UTF-16

UTF-16 (16-bitUnicode Transformation Format) is a character encoding that supports all 1,112,064 valid code points of Unicode. The encoding is variable-length as code points are encoded with one or two 16-bitcode units. UTF-16 arose from an earlier obsolete fixed-width 16-bit encoding now known as UCS-2 (for 2-byte Universal Character Set), once it became clear that more than 2 (65,536) code points were needed, including most emoji and important CJK characters such as for personal and place names.

UTF-16 is used by the Windows API, and by many programming environments such as Java and Qt. The variable-length character of UTF-16, combined with the fact that most characters are not variable-length (so variable length is rarely tested), has led to many bugs in software, including in Windows itself.

UTF-16 is the only encoding (still) allowed on the web that is incompatible with 8-bit ASCII. It has never gained popularity on the web, where it is declared by under 0.004% of public web pages (and even then, the web pages are most likely also using UTF-8). UTF-8, by comparison, gained dominance years ago and accounted for 99% of all web pages by 2025. The Web Hypertext Application Technology Working Group (WHATWG) considers UTF-8 "the mandatory encoding for all [text]" and that for security reasons browser applications should not use UTF-16.