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When it comes to computer memory, we usually learn that a kilobyte is 1024 bytes, a megabyte is 1024 kilobytes, and so on. But what if I told you that it's not necessarily true and 1 kilobyte can be 1000 bytes? And what's more, this makes even more sense.
Why do we often say 1 kilobyte = 1024 bytes?
Since computers work in a binary system (base 2), the memory is also addressed in binary. This means it's quite impractical to use memory addresses or produce RAM sticks with memory amounts that are not multiples of powers of 2. From the powers of 2 we chose 1024 (210) as the base order of magnitude, since it's very close to 1000 (2.4% difference) and it's not insanely large. So, in practice we often consider kilo as 1024 (210), mega as 1048576 (220), giga as 1073741824 (230), etc.
Why does 1000 still make more sense?
While binary kilo, mega and giga units are close to their decimal counterparts, some might already notice that the larger are the units, the more is the proportional inaccuracy. In order to illustrate, let's increase the units:
| Unit | Decimal value | Binary value | Relative difference |
|---|---|---|---|
| Kilo | 1000 | 1024 | 2.4% |
| Mega | 1000000 | 1048576 | ≈ 4.8% |
| Giga | 1000000000 | 1073741824 | ≈ 7.3% |
| Tera | 1000000000000 | 1099511627776 | ≈ 10% |
| Peta | 1015 | ~ 1.126 * 1015 | ≈ 12.6% |
| Exa | 1018 | ~ 1.153 * 1018 | ≈ 15.3% |
| Zetta | 1021 | ~ 1.181 * 1021 | ≈ 18.1% |
| Yotta | 1024 | ~ 1.209 * 1024 | ≈ 20.9% |
| Ronna | 1027 | ~ 1.238 * 1027 | ≈ 23.8% |
| Quetta | 1030 | ~ 1.268 * 1030 | ≈ 26.8% |
For 1 quettabyte the inaccuracy is already larger than a quarter. Even for 1 terabyte the difference is noticeable, around 10%. This problem often happens when hardware manufacturers (such as HDD or SSD) advertise the memory capacity in decimal units while the operating system might show in binary units.
For smaller amounts of memory the binary representation is pretty close to the decimal one, but diverges for huge amounts of memory.
What are the standard units?
This "kilobyte = 1024 bytes" rule is actually an old (often confusing) convention. In the tech industry there is still huge inertia, this old convention is still used by RAM manufacturers (JEDEC), tons of software and some operating systems (such as Windows). Interestingly, storage vendors often prefer the decimal convention, which creates even more confusion (mentioned above).
In order to solve this confusion, International Electrotechnical Commission introduced binary prefixes for binary units:
| Binary units (International Electrotechnical Commission) | Decimal units (International System of Units) | ||
|---|---|---|---|
| Unit | Value | Unit | Value |
| KiB (kibibyte) | 10241 | kB (kilobyte) | 10001 |
| MiB (mebibyte) | 10242 | MB (megabyte) | 10002 |
| GiB (gibibyte) | 10243 | GB (gigabyte) | 10003 |
| TiB (tebibyte) | 10244 | TB (terabyte) | 10004 |
| PiB (pebibyte) | 10245 | PB (petabyte) | 10005 |
| EiB (exbibyte) | 10246 | EB (exabyte) | 10006 |
| ZiB (zebibyte) | 10247 | ZB (zettabyte) | 10007 |
| YiB (yobibyte) | 10248 | YB (yottabyte) | 10008 |
| RiB (robibyte) | 10249 | RB (ronnabyte) | 10009 |
| QiB (quebibyte) | 102410 | QB (quettabyte) | 100010 |
The guidance is: SI prefixes are powers of 10 only, and if you mean powers of 2 you should use IEC binary prefixes (Ki, Mi, Gi, …).
Conclusion
There is still a lot of inertia to equate 1 kilobyte to 1024 bytes. While this is usually acceptable depending on the context, it can sometimes cause confusion, especially for non-technical people.