UTF-8 is a variable width character encoding capable of encoding all 1,112,064^[1] valid code points in Unicode using one to four 8-bit bytes.^[2] The encoding is defined by the Unicode Standard, and was originally designed by Ken Thompson and Rob Pike.^[5][6] The name is derived from Unicode (or Universal Coded Character Set) Transformation Format – 8-bit.^[7]

It was designed for backward compatibility with ASCII. Code points with lower numerical values, which tend to occur more frequently, are encoded using fewer bytes. The first 128 characters of Unicode, which correspond one-to-one with ASCII, are encoded using a single byte with the same binary value as ASCII, so that valid ASCII text is valid UTF-8-encoded Unicode as well. Since ASCII bytes do not occur when encoding non-ASCII code points into UTF-8, UTF-8 is safe to use within most programming and document languages that interpret certain ASCII characters in a special way, such as "/" (slash) in filenames, "" (backslash) in escape sequences, and "%" in printf.

Since 2009, UTF-8 has been the dominant encoding (of any kind, not just of Unicode encodings) for the World Wide Web (and declared mandatory "for all things" by WHATWG^[9]) and as of September 2019 accounts for 94.0% of all web pages (some of which are simply ASCII, as it is a subset of UTF-8) and 95% of the top 1,000 highest ranked^[10] web pages. The next-most popular multi-byte encodings, Shift JIS and GB 2312, have 0.3% and 0.3% respectively.^[11][12][8] The Internet Mail Consortium (IMC) recommended that all e-mail programs be able to display and create mail using UTF-8,^[13] and the W3C recommends UTF-8 as the default encoding in XML and HTML.^[14]

Since the restriction of the Unicode code-space to 21-bit values in 2003, UTF-8 is defined to encode code points in one to four bytes, depending on the number of significant bits in the numerical value of the code point. The following table shows the structure of the encoding. The x characters are replaced by the bits of the code point. If the number of significant bits is no more than seven, the first line applies; if no more than 11 bits, the second line applies, and so on.

The first 128 characters (US-ASCII) need one byte. The next 1,920 characters need two bytes to encode, which covers the remainder of almost all Latin-script alphabets, and also Greek, Cyrillic, Coptic, Armenian, Hebrew, Arabic, Syriac, Thaana and N'Ko alphabets, as well as Combining Diacritical Marks. Three bytes are needed for characters in the rest of the Basic Multilingual Plane, which contains virtually all characters in common use^[15], including most Chinese, Japanese and Korean characters. Four bytes are needed for characters in the other planes of Unicode, which include less common CJK characters, various historic scripts, mathematical symbols, and emoji (pictographic symbols).

Some of the important features of this encoding are as follows:

Consider the encoding of the Euro sign, €:

The three bytes 1110 0010 1000 0010 1010 1100 can be more concisely written in hexadecimal, as E2 82 AC.

The following table summarises this conversion, as well as others with different lengths in UTF-8. The colors indicate how bits from the code point are distributed among the UTF-8 bytes. Additional bits added by the UTF-8 encoding process are shown in black.

Since UTF-8 uses groups of six bits, it is sometimes useful to use octal notation which uses 3-bit groups. With a calculator which can convert between hexadecimal and octal it can be easier to manually create or interpret UTF-8 compared with using binary.

When converting UTF-8 into code points, the following rules apply:

The following table summarizes usage of UTF-8 code units (individual bytes or octets) in a code page format. The upper half (0_ to 7_) is for bytes used only in single-byte codes, so it looks like a normal code page; the lower half is for continuation bytes (8_ to B_) and leading bytes (C_ to F_), and is explained further in the legend below.

Orange cells with a large dot are continuation bytes. The hexadecimal number shown after a "+" plus sign is the value of the six bits they add.

White cells are the leading bytes for a sequence of multiple bytes, the length shown at the left edge of the row. The text shows the Unicode blocks encoded by sequences starting with this byte, and the hexadecimal code point shown in the cell is the lowest character value encoded using that leading byte.

