You buy a 1 TB drive, plug it into a Windows PC, and it shows 931 GB. Nothing is missing. The box and the computer are using the same letters for two different numbers: the manufacturer counts a terabyte as 1,000,000,000,000 bytes, while Windows divides by 1,024 at every step and still calls the result "GB."
Here's where the 931 comes from, how big the gap is for common sizes, which systems count which way, and what accounts for any space that really is used.
Two meanings of "gigabyte"
In the metric system, kilo means 1,000, mega a million, giga a billion and tera a trillion. Drive makers use those decimal values:
- 1 GB = 1,000,000,000 bytes
- 1 TB = 1,000,000,000,000 bytes
Computers work in binary, so memory and early software counted in powers of 2: 1,024 bytes was called a kilobyte because 210 = 1,024 is close to 1,000. Each step up multiplies by 1,024 again:
- 1 "GB" (binary) = 1,024 × 1,024 × 1,024 = 1,073,741,824 bytes
- 1 "TB" (binary) = 1,099,511,627,776 bytes
Apple's support article on how storage capacity is measured gives the same two sets of figures: in decimal, "1GB = 1 billion bytes, and 1TB = 1 trillion bytes," and in binary, "1GB = 1,073,741,824 bytes, and 1TB = 1,099,511,627,776 bytes."
The names that fix the confusion
To end the ambiguity, the International Electrotechnical Commission introduced separate names for the binary units. As NIST's page on prefixes for binary multiples records, "in December 1998 the International Electrotechnical Commission (IEC)" approved them for data processing:
| Prefix | Decimal (SI) | Binary (IEC) |
|---|---|---|
| kilo / kibi | kB = 1,000 bytes | KiB = 1,024 bytes |
| mega / mebi | MB = 106 | MiB = 220 |
| giga / gibi | GB = 109 | GiB = 230 |
| tera / tebi | TB = 1012 | TiB = 240 |
NIST notes that these binary prefixes "are not part of the International System of Units (SI)." So, strictly, GB means 109 bytes and GiB means 230. Windows shows GiB values but labels them GB, and that mismatch is the whole story.
The math behind 931
Take the drive's real size in bytes and divide by the size of a binary gigabyte:
1,000,000,000,000 ÷ 1,073,741,824 = 931.32 GiBOr in tebibytes:
1,000,000,000,000 ÷ 1,099,511,627,776 = 0.9095 TiBOur Unit Converter does this in its Data size category, which has both the decimal units (kB, MB, GB, TB) and the binary ones (KiB, MiB, GiB, TiB). It gives 1 TB = 931.3225746 GiB.
What common sizes show
Here's what each advertised size works out to in binary units, before any formatting or system files:
| On the box | Shown in binary units |
|---|---|
| 128 GB | 119.2 GiB |
| 256 GB | 238.4 GiB |
| 500 GB | 465.7 GiB |
| 512 GB | 476.8 GiB |
| 1 TB | 931.3 GiB (0.91 TiB) |
| 2 TB | 1.82 TiB |
| 4 TB | 3.64 TiB |
If your new 512 GB laptop shows about 476 GB in Windows, that's the conversion and nothing else.
The gap grows with every prefix
Each binary step multiplies by 1,024 instead of 1,000, so the difference compounds:
| Units | Binary unit is larger by |
|---|---|
| KiB vs kB | 2.40% |
| MiB vs MB | 4.86% |
| GiB vs GB | 7.37% |
| TiB vs TB | 9.95% |
So on a 1 TB drive the reported number is about 9% lower, and on a 20 TB drive the same percentage hides almost 2 TB of apparent "missing" space. The percentages also aren't symmetrical: a TiB is 9.95% bigger than a TB, but a TB is 9.05% smaller than a TiB. Percent Change vs Percentage Points explains why the direction of a comparison changes the percentage.
Which systems count which way
- Windows File Explorer divides by 1,024 and labels the result GB and TB. That's why it shows 931 GB for a 1 TB drive.
- macOS and iOS today report in decimal units, so a 1 TB drive shows close to 1 TB. Apple's article notes that "iOS 10 and earlier and OS X Leopard and earlier" reported storage in binary.
- Linux depends on the tool.
df -huses powers of 1,024 and prints932Gfor a 1 TB disk (it rounds up), whiledf -Huses powers of 1,000.ls -lhandls -l --siwork the same way. Many desktop file managers use decimal units.
The same file can therefore show different sizes on different computers without anything changing.
Why drive makers count in decimal
It isn't a trick. Kilo, mega, giga and tera are SI prefixes, and in the SI they always mean powers of 1,000. Storage devices also aren't built in neat powers of two the way memory chips are: a disk's capacity comes from its platters, tracks and sectors, and an SSD's from flash chips plus spare area the drive keeps for itself. Decimal units describe those capacities exactly, and they've been the industry convention for decades.
The confusion only appears when software divides by 1,024 and keeps the decimal names. Labeling the result GiB, as the IEC names intend, would make the two numbers easy to tell apart: 1 TB and 931 GiB are the same drive.
Space that is really used
The unit conversion explains most of the gap, but not all of it. A new computer also has:
- Formatting and the file system. Apple puts it simply: "the storage device's formatting uses some of this storage space."
- The operating system and preinstalled apps, which can take tens of gigabytes.
- Recovery partitions on many Windows PCs, which don't show up as a drive letter.
- Snapshots, caches and the page or swap file, which grow and shrink.
To separate the two, compare the capacity in bytes. Windows shows it in a drive's Properties window, macOS in Disk Utility. If the byte count is close to the advertised number, the "missing" space is just units.
RAM and network speeds count differently
Memory is the exception that started it all. RAM is built in powers of 2, so a "16 GB" memory module holds 16 GiB, 17,179,869,184 bytes. Nobody labels it as 17.18 GB.
Network speeds go the other way: they're decimal and in bits, not bytes. A 100 Mbit/s connection moves at most 12.5 MB per second (100 ÷ 8), before protocol overhead. The Unit Converter has bits, kilobits, megabits and gigabits in the same Data size list, which is handy for estimating download times.
FAQ
Is a 1 TB drive really 1 TB?
Yes, in decimal units: 1,000,000,000,000 bytes, as the manufacturer states. It's 0.91 TiB in binary units, which Windows displays as "931 GB."
Why does a 512 GB SSD show 476 GB?
Because 512 billion bytes is 476.8 binary gigabytes (GiB), and Windows labels GiB as GB. The operating system and recovery partition then take some of that space.
Is 1 GB 1,000 MB or 1,024 MB?
Strictly, 1 GB is 1,000 MB, and 1 GiB is 1,024 MiB. In everyday use, "GB" is used for both, so check which one a program means.
How do I convert TB to TiB?
Multiply by 0.909495 (or divide by 1.099511627776). In the Unit Converter, choose Data size, then Terabytes and Tebibytes.
