KB vs KiB — what's the difference?
A side-by-side look at the decimal (KB) and binary (KiB) interpretations of "kilobyte".
1 kB = 1,000 bytes (SI decimal prefix)
1 KiB = 1,024 bytes (IEC binary prefix)
KB (kilobyte) is the SI decimal interpretation: 1,000 bytes. KiB (kibibyte) is the IEC binary interpretation: 1,024 bytes (2¹⁰).
The decimal interpretation is the standard one for SI prefixes: kilo, mega, giga mean ×1000, ×10⁶, ×10⁹ respectively. Disk drive manufacturers use this.
The binary interpretation predated the distinction. When memory and disk were both small, no one cared about the 2.4% discrepancy. As storage grew, the gap compounded: - a 1 TB disk (10¹² bytes) reports as 931 GiB. - a "1.44 MB" floppy was 1.44 × 1,000,000 = 1,440,000 bytes but the system actually gave you 1,474,560 bytes — 1.44 MiB with the binary definition.
In 1998 the IEC standardised KiB, MiB, GiB, TiB, PiB to remove the ambiguity. Most operating systems picked a side: Windows displays binary (GiB labelled "GB"); macOS displays decimal (GB labelled "GB").
Frequently Asked Questions
Which should I use?
Use KB when talking about disk drives, file sizes on macOS, and network speeds.
When is the alternative better?
Use KiB when talking about RAM, OS-reported file sizes on Windows, or precise memory arithmetic.
How is the conversion calculated?
Every conversion is a single factor (or an affine formula). The math is identical either direction; the difference is context.