Measuring Length and Perimeter — Year 3 Lesson Plan
National Curriculum: Mathematics KS2 (Year 3) — Pupils should be taught to measure, compare, add and subtract: lengths (m/cm/mm); mass (kg/g); volume/capacity (l/ml). Pupils should measure the perimeter of simple 2D shapes.
Overview
This lesson builds on pupils prior experience of measuring length in KS1 and extends it to include millimetres, converting between metric units and calculating the perimeter of 2D shapes. Pupils move through concrete, pictorial and abstract representations, starting with practical measuring activities before moving to calculating perimeters of rectilinear shapes. The lesson uses real-world contexts throughout to make measurement meaningful and to develop pupils ability to choose appropriate units and tools.
Learning Objectives
- Measure lengths accurately to the nearest millimetre using a ruler
- Convert between metres, centimetres and millimetres
- Understand perimeter as the total distance around the boundary of a shape
- Calculate the perimeter of rectilinear shapes by adding the lengths of all sides
Key Vocabulary
Suggested Lesson Structure
Play a quick estimation game: display five everyday objects (a pencil, a book, a desk, a door, a paperclip) and ask pupils to estimate their length in an appropriate unit. Collect estimates and then measure two items together as a class. Discuss the importance of choosing the right unit: when would you use mm? When would you use cm? When would you use m? Introduce the lesson objective: today we will measure accurately and calculate perimeters.
Recap the relationships between mm, cm and m using a visual conversion chart on the board: 10 mm = 1 cm; 100 cm = 1 m; 1000 mm = 1 m. Practise converting some measurements together: 3 cm = ? mm; 250 cm = ? m; 45 mm = ? cm. Introduce perimeter using a large rectangle drawn on the board. Walk the perimeter with your finger and explain it is the total length of the boundary. Model measuring all four sides and adding them to find the perimeter. Then introduce an L-shaped rectilinear shape and model finding any missing side lengths before calculating the perimeter.
In pairs, pupils use rulers to measure the sides of shapes drawn on a worksheet to the nearest millimetre. They record each measurement and then add the sides to find the perimeter. Circulate and observe measuring technique: are pupils aligning the zero correctly? Are they recording units? Bring the class together to share answers and address any measuring errors. Introduce a challenge: two rectilinear shapes with some missing side lengths that must be calculated before finding the perimeter.
Pupils complete a set of graduated perimeter tasks independently. Task 1: Find the perimeter of four regular and irregular shapes with all sides labelled. Task 2: Find the perimeter of rectilinear shapes with missing sides (using properties of rectangles). Task 3 (extension): A rectangular garden has a perimeter of 24 m. One side is 8 m. Find the other dimensions. All tasks require pupils to show their working and include units in their answers.
Display a shape with some sides labelled and some missing. Ask: How do we find the missing sides? How do we find the perimeter? Take answers and work through the solution together. Ask pupils to write a definition of perimeter in their own words on a mini whiteboard and hold it up. Address any remaining misconceptions. Preview the next lesson where pupils will measure mass and begin to explore area.
Common Misconceptions
- Pupils often confuse perimeter (distance around the outside) with area (the space inside); use a running track analogy for perimeter (you run around the outside) and a garden lawn for area (the grass you mow)
- Pupils sometimes forget to include all sides when calculating perimeter, particularly with L-shaped or irregular shapes; encourage them to label each side as they include it in the calculation
Prior Knowledge
Pupils should already be able to:
- Ability to measure lengths in centimetres and metres using a ruler
- Understanding that 100 cm = 1 m from Year 2 measurement work
- Ability to add several numbers using column addition
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