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J277 1.2.4 Characters and text

Part 1 of 5 · J277 1.2.4-1.2.5 · Data representation

How text is stored in binary, covering character sets, ASCII, Unicode and text-file size.

Students will:

  • explain what a character set is
  • explain how characters are represented in binary
  • relate bits per character to the number of available codes
  • compare ASCII and Unicode
  • calculate the size of a simple text file

Inside: 6 explanation cells, 4 multiple-choice questions, 1 fill-in-the-blanks cell and 5 written answers. 27 marks, about 60 minutes.

Series: J277 1.2.4-1.2.5 · Data representation, part 1 of 5.

Shared by Coding PathwayVerified teacher

  • 16 cells
  • About 60 minutes
  • CC BY-SA 4.0
  • Shared 17 Aug 2026
  • Updated 9 Sept 2026

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The whole resource, exactly as a class sees it. Answers and marking are held back.

Data representation 1: characters and text

Computers store text using binary. To make this possible, each letter, digit, punctuation mark and symbol is matched to a binary code.

By the end of this worksheet, you should be able to:

  • explain what a character set is
  • explain how characters are represented in binary
  • relate the number of bits per character to the number of available codes
  • compare ASCII and Unicode
  • explain what it means for character codes to be logically ordered
  • calculate the size of a simple text file

You will not be asked to memorise individual ASCII or Unicode codes.

Work through the explanation, checks and applications in order. Use the final challenge to connect several ideas.

Multiple choice1 mark

Which statement explains why text must be represented in binary?

  • ABinary contains every letter of the alphabet
  • BComputer circuits represent two states using 0 and 1
  • CText becomes smaller whenever it is changed to binary
  • DHumans can read binary faster than text

1. Character sets

A character can be a letter, digit, punctuation mark, space or other symbol.

A character set is an agreed collection that matches each character to a unique binary code. The computer stores the code. Software uses the character set to interpret the code as the intended character.

Unique codes matter. If two characters had the same code in one character set, the computer could not tell which character was intended.

Fill in the blanks4 marks
A label 1 matches each character to a unique label 2. A character may be a letter, digit, punctuation mark or label 3. The code must be unique so the computer can label 4 different characters.
  • sample rate
  • space
  • distinguish
  • character set
  • binary code
  • pixel
Written answer3 marks

Explain what a character set is and why each character needs a unique code.

Define the mapping, then explain what would go wrong if two characters shared one code.

Students type their answer here.

2. Bits limit the number of characters

More bits per character create more possible bit patterns, so the character set can represent more different characters.

  • 1 bit provides 2 possible codes.
  • 2 bits provide 4 possible codes.
  • 4 bits provide 16 possible codes.
  • 8 bits provide 256 possible codes.

Do not confuse the number of bits with the number of characters. Four bits can represent up to 16 different characters, not four.

In OCR exam questions, an ASCII code will be shown using 8 bits.

Multiple choice1 mark

What is the maximum number of different characters that can be represented using 4 bits?

  • A8
  • B4
  • C32
  • D16

3. ASCII and Unicode

ASCII is a character set historically designed for English letters, digits, punctuation and control characters. OCR presents ASCII codes as 8 bits in the examination, giving up to 256 possible bit patterns.

Unicode is designed to represent a far wider range of characters from writing systems around the world, as well as many symbols. Unicode encodings can use different numbers of bits, so use the value supplied in a question rather than memorising that Unicode always has one fixed size.

There is a clear balance: using more bits per character allows a larger range of characters, but it can increase the storage needed for the same amount of text.

Logically ordered codes

Character sets are logically ordered. If the code for A is given, the code for B is one greater and the code for C is one greater again. OCR may provide a code and ask you to infer a nearby character. You do not need to memorise the code for A, and uppercase and lowercase characters have different codes.

Multiple choice1 mark

Which statement gives the most accurate comparison?

  • AUnicode can represent a wider range of characters, while using more bits per character can increase file size
  • BASCII represents every writing system and symbol using one bit
  • CUnicode can store only English capital letters
  • DASCII and Unicode must always use exactly the same number of bits
Multiple choice1 mark

In a logically ordered character set, the code for B is one greater than the code for A. What can be concluded about the code for C?

  • AIt must be identical to the code for A
  • BIt must contain no binary digits
  • CIt is one greater than the code for B
  • DIt is the code for lowercase c
Written answer2 marks

Part of a character set uses these 8-bit codes: A = 01000001, C = 01000011, T = 01010100. Write the binary codes needed to represent the word TACT, in the correct order.

Keep one 8-bit code for each character. Do not add the four codes together.

Students type their answer here.

4. Text-file size

For a simple uncompressed text file:

file size in bits = bits per character × number of characters

Example:

A file contains 600 characters and uses 8 bits per character.

600 × 8 = 4,800 bits

4,800 ÷ 8 = 600 bytes

If a question asks for kilobytes and states 1 KB = 1,000 bytes, divide by 1,000 again.

Written answer4 marks

A text file contains 2,500 characters. It uses 8 bits per character. Calculate the file size in bits, bytes and kilobytes. Use 1 KB = 1,000 bytes and show your working.

Multiply first, then divide by 8 and by 1,000.

Students type their answer here.

Written answer4 marks

A museum database must store names written in many different writing systems. Explain why a Unicode character set is more suitable than a limited ASCII character set.

Link the range of available codes to the range of required characters. You may also explain that using more bits per character can increase storage needs.

Students type their answer here.

Written answer6 marks

Challenge: a student says, “Using twice as many bits per character has no effect because the text still looks the same.” Evaluate this statement.

Discuss the number of available codes, the range of characters and the file-size formula before reaching a conclusion.

Students type their answer here.

Closed-book review

  1. What is a character?
  2. What is a character set?
  3. Why must codes be unique?
  4. How does adding more bits affect the number of possible characters?
  5. How do ASCII and Unicode differ?
  6. What does logically ordered mean?
  7. What is the text-file size formula?
  8. Do you need to memorise ASCII codes for OCR?

Check that you have not said “four bits means four characters” or treated uppercase and lowercase as the same character.