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Text as Numbers: ASCII to Unicode

⏱ 6 min read

In one sentence: Computers store text by giving every character a number; ASCII was an early list of 128 characters, and Unicode is today's list for almost every writing system in the world.

Why should I care?

Every message and web page you have ever read was stored as numbers. A computer stores only numbers, in Binary. So for a word, it stores one number for each letter.

You may also have seen text go wrong: a name like "José" shown with a wrong letter or strange symbols in place of the "é". This can happen when two computers do not agree on which number means which letter.

Analogy

Think of a football team. Every player wears a numbered shirt. With the team list, you know exactly who "number 10" is.

A computer does the same with text. It keeps a list where every character has a number. (A character is one letter, digit, sign or symbol, and even a space counts as one.) To store a character, it stores the number.

But another team may have a different number 10. Old computer lists had the same problem: one number meant different letters in different lists.

Where the analogy breaks down: on a football field, you can still see the player's face. In a computer, only the number is stored. Without the right list, the number tells you nothing about which character it was.

How it actually works

Every character gets a number

The list that gives characters their numbers is called a character set. The number of one character is called its code point.

When the computer stores the number, it writes it as bits in one or more bytes. The rules for turning code points into bytes, and back again, are called a character encoding.

character:     A          a
code point:    65         97
stored byte:   01000001   01100001

A and a have different numbers: to a computer, they are two different characters.

ASCII: 128 characters

ASCII is short for American Standard Code for Information Interchange. It is an early character set. It has 128 code points, numbered 0 to 127, so each one fits in 7 bits. In an 8-bit byte, the extra bit on the left is 0 (see the table above).

  • 95 of them are printable characters: the English letters A to Z and a to z, the digits 0 to 9, a space and signs like ! and ?.
  • The other 33 are control characters: codes meant to control devices, such as printers, not letters or signs.

In ASCII, the digit 0 is 48 and a space is 32. The capital letters are in order: A is 65, B is 66, and so on up to Z, which is 90.

Careful: when you type the character 0, the computer stores the number 48, not the number 0.

ASCII was made mainly for English: it has no letters with accents, like é.

One byte, many lists

Before Unicode, there were hundreds of different character encodings, and they often gave different meanings to the same numbers. Many used one byte per character, so each could hold at most 256 characters. In these, a character's number was also its byte.

For example, take two of these lists, ISO 8859-1 and ISO 8859-5. In ISO 8859-1, the number 233 means é. In ISO 8859-5, the same number 233 means щ, a letter from another alphabet, called Cyrillic. Only the list tells you which letter it is. Use the wrong list, and you see the wrong letters.

Unicode: one list for the world

Unicode solves this with one list that gives a unique number to every character, in any language. Its edition from September 2026, version 18.0, has 172,808 characters.

The first 128 Unicode characters are the ASCII characters, with the same numbers. So A is 65 in both.

Unicode numbers are often written in base 16 (you met it in Counting Systems: Base 10 and Other Bases) after the letters U+. For example, A is written U+0041. In base 16, 41 means 4 × 16 + 1 = 65. The zeros in front change nothing.

UTF-8: turning Unicode numbers into bytes

A Unicode number is not yet bytes. The rules for storing it as bytes are an encoding. The most widely used one for web pages is UTF-8.

UTF-8 uses 1 to 4 bytes per character. ASCII characters use one byte, the same byte as in ASCII. So text written only with ASCII characters is also correct UTF-8. Other characters use more bytes: é, for example, takes two.

How it came to be

In 1963, a committee of the American Standards Association published the first edition of ASCII. That first edition left 28 code points without a meaning, for later use. ASCII had a major revision in 1967 and more until 1986. This page describes the revised ASCII. ASCII is the basis of most modern character encodings.

The first volume of Unicode 1.0 was published in October 1991.

UTF-8 was designed around September 1992 by Ken Thompson, with Rob Pike supporting him. Pike later wrote that Thompson designed it on a placemat in a diner, a type of restaurant, in New Jersey, USA. In December 2007, UTF-8 became more common on the web than ASCII.

🧪 Try it in your browser

You will use the browser console, the tool from Counting Systems: Base 10 and Other Bases where you type a short instruction and see the answer.

Safety rule: only type instructions in the console that you understand or that come from a source you trust. Never paste anything a stranger sends you. The lines on this page are safe.

  1. In Chrome, Edge or Firefox, open any ordinary web page, for example https://example.com.

  2. Open the console:

    • Windows: press Ctrl + Shift + J (in Firefox: Ctrl + Shift + K)
    • Mac: press Cmd + Option + J (in Firefox: Cmd + Option + K)

  3. You may see some messages already. Click the empty line at the bottom.

  4. Type this line yourself (some browsers block pasting in the console), then press Enter:

    "A".codePointAt(0)

    The (0) means "the first character": in code, counting often starts at 0. If the browser adds a bracket or quote mark by itself, that is fine.

  5. You should see 65: the code point of A. Change "A" to "a" and press Enter again. You should see 97.

  6. Now go the other way. Type this line and press Enter:

    String.fromCodePoint(65, 90)

  7. You should see "AZ" or 'AZ': the characters with the numbers 65 and 90.

  8. Now a character outside ASCII. Type String.fromCodePoint(9731) and press Enter. You should see a snowman, maybe inside quote marks: '☃' or "☃".

  9. Close the tab when you are done.

On a phone: use paper. Capital letters in ASCII go in order from A = 65 to Z = 90. Write your name in capital letters, then write the number under each letter.

Common misunderstandings

  • "Computers store letters as small pictures." No. They store a number for each character. The shape you see comes from a font, a collection of letter shapes.
  • "Unicode and UTF-8 are the same thing." No. Unicode gives every character a number. UTF-8 is one way to store those numbers as bytes.
  • "ASCII is gone." Not really. ASCII lives on inside Unicode and UTF-8, with the same numbers and bytes.

Recap

  • A computer stores text by giving each character a number, called a code point.
  • ASCII (first published in 1963) has 128 code points. Unicode is one list with a unique number for every character in any language.
  • An encoding such as UTF-8 turns those numbers into bytes: 1 to 4 bytes per character.

Sources

Written with AI · Reviewed by AI · Not yet checked by a human

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