UTF-8

UTF-8 is a variable-width character encoding used for electronic communication. Defined by the Unicode Standard, the name is derived from Unicode Transformation Format – 8-bit.

UTF-8 is by far the most common encoding for the World Wide Web, accounting for over 96% of all web pages, and up to 100% for some languages, as of 2021. UTF-8 is the recommendation from the WHATWG for HTML and DOM specifications, and the Internet Mail Consortium recommends that all e-mail programs be able to display and create mail using UTF-8.

Encoding Rules

UTF-8 uses 1 ~ 4 bytes to encode each character, and can represent any standard Unicode character:

• Backward-compatible with ASCII, the first 128 UTF-8 characters precisely match the first 128 ASCII characters (numbered 0-127), meaning that existing ASCII text is already valid UTF-8. A ASCII character only needs 1 byte encoding (Unicode range is from U+0000 ~ U+007F)
• Latin, Greek, Cyrillic, Armenian, Hebrew, Arabic, Syrian and other letters with umlauts require 2 byte encoding (Unicode range from U+0080 ~ U+07FF)
• Characters in other languages (including Chinese, Japanese, Korean, Southeast Asian, Middle Eastern, etc.) contain most of the commonly used characters, using 3 byte encoding
• Other rarely used language characters use 4 byte encoding

UTF-8 encoding rules: if there is only one byte, the most significant bit is 0; if it is multi-byte, the first byte starts from the most significant bit, and the number of consecutive binary bits is 1. The number of bytes it encodes. The remaining bytes begin with 10.

The layout of UTF-8 byte sequences:

Number of bytes	First code point	Last code point	Byte 1	Byte 2	Byte 3	Byte 4
1	U+0000	U+007F	0xxxxxxx	-	-	-
2	U+0080	U+07FF	110xxxxx	10xxxxxx	-	-
3	U+0800	U+FFFF	1110xxxx	10xxxxxx	10xxxxxx	-
4	U+10000	U+10FFFF	11110xxx	10xxxxxx	10xxxxxx	10xxxxxx

More

• RFC 3629 - UTF-8, a transformation format of ISO 10646 (2003-11), which establishes UTF-8 as a standard Internet protocol element
• RFC 5198 - Unicode Format for Network Interchange (2008-03), defines UTF-8 NFC for Network Interchange (2008-03)
• ISO/IEC 10646:2020 - Information technology — Universal coded character set (UCS) (2020-12)