a Mac. Nothing is wrong in either case.
How much is a terabyte in practical terms?
Roughly 250,000 photos from a phone, or 250 hours of high-definition video, or several hundred thousand documents. The figure shifts constantly as file sizes grow, so treat any such comparison as a snapshot rather than a rule.
What comes after terabyte?
Petabyte, exabyte, zettabyte and yottabyte, each a thousand times the last. Petabytes are ordinary in large organisations, exabytes describe global traffic, and the units beyond exist mostly to have names ready.
Why 1024 rather than 1000?
Because memory addressing is binary, and 1024 is 2 to the power of 10 — the nearest power of two to a thousand. Early computing borrowed the metric prefixes for convenience, and the small discrepancy went unremarked until drive sizes grew large enough for 7 per cent to be noticeable.
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