Red cells must never appear in a valid UTF-8 sequence. The first two red cells (C0 and C1) could be used only for a two-byte encoding of a 7-bit ASCII character which should be encoded in one byte; as described below such "overlong" sequences are disallowed. The red cells in the F row (F5 to FD) indicate leading bytes of 4-byte or longer sequences that cannot be valid because they would encode code points larger than the U+10FFFF limit of Unicode (a limit derived from the maximum code point encodable in UTF-16), and FE and FF were never defined for any purpose in UTF-8.

Pink cells are the leading bytes for a sequence of multiple bytes, of which some, but not all, possible continuation sequences are valid. E0 and F0 could start overlong encodings, in this case the lowest non-overlong-encoded code point is shown. F4 can start code points greater than U+10FFFF which are invalid. ED can start the encoding of a code point in the range U+D800–U+DFFF; these are invalid since they are reserved for UTF-16 surrogate halves.

In principle, it would be possible to inflate the number of bytes in an encoding by padding the code point with leading 0s. To encode the Euro sign € from the above example in four bytes instead of three, it could be padded with leading 0s until it was 21 bits long – 000 000010 000010 101100, and encoded as 11110000 10000010 10000010 10101100 (or F0 82 82 AC in hexadecimal). This is called an overlong encoding.

The standard specifies that the correct encoding of a code point use only the minimum number of bytes required to hold the significant bits of the code point. Longer encodings are called overlong and are not valid UTF-8 representations of the code point. This rule maintains a one-to-one correspondence between code points and their valid encodings, so that there is a unique valid encoding for each code point. This ensures that string comparisons and searches are well-defined.

Modified UTF-8 uses the two-byte overlong encoding of U+0000 (the NUL character), 11000000 10000000 (hexadecimal C0 80), instead of 00000000 (hexadecimal 00). This allows the byte 00 to be used as a string terminator.

Not all sequences of bytes are valid UTF-8. A UTF-8 decoder should be prepared for:

Many of the first UTF-8 decoders would decode these, ignoring incorrect bits and accepting overlong results. Carefully crafted invalid UTF-8 could make them either skip or create ASCII characters such as NUL, slash, or quotes. Invalid UTF-8 has been used to bypass security validations in high-profile products including Microsoft's IIS web server^[16] and Apache's Tomcat servlet container.^[17] RFC 3629 states "Implementations of the decoding algorithm MUST protect against decoding invalid sequences."^[18] The Unicode Standard requires decoders to "...treat any ill-formed code unit sequence as an error condition. This guarantees that it will neither interpret nor emit an ill-formed code unit sequence."

Some implementations of decoders throw exceptions on errors.^[19] This has the disadvantage that it can turn what would otherwise be harmless errors (such as a "no such file" error) into a denial of service. For instance early versions of Python 3.0 would exit immediately if the command line or environment variables contained invalid UTF-8.^[20]

An alternative practice is to replace errors with a replacement character. Replacement requires defining how many bytes are in the error. Using the length defined by the lead byte (so that E1,A0,C0 is one error) is a bad idea, as a missing byte will cause the next character to become an error as well. Since Unicode 6^[21] (October 2010), the standard (chapter 3) has “recommended” a "best practice" where the error ends as soon as a disallowed byte is encountered. In these decoders E1,A0,C0 is two errors (2 bytes in the first one). This means an error is no more than three bytes long and never contains the start of a valid character, and there are 21,632 different possible errors.^[22] The standard also recommends replacing each error with the replacement character "�" (U+FFFD).

Another popular practice is to turn each byte into an error. In this case E1,A0,C0 is three errors. This requires the UTF-8 parser to back up 2 bytes when an error is detected. The primary advantage is that there are now only 128 different errors. This allows the decoder to define 128 different error replacements such as:

The large number of invalid byte sequences provides the advantage of making it easy to have a program accept both UTF-8 and legacy encodings such as ISO-8859-1. Software can check for UTF-8 correctness, and if that fails assume the input to be in the legacy encoding. It is technically true that this may detect an ISO-8859-1 string as UTF-8, but this is very unlikely if it contains any 8-bit bytes as they all have to be in unusual patterns of two or more in a row, such as "£".

Since RFC 3629 ^[79] (November 2003), the high and low surrogate halves used by UTF-16 (U+D800 through U+DFFF) and code points not encodable by UTF-16 (those after U+10FFFF) are not legal Unicode values, and their UTF-8 encoding must be treated as an invalid byte sequence.

