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SI and IEC units

SI units:
The standard SI units like KB, MB, GB and TB are powers of 1000.

Examples:
1 KB = 1000 Byte = 103 Byte
1 MB = 1000 KB = 106 Byte = 1 000 000 Byte
1 GB = 1000 MB = 109 Byte = 1 000 000 000 Byte
1 TB = 1000 GB = 1012 Byte = 1 000 000 000 000 Byte

IEC units:
On the other hand the units KiB, MiB, GiB and TiB are specified as powers of 1024. These units were standardized by IEC in 1999. Starting with Boot-US 3.0.1 these IEC units are used by default.

Examples:
1 KiB = 1024 Byte = 210 Byte
1 MiB = 1024 KiB = 220 Byte = 1 048 576 Byte
1 GiB = 1024 MiB = 230 Byte = 1 073 741 824 Byte
1 TiB = 1024 GiB = 240 Byte = 1 099 511 627 776 Byte

Since the number 1024 is close to the number 1000 it became customary to abbreviate 1024 as 1 KB although strictly speaking both numbers are different. However the difference is only about 2 percent and thus in many cases negligible. But the above table shows that the error becomes larger with increasing numbers. 1 TiB differs already by about 10 percent from 1 TB, and such a difference is usually not negligible any more. It is therefore expected that in the future more emphasis is put on using the correct units. If numbers are meant as powers of 1024 then one expects to see the IEC units.