What Is ASCII? American Standard Code for Information Interchange Explained

ASCII, short for American Standard Code for Information Interchange, is a character encoding that assigns numbers from 0 to 127 to letters, digits, punctuation and control codes. Each ASCII character fits in 7 bits, so a computer can store the letter A as the number 65 and share it with any other system that follows the same standard. ASCII is the foundation of modern text encodings such as UTF-8.

Key takeaways

• ASCII defines 128 codes: 33 control codes (0 to 31 and 127) and 95 printable characters (32 to 126, including space).
• Uppercase letters start at 65, lowercase at 97 and the digits 0 to 9 at 48.
• An uppercase letter and its lowercase form differ by 32.
• Extended ASCII uses 8 bits, but it is not one standard. Different code pages define codes 128 to 255 differently.
• Unicode includes ASCII as its first 128 code points, and UTF-8 is backward compatible with ASCII.

What is ASCII?

ASCII is a standard that maps characters to numeric codes so that computers and devices can exchange text. It was first published in 1963 by the American Standards Association, revised in later years and is formally known as ANSI X3.4. Before character encodings like this, different makers used incompatible codes, which made it hard to move text between machines. ASCII solved that problem for English text and basic symbols.

How does ASCII work?

ASCII works by giving every character a fixed number, which a computer stores in binary. The capital letter A is code 65, which is 1000001 in binary. When you type the letter, the keyboard sends the code, and when software displays text it converts the code back to a shape. The diagram shows how the 128 codes divide into control codes, digits, letters and symbols.

What is in the ASCII table?

The ASCII table lists every code with its character. These ranges and common codes cover most daily use:

Notice that a lowercase letter is its uppercase form plus 32, which in binary is a single bit flip. That design makes case conversion fast.

What is extended ASCII?

Extended ASCII refers to 8-bit encodings that keep codes 0 to 127 the same as ASCII and use codes 128 to 255 for extra characters, such as accented letters. There is no single extended ASCII standard. ISO 8859-1 (Latin-1) and Windows-1252 are common examples, and they differ in places, which is why text sometimes shows wrong symbols when opened with the wrong encoding.

ASCII vs Unicode: what is the difference?

ASCII covers 128 characters, mostly English letters, digits and symbols, while Unicode covers well over a hundred thousand characters from many languages and emoji. The first 128 Unicode code points are identical to ASCII. UTF-8, the most common Unicode encoding on the web, stores ASCII characters as the same single bytes, so a plain ASCII file is also valid UTF-8.

How do you convert text to ASCII codes?

You can convert characters to ASCII codes with a short function in almost any language:

• Python: ord("A") returns 65 and chr(65) returns A.
• JavaScript: "A".charCodeAt(0) returns 65 and String.fromCharCode(65) returns A.
• PowerShell: [int][char]"A" returns 65 and [char]65 returns A. Our guide to the PowerShell do while loop shows how to use that kind of conversion in a loop.

In loops that walk through characters, a missing exit condition can run forever. Read about the causes and fixes in our guide to infinite loops in programming.

Where is ASCII still used?

ASCII is still used for plain text files, source code, configuration files, email headers, protocol commands such as HTTP and SMTP, terminal control codes and ASCII art. Identifiers such as a URN (uniform resource name) are also limited to a safe set of characters, many of which come from ASCII. For some history of how early communication systems agreed on shared standards, see our history of telephony.

Frequently asked questions

What does ASCII stand for?

ASCII stands for American Standard Code for Information Interchange. It is a character encoding standard for text.

How many characters are in ASCII?

ASCII has 128 codes, numbered 0 to 127. Of these, 95 are printable characters and 33 are control codes.

What is the ASCII code for A?

Uppercase A is 65, and lowercase a is 97. The digit 0 is 48.

Is ASCII the same as UTF-8?

No, but they are compatible. UTF-8 encodes the first 128 Unicode characters exactly like ASCII, then uses more bytes for other characters.

Why does text sometimes show strange symbols?

This usually happens when a file is read with the wrong encoding, such as a Latin-1 file opened as UTF-8. Choose the correct encoding or save the file as UTF-8.

Conclusion

ASCII is the simple, durable base layer of digital text: 128 codes that turned letters and symbols into numbers every computer could share. Understanding it helps you debug encoding problems, work with text in code and see why UTF-8 became the standard for the modern web.