Comparison of Unicode encodings

A UTF-8 file that contains only ASCII characters is identical to an ASCII file. Legacy programs can generally handle UTF-8 encoded files, even if they contain non-ASCII characters. For instance, the C printf function can print a UTF-8 string, as it only looks for the ASCII '%' character to define a formatting string, and prints all other bytes unchanged, thus non-ASCII characters will be output unchanged.

UTF-16 and UTF-32 are incompatible with ASCII files, and thus require Unicode-aware programs to display, print and manipulate them, even if the file is known to contain only characters in the ASCII subset. Because they contain many zero bytes, the strings cannot be manipulated by normal null-terminated string handling for even simple operations such as copy.[lower-alpha 1]

Therefore, even on most UTF-16 systems such as Windows and Java, UTF-16 text files are not common; older 8-bit encodings such as ASCII or ISO-8859-1 are still used, forgoing Unicode support; or UTF-8 is used for Unicode. One rare counter-example is the "strings" file used by macOS (Mac OS X 10.3 Panther and later) applications for lookup of internationalized versions of messages which defaults to UTF-16, with "files encoded using UTF-8 ... not guaranteed to work."[1]

XML is, by convention, encoded as UTF-8, and all XML processors must at least support UTF-8 (including US-ASCII by definition) and UTF-16.[2]

UTF-8 requires 8, 16, 24 or 32 bits (one to four bytes) to encode a Unicode character, UTF-16 requires either 16 or 32 bits to encode a character, and UTF-32 always requires 32 bits to encode a character. The first 128 Unicode code points, U+0000 to U+007F, used for the C0 Controls and Basic Latin characters and which correspond one-to-one to their ASCII-code equivalents, are encoded using 8 bits in UTF-8, 16 bits in UTF-16, and 32 bits in UTF-32.

The next 1,920 characters, U+0080 to U+07FF (encompassing the remainder of almost all Latin-script alphabets, and also Greek, Cyrillic, Coptic, Armenian, Hebrew, Arabic, Syriac, Tāna and N'Ko), require 16 bits to encode in both UTF-8 and UTF-16, and 32 bits in UTF-32. For U+0800 to U+FFFF, i.e. the remainder of the characters in the Basic Multilingual Plane (BMP, plane 0, U+0000 to U+FFFF), which encompasses the rest of the characters of most of the world's living languages, UTF-8 needs 24 bits to encode a character, while UTF-16 needs 16 bits and UTF-32 needs 32. Code points U+010000 to U+10FFFF, which represent characters in the supplementary planes (planes 1–16), require 32 bits in UTF-8, UTF-16 and UTF-32.

Therefore, a file is shorter in UTF-8 than in UTF-16 if there are more ASCII code points than there are code points in the range U+0800 to U+FFFF. A surprising result is that real-world documents written in languages that use characters only in the high range are still often shorter in UTF-8, due to the extensive use of spaces, digits, punctuation, newlines, html markup, and embedded words and acronyms written with Latin letters.[3] UTF-32 is always longer unless there are no code points less than U+10000.

All printable characters in UTF-EBCDIC use at least as many bytes as in UTF-8, and most use more, due to a decision made to allow encoding the C1 control codes as single bytes. For seven-bit environments, UTF-7 is more space efficient than the combination of other Unicode encodings with quoted-printable or base64 for almost all types of text (see "Seven-bit environments" below).

For processing, a format should be easy to search, truncate, and generally process safely. All normal Unicode encodings use some form of fixed size code unit. Depending on the format and the code point to be encoded, one or more of these code units will represent a Unicode code point. To allow easy searching and truncation, a sequence must not occur within a longer sequence or across the boundary of two other sequences. UTF-8, UTF-16, UTF-32 and UTF-EBCDIC have these important properties but UTF-7 and GB 18030 do not.

Fixed-size characters can be helpful, but even if there is a fixed byte count per code point (as in UTF-32), there is not a fixed byte count per displayed character due to combining characters. Considering these incompatibilities and other quirks among different encoding schemes, handling unicode data with the same (or compatible) protocol throughout and across the interfaces (e.g. using an API/library, handling unicode characters in client/server model, etc.) can in general simplify the whole pipeline while eliminating a potential source of bugs at the same time.

UTF-16 is popular because many APIs date to the time when Unicode was 16-bit fixed width (referred as UCS-2). However, using UTF-16 makes characters outside the Basic Multilingual Plane a special case which increases the risk of oversights related to their handling. That said, programs that mishandle surrogate pairs probably also have problems with combining sequences, so using UTF-32 is unlikely to solve the more general problem of poor handling of multi-code-unit characters.

If any stored data is in UTF-8 (such as file contents or names), it is very difficult to write a system that uses UTF-16 or UTF-32 as an API. This is due to the oft-overlooked fact that the byte array used by UTF-8 can physically contain invalid sequences. For instance, it is impossible to fix an invalid UTF-8 filename using a UTF-16 API, as no possible UTF-16 string will translate to that invalid filename. The opposite is not true: it is trivial to translate invalid UTF-16 to a unique (though technically invalid) UTF-8 string, so a UTF-8 API can control both UTF-8 and UTF-16 files and names, making UTF-8 preferred in any such mixed environment. An unfortunate but far more common workaround used by UTF-16 systems is to interpret the UTF-8 as some other encoding such as CP-1252 and ignore the mojibake for any non-ASCII data.

UTF-16 and UTF-32 do not have endianness defined, so a byte order must be selected when receiving them over a byte-oriented network or reading them from a byte-oriented storage. This may be achieved by using a byte-order mark at the start of the text or assuming big-endian (RFC 2781). UTF-8, UTF-16BE, UTF-32BE, UTF-16LE and UTF-32LE are standardised on a single byte order and do not have this problem.

If the byte stream is subject to corruption then some encodings recover better than others. UTF-8 and UTF-EBCDIC are best in this regard as they can always resynchronize after a corrupt or missing byte at the start of the next code point; GB 18030 is unable to recover until the next ASCII non-number. UTF-16 can handle altered bytes, but not an odd number of missing bytes, which will garble all the following text (though it will produce uncommon and/or unassigned characters).[lower-alpha 2] If bits can be lost all of them will garble the following text, though UTF-8 can be resynchronized as incorrect byte boundaries will produce invalid UTF-8 in almost all text longer than a few bytes.

The tables below list the number of bytes per code point for different Unicode ranges. Any additional comments needed are included in the table. The figures assume that overheads at the start and end of the block of text are negligible.

N.B. The tables below list numbers of bytes per code point, not per user visible "character" (or "grapheme cluster"). It can take multiple code points to describe a single grapheme cluster, so even in UTF-32, care must be taken when splitting or concatenating strings.