Not decoding unpaired surrogate halves makes it impossible to store invalid UTF-16 (such as Windows filenames or UTF-16 that has been split between the surrogates) as UTF-8. To preserve these invalid UTF-16 sequences, their corresponding UTF-8 encodings are sometimes allowed by implementations despite the above rule. There are attempts to define this behavior formally (see WTF-8 and CESU below).

The official Internet Assigned Numbers Authority (IANA) code for the encoding is "UTF-8".^[25] All letters are upper-case, and the name is hyphenated. This spelling is used in all the Unicode Consortium documents relating to the encoding.

Alternatively, the name "utf-8" may be used by all standards conforming to the IANA list (which include CSS, HTML, XML, and HTTP headers),^[26] as the declaration is case insensitive.^[25]

Other descriptions, such as those that omit the hyphen or replace it with a space, i.e. "utf8" or "UTF 8", are not accepted as correct by the governing standards.^[18] Despite this, most agents such as browsers can understand them, and so standards intended to describe existing practice (such as HTML5) may effectively require their recognition.^[27]

Unofficially, UTF-8-BOM and UTF-8-NOBOM are sometimes used to refer to text files which respectively contain and lack a byte order mark (BOM). In Japan especially, UTF-8 encoding without BOM is sometimes called "UTF-8N".^[28][29]

Supported Windows versions, i.e. Windows 7 and later, have codepage 65001, as a synonym for UTF-8 (with better support than in older Windows),^[30] and Microsoft has a script for Windows 10, to enable it by default for its notepad program.^[31]

In PCL, UTF-8 is called Symbol-ID "18N" (PCL supports 183 character encodings, called Symbol Sets, which potentially could be reduced to one, 18N, that is UTF-8).^[32]

The following implementations show slight differences from the UTF-8 specification. They are incompatible with the UTF-8 specification and may be rejected by conforming UTF-8 applications.

Many programs added UTF-8 conversions for UCS-2 data and did not alter this UTF-8 conversion when UCS-2 was replaced with the surrogate-pair using UTF-16. In such programs each half of a UTF-16 surrogate pair is encoded as its own three-byte UTF-8 encoding, resulting in six-byte sequences rather than four bytes for characters outside the Basic Multilingual Plane. Oracle and MySQL databases use this, as well as Java and Tcl as described below, and probably many Windows programs where the programmers were unaware of the complexities of UTF-16. Although this non-optimal encoding is generally not deliberate, a supposed benefit is that it preserves UTF-16 binary sorting order when CESU-8 is binary sorted.

In Modified UTF-8 (MUTF-8),^[33] the null character (U+0000) uses the two-byte overlong encoding 11000000 10000000 (hexadecimal C0 80), instead of 00000000 (hexadecimal 00). Modified UTF-8 strings never contain any actual null bytes but can contain all Unicode code points including U+0000,^[34] which allows such strings (with a null byte appended) to be processed by traditional null-terminated string functions.

All known Modified UTF-8 implementations also treat the surrogate pairs as in CESU-8.

In normal usage, the Java programming language supports standard UTF-8 when reading and writing strings through InputStreamReader ^[80] and OutputStreamWriter ^[81] (if it is the platform's default character set or as requested by the program). However it uses Modified UTF-8 for object serialization^[35] among other applications of DataInput ^[82] and DataOutput ^[83] , for the Java Native Interface,^[36] and for embedding constant strings in class files.^[37] The dex format defined by Dalvik also uses the same modified UTF-8 to represent string values.^[38] Tcl also uses the same modified UTF-8^[39] as Java for internal representation of Unicode data, but uses strict CESU-8 for external data.

WTF-8 (Wobbly Transformation Format – 8-bit) is an extension of UTF-8 where the encodings of unpaired surrogate halves (U+D800 through U+DFFF) are allowed.^[40] This is necessary to store possibly-invalid UTF-16, such as Windows filenames. Many systems that deal with UTF-8 work this way without considering it a different encoding, as it is simpler.

