LNF Cymru Literacy and numeracy for Wales

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GCSE Computer Science

Numeracy is heavy and specific: learners convert between denary, binary and hexadecimal, add in binary, work with base 2 storage units, calculate image, sound and compressed file sizes and routing costs, and reason with truth tables and DIV and MOD. Literacy centres on close reading of the Unit 2 scenario and requirements, precise technical vocabulary, short and extended judgement answers in Unit 1, and code annotation and test commentaries. The single biggest thing Key Stage 3 should build is fluent place value in base 2 and base 16, so that storage and file size calculations become routine.

First taught
September 2025
Amount of literacy work
Medium
Amount of numeracy work
High
Specification
Open the WJEC specification (PDF)
How this GCSE is assessed
  • Unit 1: Understanding Computer Science: Digital examination 1 hour 30 minutes, 50%, 80 marks
  • Unit 2: Computer Programming: On-screen examination based on a pre-released brief, 2 hours, 50%, 80 marks, set and marked by WJEC

Skills that use Relationships within the number system

  1. Converting between denary, binary and hexadecimal

    Assessed: In the exam

    Learners convert between denary, binary and hexadecimal, perform binary addition, use arithmetic shifts and explain overflow and underflow. All of this rests on secure place value and powers of 2.

    Numeracy: The number system, Calculation, Relationships within the number system, Conceptual understanding, Fluency, Logical reasoning

    Getting pupils ready

    Year 7
    Pupils use binary place-value cards from 128 down to 1 to make their age and house number, write each as 8-bit binary and check by adding the place values.
    Year 8
    Pupils add two 8-bit binary numbers in columns, convert the answer to denary to check it, and explain in one sentence what happens when a ninth bit is needed.
    Year 9
    Pupils convert web colour codes such as FF8800 from hexadecimal to denary and binary, and show that a left shift of one place doubles a binary number.
    Show the specification wording and the LNF statements

    Where it is in the specification: Unit 1, Section 1.2.1, page 13

    “convert between, denary, binary and hexadecimal counting systems”
    Progression step 3

    • I can read and write numbers to 1 million and numbers to 3 decimal places.

    • I can add and subtract numbers using whole numbers and decimals.

    • I can use a range of strategies to check calculations including the use of inverse operations, equivalent calculations and the rules of divisibility.

    Progression step 4

    • I can use powers and understand the importance of powers of 10.

    • I can represent a concept in different ways, flowing between different representations including verbal, concrete, visual, digital and abstract.

    • I can use the four operations and the connections between them, e.g. apply division as the inverse of multiplication.

    Progression step 5

    • I can represent a concept in different ways, flowing between different representations including verbal, concrete, visual, digital and abstract.

    • I can use firmly established, memorable and usable facts and techniques in order to apply the most efficient methods.

    • I can verify and prove results and solutions.

  2. Data storage units and capacity

    Assessed: In the exam

    Learners must know the base 2 storage units from bit and nibble to petabyte, where each step is 1,024 not 1,000, and calculate the capacity needed to store data.

    Numeracy: Measurement, Calculation, The number system, Relationships within the number system, Communicating with symbols, Fluency, Conceptual understanding

    Getting pupils ready

    Year 7
    Pupils order bit, nibble, byte, kilobyte, megabyte and gigabyte on a class washing line and work out how many bits are in one kilobyte.
    Year 8
    Pupils work out how many 3 MB photos fit on a 16 GB memory stick, first estimating with 1,000 and then calculating with 1,024, and explain the difference.
    Year 9
    Pupils total the file sizes in a sample project folder, convert the total into the most sensible unit and judge whether it fits their school network storage allowance.
    Show the specification wording and the LNF statements

    Where it is in the specification: Unit 1, Section 1.2.1, pages 13 to 14

    “the relationship between data storage units using the base 2 prefixes”
    Progression step 3

    • I can convert metric units of length to smaller units, e.g. cm to mm, m to cm, km to m.

    • I can multiply numbers and decimals by a multiple of 10, e.g. 15 x 30, 1.4cm x 20.

    • I can read and write numbers to 1 million and numbers to 3 decimal places.

    Progression step 4

    • I can use the common units of measure, convert between related units of the metric system and carry out calculations.

    • I can use powers and understand the importance of powers of 10.

    • I can read and write numbers of any size.

    Progression step 5

    • I can use appropriate notation, symbols and units of measurement, including compound measures.

    • I can use firmly established, memorable and usable facts and techniques in order to apply the most efficient methods.

    • I can interpret answers within the context of the problem and consider whether answers, including calculator, analogue and digital displays, are sensible.

  3. Compression ratios and file size reduction

    Assessed: In the exam

    Learners calculate compression ratios and the reduction in file size after lossy or lossless compression, and judge whether the saving is worth the loss of quality.

    Numeracy: Calculation, Relationships within the number system, Conceptual understanding, Logical reasoning

    Getting pupils ready

    Year 7
    Pupils run-length encode rows of their pixel art, for example 5 white then 3 black, and count how many characters they save compared with listing every pixel.
    Year 8
    Pupils zip a folder of text files, record the size before and after, and write the saving as a ratio in its simplest form.
    Year 9
    Pupils save one photo as PNG and at three JPEG quality settings, calculate the percentage reduction for each and explain which methods are lossy.
    Show the specification wording and the LNF statements

    Where it is in the specification: Unit 1, Section 1.2.1, page 14

    “calculation of compression ratios and the reduction in file sizes following compression.”
    Progression step 3

    • I can use ratio and proportion to calculate quantities.

    • I can calculate a percentage, fraction and decimal of any quantity with a calculator where appropriate.

    • I can use equivalence of fractions, decimals and percentages to compare proportions.

    Progression step 4

    • I can express one quantity as a percentage of another.

    • I can calculate a percentage increase or decrease.

    • I can use equivalence of fractions, decimals and percentages to select the most appropriate one for a calculation.

    Progression step 5

    • I can use multipliers as an efficient method when working with percentages, e.g. multiply by 1.2 to increase an amount by 20%.

    • I can interpret answers within the context of the problem and consider whether answers, including calculator, analogue and digital displays, are sensible.

    • I can explain results and procedures precisely using appropriate mathematical language.

Good to know

Links with other GCSEs

Place value, powers, ratio and percentage reduction are taught in Mathematics, so agreeing shared language (for example 'column value' and 'simplest form') with the Mathematics department in Years 7 to 9 will help pupils transfer these skills to binary and file size work.

Source: WJEC GCSE Computer Science specification, teaching from 2025, for award from 2027, Version 3, September 2025.