Understanding Character Encoding: ASCII vs Unicode vs UTF-8

“ASCII,” “Unicode,” and “UTF-8” get used almost interchangeably in casual conversation, but they answer three different questions: which characters exist, which number represents each one, and how that number gets stored as actual bytes.

Three Different Layers

• What Is ASCII?: an old, limited character set defining 128 characters, each with a fixed number.
• Understanding Unicode: a much larger character set (well over 140,000 characters) that includes ASCII’s 128 characters as its starting range, then extends far beyond it.
• UTF-8: a specific method for encoding Unicode code points as actual bytes, using a variable number of bytes per character.

ASCII and Unicode are character sets, definitions of which number stands for which character. UTF-8 is an encoding, a rule for turning those numbers into bytes that can actually be stored or transmitted.

Why UTF-8 Is Backward-Compatible With ASCII

Common Mistakes

• Saying “UTF-8” when you mean “Unicode,” or vice versa. One is a character set, the other is an encoding of that character set; they answer different questions.
• Assuming every character takes the same number of bytes. UTF-8 is variable-length; basic Latin characters take 1 byte, while many other scripts and emoji take 2 to 4.
• Forgetting other Unicode encodings exist. UTF-16 and UTF-32 also encode the same Unicode code points, just using different byte-length rules, so “Unicode” alone doesn’t specify a byte format.

Use the Text Encoding Tools

The ASCII Text Converter, Unicode Text Converter, Hex to Text Converter, and Binary to Text Converter all help work with these representations directly.