DECIMAL
SI prefixes count in tens
The SI Brochure says SI prefixes refer strictly to powers of 10, so one kilobit is 1,000 bits, not 1,024.
Compare units- SI counts in tens
- Binary counts in twos
- The gap grows each step
- Two systems, one byte
00WHAT THE STANDARDS SAY
1,000 or 1,024?
It depends on the prefix.
Two counting systems share almost the same names.
The metric prefixes kilo, mega and giga mean powers of ten. Computers count in twos, so in 1998 the IEC approved separate names, kibi, mebi and gibi, for powers of two. Mixing them up is why sizes rarely match.
Compare units01Principles
02THE SHORT ANSWER
Kilo means 1,000.
Kibi means 1,024.
The SI prefixes are strictly powers of ten.
Binary prefixes have an extra i in the symbol.
BINARY
Kibi, mebi, gibi
In December 1998 the IEC approved prefixes for binary multiples: kibi (Ki) for 2¹⁰, mebi (Mi) for 2²⁰, gibi (Gi) for 2³⁰ and onwards.
See the numbers
BIG NUMBERS
Ronna and quetta
In 2022 the CGPM added ronna (10²⁷) and quetta (10³⁰), citing the needs of data science to express quantities of digital information.
The newest prefixes03A QUICK UNIT CHECK
How many bytes
is that?
Pick a unit to see exactly how many bytes it holds. The bar shows the size, from smallest to largest.
Values from NIST's page on prefixes for binary multiples and the BIPM SI Brochure (9th edition).
04TWO NUMBERS WORTH KNOWING
A small gap
that keeps growing.
One figure for the binary step,
one for the year the names became standard.
1,024
Bits in a kibibit
A kibibit is 2¹⁰ bits, or 1,024, while a kilobit is 10³, or 1,000. By the gibibyte the binary unit is 1,073,741,824 bytes against a gigabyte's 1,000,000,000.
BINARY STEP · NIST
1998
Year the IEC approved binary prefixes
The IEC approved the names in December 1998, first published as an amendment to IEC 60027-2 in January 1999. NIST notes they are not part of the SI.
THE STANDARD · NIST
Each binary name keeps the first two letters of the SI prefix and adds "bi", for binary; each symbol adds an "i". NIST suggests pronouncing the second syllable "bee".
05READING A STORAGE SIZE
Look for
the little i.
Four quick checks, based on the definitions, for when sizes don't seem to match.
Check the symbol
kB, MB, GB and TB are decimal. KiB, MiB, GiB and TiB, with an i, are binary.
Know the factor
Decimal prefixes step by 1,000; binary prefixes step by 1,024.
Expect a bigger gap at bigger sizes
A kibibyte is 2.4% bigger than a kilobyte, but a gibibyte is about 7% bigger than a gigabyte.
Bits are not bytes
A byte is 8 bits, so a figure in bits is eight times a figure in bytes.
06THE NEWEST PREFIXES
Room to grow,
officially.
The SI now has 24 prefixes, running from quecto (10⁻³⁰) to quetta (10³⁰). The CGPM added ronna, quetta, ronto and quecto in 2022, noting the needs of data science to express quantities of digital information beyond 10²⁴.
The binary list has grown to match. The SI Brochure lists kibi through exbi, then zebi (2⁷⁰), yobi (2⁸⁰), robi (2⁹⁰) and quebi (2¹⁰⁰), pointing to the standard IEC 80000-13 for details.
07Questions
Questions people ask
Is a kilobyte 1,000 or 1,024 bytes?
Under the SI, kilo means 1,000, so a kilobyte is 1,000 bytes. The binary unit of 1,024 bytes is properly called a kibibyte, symbol KiB.
Why does my drive seem smaller than advertised?
Often because two systems are being compared: the label uses decimal units such as GB, while some software counts in binary units of 1,073,741,824 bytes. The same bytes then give a smaller number.
Who decided on kibi, mebi and gibi?
The International Electrotechnical Commission approved them in December 1998, with support from the CIPM and IEEE, according to NIST.
What comes after yotta?
Ronna (10²⁷) and quetta (10³⁰), added to the SI in 2022. Their binary counterparts are robi (2⁹⁰) and quebi (2¹⁰⁰).
Are binary prefixes part of the metric system?
No. NIST says the binary prefixes are not part of the SI, although their names were derived from the SI prefixes.
08Sources