# What is ASCII?

> ASCII (American Standard Code for Information Interchange) is a 7-bit character encoding standard that maps 128 characters — including English letters, digits, punctuation, and control codes — to numeric values.

- URL: https://www.browserutils.dev/glossary/ascii
- Published: 2026-03-21
- Updated: 2026-03-16

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**ASCII (American Standard Code for Information Interchange)** is a 7-bit character encoding standard that maps 128 characters — including English letters, digits, punctuation, and control codes — to numeric values 0 through 127. Published in 1963, ASCII is the foundation that virtually all modern character encodings build upon.

## The character set

ASCII defines 128 characters split into two groups:

- **Control characters (0-31, 127)**: Non-printable characters like null (`\0`), tab (`\t`), newline (`\n`), carriage return (`\r`), escape (`\x1B`), and delete. These originally controlled teletype machines.
- **Printable characters (32-126)**: Space, digits 0-9, uppercase A-Z, lowercase a-z, and punctuation/symbols.

Key code points developers memorize:
```
48-57:   0-9
65-90:   A-Z
97-122:  a-z
32:      space
10:      newline (LF)
13:      carriage return (CR)
```

The difference between uppercase and lowercase letters is always 32 — a deliberate design choice that makes case conversion a single bit flip.

## Why ASCII still matters

ASCII is a subset of UTF-8. Every ASCII character has the same byte value in UTF-8, which is why ASCII compatibility was crucial for UTF-8's adoption. When a specification says "ASCII-safe," it means only bytes 0-127 are used, guaranteeing compatibility with any encoding.

ASCII is the baseline for:
- **Protocol headers**: HTTP, SMTP, and FTP headers are ASCII
- **Source code**: Most programming languages use ASCII for syntax (keywords, operators, identifiers)
- **Hostname rules**: Domain names originally required ASCII (before internationalized domain names)
- **File paths**: Many file systems work best with ASCII filenames

## Extended ASCII

"Extended ASCII" refers to various 8-bit encodings (like ISO 8859-1, Windows-1252) that use values 128-255 for additional characters. These are not standardized — different encodings map the upper 128 values to different characters. This incompatibility is exactly what Unicode and UTF-8 were designed to solve.

## Practical uses

Developers frequently need to look up ASCII values for debugging character encoding issues, writing binary protocols, or understanding escape sequences. The classic `man ascii` command on Unix systems remains useful.

Look up character codes in the [ASCII Table](/tools/ascii-table), convert between representations with [ASCII to Hex](/tools/ascii-to-hex), or create text art with [Image to ASCII](/tools/image-to-ascii).