Plug a new 1 TB drive into a Windows PC and Explorer reports 931 GB. No space has gone missing. The box and the operating system are using the same two letters for two different numbers, and the difference between them grows with every step up the scale.
Two Numbers Behind One Abbreviation
Everywhere else in measurement, kilo means a thousand. A kilometre is 1,000 metres and a kilogram is 1,000 grams. Storage makers follow that rule: a gigabyte on the box is 10⁹ bytes, a terabyte 10¹² bytes.
Memory does not fit that rule. RAM chips are addressed in binary, so their natural sizes are powers of two, and 2¹⁰ is 1,024, close enough to a thousand that early programmers borrowed the word. A "kilobyte" of memory became 1,024 bytes, a "megabyte" 1,048,576, and the habit spread from memory to file sizes and then to whole disks.
For small numbers the difference hardly matters. At the kilobyte level the two meanings differ by 2.4%. But the error compounds with each prefix, because each binary step multiplies by another 1.024.
How Far Apart the Two Units Drift
Every binary unit is exactly a power of 1,024 bytes; the decimal unit is the same power of 1,000.
| Decimal | Binary | Binary in bytes | Binary is larger by |
|---|---|---|---|
| 1 kB | 1 KiB | 1,024 | 2.4% |
| 1 MB | 1 MiB | 1,048,576 | 4.9% |
| 1 GB | 1 GiB | 1,073,741,824 | 7.4% |
| 1 TB | 1 TiB | 1,099,511,627,776 | 10.0% |
| 1 PB | 1 PiB | 1,125,899,906,842,624 | 12.6% |
The Standard Gave the Binary Units Their Own Names
The fix has existed since January 1999, when the International Electrotechnical Commission published Amendment 2 to IEC 60027-2. It kept kilo, mega and giga for powers of ten, as the SI requires, and created new prefixes for powers of two: kibi (Ki) for 2¹⁰, mebi (Mi) for 2²⁰, gibi (Gi) for 2³⁰, tebi (Ti) for 2⁴⁰. Later editions added pebi and exbi, and the set now lives in ISO/IEC 80000-13.
The IEEE adopted the same prefixes in IEEE 1541-2002, and the international vocabulary of metrology (VIM3) states the rule plainly: a kilobit is 1,000 bits, and 1,024 bits is a kibibit. NIST publishes the same guidance for the United States.
So by every standards body, a 1 TB drive is 1 TB, and the 931 figure is 931 GiB. Windows still prints GB.
The difference reached a courtroom
In 2006 Western Digital settled a class action brought by buyers who said its drives held less than advertised. A 200 GB drive held 200 billion bytes, as labelled, but Windows XP reported it as 186.26 GB. The company did not change its arithmetic. It agreed to give eligible buyers backup software and to print a disclaimer explaining that one gigabyte means one billion bytes.
Who Counts in Thousands and Who Counts in 1,024s
There is no single answer to "how big is a gigabyte" because the industry never picked one. These are the conventions you will actually meet.
Drive and USB stick makers: decimal
Hard drives, SSDs, memory cards and flash drives are all sold in powers of ten, with the definition in the small print on the packaging.
Windows: binary, labelled as decimal
Explorer divides by 1,024 and shows the result as KB, MB, GB or TB. This is the main source of the confusion, since the units on screen look identical to the ones on the box.
macOS: decimal since 2009
Mac OS X 10.6 Snow Leopard switched Finder to powers of ten, so a 1 TB drive shows as 1 TB on a Mac. File sizes changed with it: a file Finder called 93.6 MB under 10.5 shows as 98.2 MB under 10.6, with not a single byte changed.
RAM: binary, always
A memory module sold as 16 GB holds 16 GiB, because memory is built in powers of two. JEDEC's memory standards keep the older binary meaning of KB, MB and GB for exactly this reason.
Networks: decimal, and in bits
Line speeds are quoted in bits per second with decimal prefixes. A megabit is 1,000,000 bits, and there is no binary meaning in networking at all.
Convert between GB, GiB, MB and bits
Decimal and binary side by side, with what Windows and macOS will each show.
Bits, Bytes and Download Speeds
The other common trap is the letter b. A capital B is a byte and a lowercase b is a bit, and there are eight bits in a byte. Internet plans are sold in megabits per second (Mbps), while browsers and download managers report megabytes per second (MB/s).
Divide by eight to move from one to the other. A 500 Mbps connection tops out at 62.5 MB/s, and a 1 Gbps fibre line at 125 MB/s. Real transfers land somewhat below those ceilings, because every packet carries headers and some get resent.
When a speed and a file size meet, convert both to bytes before dividing. A 50 GB game on a 500 Mbps line takes at least 800 seconds, a little over thirteen minutes, not the hundred seconds you get by dividing 50,000 by 500.
Where the Rest of the Space Goes
Even after converting to GiB, a new drive shows slightly less free space than the arithmetic predicts. The file system needs room for its own tables, journals and metadata, and the share depends on the format and the drive size.
Computers and phones go further. A laptop's internal drive often carries a recovery partition, and a phone sold as 128 GB spends a sizeable slice of it on the operating system before you install anything. None of that is a unit problem, but it lands on top of one, which is why the figure on screen can sit well below the one on the box.
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