https://en.wikipedia.org/wiki/UTF-8
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Encoding
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.

Code point <-> UTF-8 conversion
First code point	Last code point	Byte 1	Byte 2	Byte 3	Byte 4
U+0000	U+007F	0xxxxxxx	
U+0080	U+07FF	110xxxxx	10xxxxxx	
U+0800	U+FFFF	1110xxxx	10xxxxxx	10xxxxxx	
U+10000	[nb 2]U+10FFFF	11110xxx	10xxxxxx	10xxxxxx	10xxxxxx
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,[20] 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).

A "character" can actually take more than 4 bytes, e.g. an emoji flag character takes 8 bytes since it's "constructed from a pair of Unicode scalar values".[21] Byte-count can go up to at least 17 for valid sets of combining characters.[22]
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