UTF-16
UTF-16 (16-bit Unicode Transformation Format) is a character encoding capable of encoding all 1,112,064 valid code points of Unicode (in fact this number of code points is dictated by the design of UTF-16). The encoding is variable-length, as code points are encoded with one or two 16-bit code 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 216 (65,536) code points were needed, including most emoji and important CJK characters such as for personal and place names.
The first 216 Unicode code points. The white stripe near the bottom are the surrogate halves used by UTF-16. | |
| Language(s) | International |
|---|---|
| Standard | Unicode Standard |
| Classification | Unicode Transformation Format, variable-width encoding |
| Extends | UCS-2 |
| Transforms / Encodes | ISO/IEC 10646 (Unicode) |
UTF-16 is used by systems such as the Microsoft Windows API, the Java programming language and JavaScript/ECMAScript. It is also sometimes used for plain text and word-processing data files on Microsoft Windows. It is used by SMS (the SMS standard specifies UCS-2, but almost all users actually implement UTF-16 so that emojis work).
UTF-16 is the only encoding (still) allowed on the web that is incompatible with ASCII and never gained popularity on the web, where it is declared by under 0.004% of web pages (and many of these are actually UTF-8 but incorrectly marked). UTF-8, by comparison, accounts for over 98% of all web pages. 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.