What is a Kilobyte?

The kilobyte (KB) equals 1,000 bytes (decimal) or sometimes 1,024 bytes (binary, properly KiB). In modern strict usage, KB = 10³ bytes and KiB = 2¹⁰ bytes, but historical software and operating systems often interchanged them. The kilobyte was the standard file-size unit in early computing: a typical floppy disk held 360–1,440 KB, early word-processor documents were a few KB. Today, the kilobyte is rarely the primary user-facing unit (megabytes and gigabytes dominate), but it remains relevant for small files, source-code text, and embedded systems memory. The original Apple Macintosh (1984) shipped with 128 KB of RAM; the original IBM PC had 16–640 KB. The kilobyte relates to the byte (1,000 bytes = 1 KB decimal, 1,024 bytes = 1 KiB binary), the megabyte (1,000 KB = 1 MB), and the kilobit (1 KB = 8 kbit).

Visual reference for how the kilobyte relates to other data units. Each row links to the full converter for that pair.

• Small file sizes (icons, short documents)
• Network packet sizes
• Early-computing memory specifications

• ✓SI: 1 KB = 1000 B. Binary: 1 KB = 1024 B (KiB).
• ✓Storage makers use SI; RAM typically binary.
• ✓Old BIOS messages may show memory as "640K" meaning KiB.

• ✗Assuming 1 KB always = 1024 B — varies by context.
• ✗Confusing KB (storage) with Kb (kilobit, 1/8 of KB).
• ✗Mixing SI and binary without noting which.