Kilobyte: 1000 or 1024 bytes?
Published
In a nutshell
In the International System of Units the prefix kilo always means 1000, so a kilobyte (kB) is 1000 bytes. The value 1024 bytes has a separate name and symbol: the kibibyte (KiB).
The difference is small for kilo and grows with every step: about 2.4 % for kilo, but already about 10 % for tera.
Why the confusion arose
Computers count in powers of two, and 210 = 1024 is close to 1000. That is why the SI prefixes kilo, mega and giga came to be used in computing for 1024, 1 048 576 and 1 073 741 824.
The SI does not allow this. SI prefixes always stand for powers of ten, and the NIST guide to the SI states explicitly that kilo, mega and giga must not be redefined to mean powers of two. More on prefixes on the Overview page.
Prefixes for powers of two
In December 1998 the International Electrotechnical Commission (IEC) approved separate prefixes for binary multiples: kibi (Ki, 210), mebi (Mi, 220), gibi (Gi, 230), tebi (Ti, 240), pebi (Pi, 250) and exbi (Ei, 260). The names join the start of the SI prefix with “bi” for binary.
These prefixes are not part of the SI, but they remove the ambiguity: 1 kibibit is 1024 bit, while 1 kilobit is 1000 bit.
How big is the gap
One kibibyte is 2.4 % more than one kilobyte. One mebibyte (1 048 576 B) is about 4.9 % more than one megabyte, one gibibyte (1 073 741 824 B) about 7.4 % more than one gigabyte, and one tebibyte about 10.0 % more than one terabyte.
So a capacity of 1 TB, meaning 1012 bytes, is about 0.91 TiB. Neither number is wrong; they use different prefixes.
How to write it
The symbol for kilo is a lower-case k, so a kilobyte is kB, not KB. The binary prefix for kibi starts with a capital K: KiB. The symbol for the byte is B and for the bit is bit.
Sources
- BIPM, The International System of Units (SI Brochure), 9th edition, checked 28 October 2026
- NIST, Guide for the Use of the International System of Units (SP 811), Chapter 4: Two classes of SI units and the SI prefixes, checked 28 October 2026
- NIST, Prefixes for binary multiples, checked 28 October 2026