UTF-8

UTF-8 is a variable-width character encoding that stores each Unicode code point in one to four bytes. The name is short for Unicode Transformation Format, where the 8 refers to its 8-bit code units. Ken Thompson and Rob Pike designed the encoding in 1992 for the Plan 9 operating system, and RFC 3629 later standardized it.

Code points up to 127 occupy a single byte with the same value as in ASCII, so every ASCII file is already valid UTF-8. Higher code points take two, three, or four bytes, and the leading byte’s bit pattern announces how many continuation bytes follow.

That design lets a decoder find the start of the next character after corrupted input, and since UTF-8 works byte by byte, it avoids the byte-order ambiguity of UTF-16 and UTF-32. Each code point’s range alone decides how many bytes it needs:

Flowchart of a teal Unicode code point box fanning out to four boxes pairing each code point range with its byte width, from the highlighted one byte ASCII range up to four bytes.
ASCII stays a single byte, while higher ranges take two, three, or four.

UTF-8 dominates modern computing. It encodes the overwhelming majority of web pages and is the required text encoding for JSON exchanged between systems. In Python, source files are UTF-8 by default, and str.encode() and bytes.decode() assume it when no other encoding is named.

Under a variable-width encoding, a string’s length in characters and its size in bytes are different numbers. Say a program needs to know how many bytes a message will occupy on disk. Encoding individual characters shows that each one’s width depends on its code point:

Python 
>>> "A".encode("utf-8")
b'A'
>>> "é".encode("utf-8")
b'\xc3\xa9'
>>> "🐍".encode("utf-8")
b'\xf0\x9f\x90\x8d'

>>> len("café"), len("café".encode("utf-8"))
(4, 5)

Calling .decode("utf-8") on the bytes restores the original string. Decoding them with a different encoding instead raises an error or produces garbled text, commonly called mojibake.

Unicode & Character Encodings in Python: A Painless Guide

Tutorial

Unicode & Character Encodings in Python: A Painless Guide

In this tutorial, you'll get a Python-centric introduction to character encodings and unicode. Handling character encodings and numbering systems can at times seem painful and complicated, but this guide is here to help with easy-to-follow Python examples.

advanced python

For additional information on related topics, take a look at the following resources:

By Martin Breuss • Updated July 12, 2026
Computer Science Glossary Share Feedback