Converting cm to mm

National Curriculum Alignment

Subject: Mathematics
Year Group: 4 (Age 8-9)
Duration: 45 minutes
Class size: 30 pupils

Relevant National Curriculum Programmes of Study:

• Measurement – Key Stage 2
  • Use, read, write and convert between standard units, converting measurements of length from cm to mm and vice versa (Year 4).
• White Rose Maths Scheme: Summer Term, Week 3, Lesson 3 focus on metric measures conversion.

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Learning Objectives

By the end of the lesson, pupils will be able to:

• Understand and explain the relationship between centimetres and millimetres.
• Convert measurements from centimetres to millimetres and vice versa accurately.
• Apply understanding of metric conversions to solve problems involving length measurements.

Success Criteria

Pupils will:

• Describe how 1 cm equals 10 mm.
• Accurately convert between cm and mm in a variety of contexts with 90% accuracy.
• Justify their conversion choices using place value reasoning.
• Apply conversions confidently to solve word problems and real-life measurements.

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Lesson Outline

1. Engaging Introduction (10 minutes)

Hook:
Show a 30 cm ruler and a 300 mm ruler side-by-side. Ask:

• “Do these rulers measure the same length?”
• “What does a centimetre look like compared to a millimetre?”

Teacher Demonstration:
Use a metre stick or ruler to physically show 1 cm and 1 mm markings. Use counters or cubes to model how 10 mm cubes lined up equal 1 cm cube length.

Key Question:

• "How can we be sure how many millimetres are in a centimetre?"

Discussion: Lead pupils to discover that 10 mm = 1 cm. Emphasise the idea of “ten millimetres make one centimetre” and link to place value (10 ones = 1 ten).

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2. Whole Class Questioning & Formative Assessment

Use Bloom’s Taxonomy to challenge thinking beyond recall:

Question Type	Sample Question	Purpose
Remembering	“What is the relationship between centimetres and millimetres?”	Check basic recall of conversion fact.
Understanding	“Why does 1 cm equal 10 mm?”	Check comprehension of the concept.
Applying	“If you have 5 cm, how many millimetres is that? Show how you worked it out.”	Apply conversion knowledge.
Analysing	“If a pencil measures 57 mm, how many centimetres and millimetres does this represent? Explain your steps.”	Assess ability to break down mm into cm/mm.
Evaluating	“Which is better to measure a small screw, cm or mm? Why?”	Encourage reasoning and justification.
Creating	“Can you create your own problem involving conversion between cm and mm for your partner to solve?”	Extend learning and encourage deeper thinking.

AfL Point (Formative):
During questioning, use mini whiteboards for instant responses to see understanding across the class. Identify pupils struggling to convert or explain reasoning and target with further support.

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3. Modelling & Examples (10 minutes)

Teacher Modelled Example (to copy into books):

Example: Convert 8.3 cm into mm.

• Step 1: Understand 1 cm = 10 mm
• Step 2: Multiply 8.3 by 10 because each centimetre is 10 millimetres.
• 8.3 x 10 = 83 mm
• Answer: 8.3 cm = 83 mm

Write this clearly in the book as:

8.3 cm = 8.3 x 10 mm = 83 mm

Use place value chart to show:

• 8 cm = 80 mm
• 0.3 cm = 3 mm

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4. Guided Practice / Task with Increasing Complexity (15 minutes)

Stage 1 (Simple conversions):
Convert the following cm measurements into mm:

• 4 cm
• 12 cm
• 0.7 cm

Stage 2 (Decimals):
Convert these:

• 6.5 cm
• 9.8 cm
• 0.03 cm

Stage 3 (Problem-solving applications):

• A ribbon is 125 mm long. How many centimetres is it?
• A bookmark is 14.6 cm long. How many millimetres is that?
• Sally measured her book width as 7.2 cm and height as 105 mm. Which is longer? Explain how you know.

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5. Common Misconceptions & Difficulties

• Mistaking the scale direction: Pupils sometimes multiply when they should divide or vice versa when converting back from mm to cm.
• Ignoring decimal places or place value: Treating 8.3 cm as 83 cm or 0.7 as 7 mm.
• Thinking mm and cm can be added/subtracted without converting (e.g., 5 cm + 10 mm = 15 cm) without conversion.
• Confusing cm and mm abbreviations.

To address:
Careful focus on place value during modelling, use of visual aids, and constant questioning to check understanding.

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6. Plenary (5 minutes)

Exit Ticket Question (on mini whiteboards):

• “Explain how you would convert 9.6 cm to millimetres.”
• “True or False: 1 mm is bigger than 1 cm. Explain your answer.”
• “If something measures 150 mm, how many centimetres is that?”

Peer Discussion:

• Pupils share answers with a partner and justify their thinking. Teacher randomly selects pupils to explain to the class, reinforcing reasoning skills.

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Assessment for Learning (AfL) Summary:

Lesson Stage	AfL Technique	Purpose
Introduction	Mini whiteboard questions to check prior knowledge	Identify preconceptions and engage interest.
Whole class questioning	Verbal explanations, reasoning prompts	Gauge depth of understanding beyond recall.
Modelling	Think-alouds, questioning reasoning	Clarify misconceptions as they arise.
Guided Practice	Teacher checking books, targeted verbal questioning	Inform next steps and identify pupils needing support.
Plenary	Exit ticket verbal & written explanations	Summative formative check before lesson closure.

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Resources Needed

• Rulers with cm and mm markings (one per pupil if possible)
• Whiteboards and pens for mini whiteboard responses
• Counters/cubes or visual objects to model 1 cm = 10 mm
• Place value charts
• Workbook pages or exercise sheets aligned with White Rose Maths Scheme

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This detailed, scaffolded plan ensures pupils not only achieve the Year 4 National Curriculum expectations by understanding and converting between cm and mm but also develop reasoning and problem-solving skills through deep questioning and application. The use of progressive tasks supports varied learner needs while embedding measurement fluency for real-life maths confidence.