The term "WTF-8" has also been used humorously to refer to erroneously doubly-encoded UTF-8^[41][42] sometimes with the implication that CP1252 bytes are the only ones encoded.^[43]

Many Windows programs (including Windows Notepad) add the bytes 0xEF, 0xBB, 0xBF at the start of any document saved as UTF-8. This is the UTF-8 encoding of the Unicode byte order mark (BOM), and is commonly referred to as a UTF-8 BOM, even though byte order is irrelevant to UTF-8. While ASCII text encoded using UTF-8 normally is backwards compatible with ASCII, this is not true when Unicode Standard recommendations are ignored and a BOM is added. Non-UTF-8 software may show the BOM as three garbage characters, e.g. "" in software interpreting the document as ISO 8859-1 or Windows-1252, and "" if interpreted as code page 437.

The Unicode Standard neither requires nor recommends the use of the BOM for UTF-8, but warns that it may be encountered at the start of a file transcoded from another encoding.^[44] The presence of the UTF-8 BOM may cause problems with existing software that can handle UTF-8, for example:

Nonetheless, some software such as text editors will refuse to correctly display or interpret UTF-8 unless the text starts with a byte order mark or it contains only ASCII.

By early 1992, the search was on for a good byte-stream encoding of multi-byte character sets. The draft ISO 10646 standard contained a non-required annex called UTF-1 that provided a byte stream encoding of its 32-bit code points. This encoding was not satisfactory on performance grounds, among other problems, and the biggest problem was probably that it did not have a clear separation between ASCII and non-ASCII: new UTF-1 tools would be backward compatible with ASCII-encoded text, but UTF-1-encoded text could confuse existing code expecting ASCII (or extended ASCII), because it could contain continuation bytes in the range 0x21–0x7E that meant something else in ASCII, e.g., 0x2F for '/', the Unix path directory separator, and this example is reflected in the name and introductory text of its replacement. The table below was derived from a textual description in the annex.

In July 1992, the X/Open committee XoJIG was looking for a better encoding. Dave Prosser of Unix System Laboratories submitted a proposal for one that had faster implementation characteristics and introduced the improvement that 7-bit ASCII characters would only represent themselves; all multi-byte sequences would include only bytes where the high bit was set. The name File System Safe UCS Transformation Format (FSS-UTF) and most of the text of this proposal were later preserved in the final specification.^{[45][46][47][48]}

In August 1992, this proposal was circulated by an IBM X/Open representative to interested parties. A modification by Ken Thompson of the Plan 9 operating system group at Bell Labs made it somewhat less bit-efficient than the previous proposal but crucially allowed it to be self-synchronizing, letting a reader start anywhere and immediately detect byte sequence boundaries. It also abandoned the use of biases and instead added the rule that only the shortest possible encoding is allowed; the additional loss in compactness is relatively insignificant, but readers now have to look out for invalid encodings to avoid reliability and especially security issues. Thompson's design was outlined on September 2, 1992, on a placemat in a New Jersey diner with Rob Pike. In the following days, Pike and Thompson implemented it and updated Plan 9 to use it throughout, and then communicated their success back to X/Open, which accepted it as the specification for FSS-UTF.^[47]

UTF-8 was first officially presented at the USENIX conference in San Diego, from January 25 to 29, 1993.

In November 2003, UTF-8 was restricted by RFC 3629 ^[84] to match the constraints of the UTF-16 character encoding: explicitly prohibiting code points corresponding to the high and low surrogate characters removed more than 3% of the three-byte sequences, and ending at U+10FFFF removed more than 48% of the four-byte sequences and all five- and six-byte sequences.

Google reported that in 2008, UTF-8 (labelled "Unicode") became the most common encoding for HTML files.^[49] By 2018, most languages have use of UTF-8 up in the low to high 90%, including Greek at 97.1%.^[50] A few have even 100.0% use such as Kurdish, Pashto, Javanese, Kalaallisut (Greenlandic) and Iranian languages^[51] and Sign Languages.^[52] Exceptions include mainly Asian languages with Chinese at 88.0%,^[53] Japanese at 86.7% (while Mongolian is at 99.7%^[54]) and Breton at 70%.^[55]

International Components for Unicode (ICU) has historically used UTF-16, and still does only for Java; while for C/C++ UTF-8 is now supported as the "Default Charset",^[56] including the correct handling of "illegal UTF-8".^[57]

See also Comparison of Unicode encodings