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Unicode does not mean 2 bytes. Unicode defines code points that can be stored in many different ways (UCS-2, UTF-8, UTF-7, etc.). Encodings vary in simplicity and efficiency. Unicode has more than 65,535 (16 bits) worth of characters.
"Simple answer": A unicode character takes 1-4 bytes.
A byte is the smallest unit of data on a system. In general, 1 byte = 1 ASCII character. 2 bytes = 1 UTF-16 character. An unsigned byte can old the values 0-255.
Strangely enough, nobody pointed out how to calculate how many bytes is taking one Unicode char. Here is the rule for UTF-8 encoded strings:
Binary Hex Comments
0xxxxxxx 0x00..0x7F Only byte of a 1-byte character encoding
10xxxxxx 0x80..0xBF Continuation byte: one of 1-3 bytes following the first
110xxxxx 0xC0..0xDF First byte of a 2-byte character encoding
1110xxxx 0xE0..0xEF First byte of a 3-byte character encoding
11110xxx 0xF0..0xF7 First byte of a 4-byte character encoding
So the quick answer is: it takes 1 to 4 bytes, depending on the first one which will indicate how many bytes it'll take up.
You won't see a simple answer because there isn't one.
First, Unicode doesn't contain "every character from every language", although it sure does try.
Unicode itself is a mapping, it defines codepoints and a codepoint is a number, associated with usually a character. I say usually because there are concepts like combining characters. You may be familiar with things like accents, or umlauts. Those can be used with another character, such as an a or a u to create a new logical character. A character therefore can consist of 1 or more codepoints.
To be useful in computing systems we need to choose a representation for this information. Those are the various unicode encodings, such as utf-8, utf-16le, utf-32 etc. They are distinguished largely by the size of of their codeunits. UTF-32 is the simplest encoding, it has a codeunit that is 32bits, which means an individual codepoint fits comfortably into a codeunit. The other encodings will have situations where a codepoint will need multiple codeunits, or that particular codepoint can't be represented in the encoding at all (this is a problem for instance with UCS-2).
Because of the flexibility of combining characters, even within a given encoding the number of bytes per character can vary depending on the character and the normalization form. This is a protocol for dealing with characters which have more than one representation (you can say "an 'a' with an accent" which is 2 codepoints, one of which is a combining char or "accented 'a'" which is one codepoint).
I know this question is old and already has an accepted answer, but I want to offer a few examples (hoping it'll be useful to someone).
As far as I know old ASCII characters took one byte per character.
Right. Actually, since ASCII is a 7-bit encoding, it supports 128 codes (95 of which are printable), so it only uses half a byte (if that makes any sense).
How many bytes does a Unicode character require?
Unicode just maps characters to codepoints. It doesn't define how to encode them. A text file does not contain Unicode characters, but bytes/octets that may represent Unicode characters.
I assume that one Unicode character can contain every possible character from any language - am I correct?
No. But almost. So basically yes. But still no.
So how many bytes does it need per character?
Same as your 2nd question.
And what do UTF-7, UTF-6, UTF-16 etc mean? Are they some kind Unicode versions?
No, those are encodings. They define how bytes/octets should represent Unicode characters.
A couple of examples. If some of those cannot be displayed in your browser (probably because the font doesn't support them), go to http://codepoints.net/U+1F6AA (replace 1F6AA with the codepoint in hex) to see an image.
a
©
®
ጷ
—
‰
€
™
☃
☎
☔
☺
⚑
⚛
✈
✞
〠
肉
💩
🚀
Okay I'm getting carried away...
Fun facts:
Simply speaking Unicode is a standard which assigned one number (called code point) to all characters of the world (Its still work in progress).
Now you need to represent this code points using bytes, thats called character encoding. UTF-8, UTF-16, UTF-6 are ways of representing those characters.
UTF-8 is multibyte character encoding. Characters can have 1 to 6 bytes (some of them may be not required right now).
UTF-32 each characters have 4 bytes a characters.
UTF-16 uses 16 bits for each character and it represents only part of Unicode characters called BMP (for all practical purposes its enough). Java uses this encoding in its strings.
In UTF-8:
1 byte: 0 - 7F (ASCII)
2 bytes: 80 - 7FF (all European plus some Middle Eastern)
3 bytes: 800 - FFFF (multilingual plane incl. the top 1792 and private-use)
4 bytes: 10000 - 10FFFF
In UTF-16:
2 bytes: 0 - D7FF (multilingual plane except the top 1792 and private-use )
4 bytes: D800 - 10FFFF
In UTF-32:
4 bytes: 0 - 10FFFF
10FFFF is the last unicode codepoint by definition, and it's defined that way because it's UTF-16's technical limit.
It is also the largest codepoint UTF-8 can encode in 4 byte, but the idea behind UTF-8's encoding also works for 5 and 6 byte encodings to cover codepoints until 7FFFFFFF, ie. half of what UTF-32 can.
In Unicode the answer is not easily given. The problem, as you already pointed out, are the encodings.
Given any English sentence without diacritic characters, the answer for UTF-8 would be as many bytes as characters and for UTF-16 it would be number of characters times two.
The only encoding where (as of now) we can make the statement about the size is UTF-32. There it's always 32bit per character, even though I imagine that code points are prepared for a future UTF-64 :)
What makes it so difficult are at least two things:
U+20AC can be represented either as three-byte sequence E2 82 AC or four-byte sequence F0 82 82 AC.There is a great tool for calculating the bytes of any string in UTF-8: http://mothereff.in/byte-counter
Update: @mathias has made the code public: https://github.com/mathiasbynens/mothereff.in/blob/master/byte-counter/eff.js
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