UTF-1 is a way of transforming ISO 10646/Unicode into a stream of bytes. Due to the design it is not possible to resynchronise if decoding starts in the middle of a character (this makes truncation hard, among other things) and simple byte-oriented search routines cannot be reliably used with it. UTF-1 is also fairly slow due to its use of division. Due to these issues, UTF-1 never gained wide acceptance and has been almost totally replaced by UTF-8. Design UTF-1 is a multi-byte encoding like UTF-8; a single Unicode code point can be encoded in one, two, three, or five octets. While the ASCII range is encoded as one octet as in UTF-8 the ASCII octets 0x21 - 0x7E (decimal 33 - 126) are also used in UTF-1 multi-byte encodings, therefore UTF-1 is unsuited for many Internet protocols including MIME. UTF-1 does not use the C0 and C1 control codes in other encodings – any 0x00–0x20 or 0x7F–0x9F octet stands for the corresponding code points in ISO-8859-1 (U+0000–0020 and U+007F–009F, respectively). This design with 66 protected octets tried to be ISO 2022 compatible.

The UTF-1 encoding scheme uses "modulo 190" arithmetic (256 − 66 = 190), it was designed to encode the complete 31 bits of the original Universal Character Set (UCS-4). For comparison, UTF-8 protects all 128 ASCII octets, and needs two bits in trail bytes of multi-byte encodings for this purpose, resulting in "modulo 64" arithmetic (8 − 2 = 6, 26 = 64). BOCU-1 protects only the minimal set required for MIME-compatibility (0x00, 0x07–0x0F, 0x1A–0x1B, and 0x20), resulting in "modulo 243" arithmetic (256 − 13 = 243). codepoint UTF-16BE UTF-16LE UTF-8 UTF-1 U+007F 007F 7F00 7F 7F U+0080 0080 8000 C280 80 U+009F 009F 9F00 C29F 9F U+00A0 00A0 A000 C2A0 A0A0 U+00BF 00BF BF00 C2BF A0BF U+00C0 00C0 C000 C380 A0C0 U+00FF 00FF FF00 C3BF A0FF U+0100 0100 0001 C480 A121 U+015D 015D 5D01 C59D A17E U+015E 015E 5E01 C59E A1A0 U+01BD 01BD BD01 C6BD A1FF U+01BE 01BE BE01 C6BE A221 U+07FF 07FF FF07 DFBF AA72 U+0800 0800 0008 E0A080 AA73 U+0FFF 0FFF FF0F E0BFBF B548 U+1000 1000 0010 E18080 B549 U+4015 4015 1540 E48095 F5FF U+4016 4016 1640 E48096 F62121 U+D7FF D7FF FFD7 ED9FBF F72FC3 U+E000 E000 00E0 EE8080 F73A79 U+F8FF F8FF FFF8 EFA3BF F75C3C U+FDD0 FDD0 D0FD EFB790 F762BA U+FDEF FDEF EFFD EFB7AF F762D9 U+FEFF FEFF FFFE EFBBBF F7644C U+FFFD FFFD FDFF EFBFBD F765AD U+FFFE FFFE FEFF EFBFBE F765AE U+FFFF FFFF FFFF EFBFBF F765AF U+10000 D800DC00 00D800DC F0908080 F765B0 U+38E2D D8A3DE2D A3D82DDE F0B8B8AD FBFFFF U+38E2E D8A3DE2E A3D82EDE F0B8B8AE FC21212121 U+FFFFF DBBFDFFF BFDBFFDF F3BFBFBF FC2137B27A U+100000 DBC0DC00 C0DB00DC F4808080 FC2137B27B U+10FFFF DBFFDFFF FFDBFFDF F48FBFBF FC21396E6C See also Comparison of Unicode encodings Universal Character Set References ISO IR 178 (PDF, 256 KB, the retired UTF-1 specification) v · d · eUnicode Unicode Unicode Consortium · ISO/IEC 10646 (Universal Character Set) Code points Code point · Plane · Block · Mapping characters · Character property · Character charts Characters Special purpose BOM · Combining grapheme joiner · Left-to-right mark and Right-to-left mark · Zero-width non-breaking space · Zero-width joiner · Zero-width non-joiner · Zero-width space Miscellaneous lists Combining character · Duplicate characters · Graphic characters Processing Algorithms Bi-directional text · Collation (ISO 14651) · Equivalence Transformation BOCU-1 · CESU-8 · UTF-1 · UTF-7 · UTF-8 · UTF-9/UTF-18 · UTF-16/UCS-2 · UTF-32/UCS-4 · UTF-EBCDIC · Punycode · SCSU · Comparison On pairs of code points Equivalence · Combining character · Duplicates · Homoglyph · Precomposed character (List) · Compatibility characters · Z-variant Usage Unicode and e-mail · Unicode and HTML · Character entity references · Unicode input · Internationalized domain name · Numeric character reference · Private Use U+F8FF · Typefaces (fonts) · Script (Unicode) Related standards Common Locale Data Repository (CLDR) · GB 18030 · Han unification · ISO/IEC 8859 (8-bit encodings) · ISO 14651 (Collation) · ISO 15924 (Script codes) Related topics Anomalies · ConScript Unicode Registry · Ideographic Rapporteur Group · International Components for Unicode · MUFI · People related to Unicode  Scripts and symbols in Unicode Common and inherited scripts Combining marks · Diacritics · Punctuation · Space Modern scripts Arabic (diacritics · Unicode blocks) · Armenian · Balinese · Batak · Bamum · Bengali · Bopomofo · Braille · Buginese · Buhid · Canadian Aboriginal · Cham · Cherokee · CJK Unified Ideographs (Han) · Cyrillic · Deseret · Devanagari · Ethiopic · Georgian · Greek · Gujarati · Gurmukhi · Kanji · Hanja · Hán tự · Hangul · Hanunoo · Hebrew (diacritics) · Hiragana · Javanese · Kannada · Katakana · Kayah Li · Khmer · Lao · Latin · Lepcha · Limbu · Lisu · Malayalam · Mandaic · Meetei Mayek · Mongolian · Manchu · Myanmar · N'Ko · New Tai Lue · Ol Chiki · Oriya · Osmanya · Rejang · Samaritan · Saurashtra · Shavian · Sinhala · Sundanese · Syloti Nagri · Syriac · Tagalog · Tagbanwa · Tai Le · Tai Tham · Tai Viet · Tamil · Telugu · Thaana · Thai · Tibetan · Tifinagh · Vai · Yi Ancient and historic scripts Avestan · Brāhmī · Carian · Coptic · Sumero-Akkadian · Cypriot · Egyptian Hieroglyphs · Glagolitic · Gothic · Imperial Aramaic · Inscriptional Pahlavi · Inscriptional Parthian · Kaithi · Kharoshthi · Linear B · Lycian · Lydian · Ogham · Old Italic · Old Persian · Phags-pa · Phoenician · Old South Arabian · Old Turkic · Runic · Ugaritic Symbols Cultural, political, and religious symbols · Currency · Mathematical operators and symbols · Phonetic symbols (including IPA) v · d · eCharacter encodings Character sets Early telecommunications ASCII · ISO/IEC 646 · ISO/IEC 6937 · T.61 · sixbit code pages · Baudot code · Morse code ISO/IEC 8859 -1 · -2 · -3 · -4 · -5 · -6 · -7 · -8 · -9 · -10 · -11 · -12 · -13 · -14 · -15 · -16 Bibliographic use ANSEL · ISO 5426 / 5426-2 / 5427 / 5428 / 6438 / 6861 / 6862 / 10585 / 10586 / 10754 / 11822 · MARC-8 National standards ArmSCII · CNS 11643 · GOST 10859 · GB 2312 · HKSCS · ISCII · JIS X 0201 · JIS X 0208 · JIS X 0212 · JIS X 0213 · KPS 9566 · KS X 1001 · PASCII · TIS-620 · TSCII · VISCII · YUSCII EUC CN · JP · KR · TW ISO/IEC 2022 CN · JP · KR · CCCII MacOS codepages ("scripts") Arabic · CentralEurRoman · ChineseSimp / EUC-CN · ChineseTrad / Big5 · Croatian · Cyrillic · Devanagari · Dingbats · Farsi · Greek · Gujarati · Gurmukhi · Hebrew · Icelandic · Japanese / ShiftJIS · Korean / EUC-KR · Roman · Romanian · Symbol · Thai / TIS-620 · Turkish · Ukrainian DOS codepages 437 · 720 · 737 · 775 · 850 · 852 · 855 · 857 · 858 · 860 · 861 · 862 · 863 · 864 · 865 · 866 · 869 · Kamenický · Mazovia · MIK · Iran System Windows codepages 874 / TIS-620 · 932 / ShiftJIS · 936 / GBK · 949 / EUC-KR · 950 / Big5 · 1250 · 1251 · 1252 · 1253 · 1254 · 1255 · 1256 · 1257 · 1258 · 1361 · 54936 / GB18030 EBCDIC codepages 37/1140 · 273/1141 · 277/1142 · 278/1143 · 280/1144 · 284/1145 · 285/1146 · 297/1147 · 420/16804 · 424/12712 · 500/1148 · 838/1160 · 871/1149 · 875/9067 · 930/1390 · 933/1364 · 937/1371 · 935/1388 · 939/1399 · 1025/1154 · 1026/1155 · 1047/924 · 1112/1156 · 1122/1157 · 1123/1158 · 1130/1164 · JEF · KEIS Platform specific ATASCII · CDC display code · DEC-MCS · DEC Radix-50 · Fieldata · GSM 03.38 · HP roman8 · PETSCII · TI calculator character sets · ZX Spectrum character set Unicode / ISO/IEC 10646 UTF-8 · UTF-16/UCS-2 · UTF-32/UCS-4 · UTF-7 · UTF-EBCDIC · GB 18030 · SCSU · BOCU-1 Miscellaneous codepages APL · Cork · HZ · IBM code page 1133 · KOI8 · TRON Related topics control character (C0 C1) · CCSID · Character encodings in HTML · charset detection · Han unification · ISO 6429/IEC 6429/ANSI X3.64 · mojibake