Apr
30
2012

What are the prefixes for binary multiples?

Computed art by Jared Tarbell (CC-BY)

The SI prefixes provide a convenient way to apply a decimal scale factor to a unit of measurement. For example, 1000 meters is a kilometer (km). However, the SI prefixes are frequently misused to apply a binary scale factor that happens to be close to a decimal factor.

For example, 1024 bytes is often incorrectly called a kilobyte (kB). Strictly speaking, a kilobyte is exactly 1000 bytes (because the SI prefix kilo denotes a factor of 1000).

The IEC (International Electrotechnical Commission) has standardized a set of binary prefixes. These prefixes can be used to unambiguously specify quantities which by their nature are most easily measured using binary scale factors.

Definitions

Name Symbol Factor Multiple
kibi Ki 210 1024
mebi Mi 220 1,048,576
gibi Gi 230 1,073,741,824
tebi Ti 240 1,099,511,627,776
pebi Pi 250 1,125,899,906,842,624
exbi Ei 260 1,152,921,504,606,846,976

Examples

2 KiB, or 2 kibibytes, is the same as 2048 bytes
1 Mi bit, or 1 mebibit, is the same as 1048576 bits

Notes

The prefixes pebi and exbi are not in common use.

Reference

IEC 60027-2, Second edition, 2000-11, Letter symbols to be used in electrical technology—Part 2: Telecommunications and electronics.

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