How many characters can UTF-8 encode?

If UTF-8 is 8 bits, does it not mean that there can be only maximum of 256 different characters?

The first 128 code points are the same as in ASCII. But it says UTF-8 can support up to million of characters?

How does this work?

回答1

UTF-8 does not use one byte all the time, it's 1 to 4 bytes.

The first 128 characters (US-ASCII) need one byte.

The next 1,920 characters need two bytes to encode. This covers the remainder of almost all Latin alphabets, and also Greek, Cyrillic, Coptic, Armenian, Hebrew, Arabic, Syriac and Tāna 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[12] including most Chinese, Japanese and Korean [CJK] 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).

source: Wikipedia

回答2

2,164,864 “characters” can be potentially coded by UTF-8.

This number is 2⁷ + 2¹¹ + 2¹⁶ + 2²¹, which comes from the way the encoding works:

1-byte chars have 7 bits for encoding 0xxxxxxx (0x00-0x7F)
2-byte chars have 11 bits for encoding 110xxxxx 10xxxxxx (0xC0-0xDF for the first byte; 0x80-0xBF for the second)
3-byte chars have 16 bits for encoding 1110xxxx 10xxxxxx 10xxxxxx (0xE0-0xEF for the first byte; 0x80-0xBF for continuation bytes)
4-byte chars have 21 bits for encoding 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx (0xF0-0xF7 for the first byte; 0x80-0xBF for continuation bytes)

As you can see this is significantly larger than current Unicode (1,112,064 characters).

UPDATE

My initial calculation is wrong because it doesn't consider additional rules. See comments to this answer for more details.