Area and Perimeter: Calculating Perimeter — Year 4 Lesson Plan
National Curriculum: Mathematics — Measurement, Year 4: measure and calculate the perimeter of a rectilinear figure (including squares) in centimetres and metres; notes and guidance: perimeter can be expressed algebraically as 2(a + b)
Overview
The second of four Area and Perimeter lessons. Pupils calculate perimeters without measuring. They use the fact that opposite sides of a rectangle are equal, find missing side lengths on rectilinear shapes, and use a shortcut for rectangles: add the length and width, then double. Problems use both centimetres and metres.
Area and Perimeter: Calculating Perimeter learning objectives
- Calculate the perimeter of rectangles and squares from their side lengths.
- Find missing side lengths on rectilinear shapes using the lengths of the opposite sides.
- Calculate the perimeter of rectilinear shapes in centimetres and metres.
- Use 2 × (length + width) as a shortcut for the perimeter of a rectangle.
Success criteria
- I can use the equal opposite sides of a rectangle to find its perimeter.
- I can find a missing side length on an L-shape.
- I can calculate the perimeter of rectilinear shapes in cm and m.
- I can use (length + width) × 2 for a rectangle.
Key Vocabulary
Suggested Lesson Structure
Quick-fire doubling and adding on mini whiteboards: double 7, double 15, 7 + 4 then double, 12 + 5 then double. Then show a rectangle with sides labelled 9 cm and 4 cm on only two sides and ask: can we find the perimeter without a ruler?
Establish that opposite sides of a rectangle are equal, so 9 + 4 + 9 + 4 = 26 cm. Show the shortcut: add the length and width, then double: (9 + 4) × 2 = 26 cm, and that for a square of side 6 cm, 4 × 6 = 24 cm. Then model an L-shape with a 10 cm base, an 8 cm left side, a 4 cm top and a 5 cm inner step, with two sides unlabelled. The missing horizontal side is 10 − 4 = 6 cm and the missing vertical side is 8 − 5 = 3 cm, so the perimeter is 8 + 10 + 3 + 6 + 5 + 4 = 36 cm. Finish with metres: fencing around a garden 12 m by 7 m needs 38 m.
Pairs solve missing-side puzzles on rectilinear shapes, first labelling every missing length and then calculating the perimeter. Discuss checking strategies: the horizontal sides along the top must add up to the same as along the bottom, and the same for the vertical sides.
Pupils calculate the perimeters of rectangles, squares and L-shapes with some unlabelled sides, then solve word problems (ribbon around a card, fencing around a vegetable patch in metres). Challenge: find every rectangle with whole-number sides and a perimeter of 24 cm.
True or false: to find the perimeter of a square, multiply the side by 4 (true). An L-shape has four sides (false: six). 2 × 9 + 4 is the perimeter of a 9 cm by 4 cm rectangle (false: that doubles only the length; it should be (9 + 4) × 2).
Video for this lesson
Calculating Perimeter | Year 4 Area and Perimeter, Lesson 2 | KS2 Maths (No Narration)
Equal opposite sides, the (length + width) × 2 shortcut, squares, finding missing sides on an L-shape step by step, and perimeter in metres, plus the common bracket mistake. The words are on screen, so you can pause and talk it through. Ends with a three-question quiz.
Watch on YouTube →Support and extension for area and perimeter: calculating perimeter
Support
- Give shapes drawn on squared paper so missing lengths can be checked by counting.
- Colour the horizontal sides one colour and the vertical sides another, so pupils see which sides add up to which.
- Use a part-whole model to find a missing side: the whole side is split into two parts.
Extension
- Find every rectangle with whole-number sides and a perimeter of 24 cm.
- A rectangle has a perimeter of 30 cm and one side of 9 cm. How long are the other sides?
- Explain why an L-shape has the same perimeter as the rectangle drawn around it.
Check for understanding
Quick questions for the plenary or an exit ticket. Tap a question to see the answer.
1. A rectangle is 9 cm long and 4 cm wide. What is its perimeter?
Answer: (9 + 4) × 2 = 26 cm.
2. A square has sides of 6 cm. What is its perimeter?
Answer: 4 × 6 = 24 cm.
3. An L-shape has a 10 cm base and a 4 cm top. How long is the missing horizontal side?
Answer: 10 − 4 = 6 cm.
4. How much fencing goes around a garden 12 m by 7 m?
Answer: 38 m.
Resources you will need
- Rectangles and L-shapes with some sides unlabelled
- Squared paper for checking
- Mini whiteboards for the starter
- Word problem cards (ribbon, fencing, frames)
Common Misconceptions
- Leaving out unlabelled sides. Label every missing length before adding.
- Adding instead of subtracting to find a missing side on an L-shape.
- Writing 2 × 9 + 4 for a 9 cm by 4 cm rectangle, which doubles only the length. Add the two sides first, then double.
- Mixing centimetres and metres in the same calculation.
Prior Knowledge
Pupils should already be able to:
- Measure the perimeter of rectilinear shapes by adding all the sides (lesson 1).
- Double numbers and add and subtract two-digit numbers mentally.
- Know that a rectangle has two pairs of equal opposite sides and a square has four equal sides.
Make it yours
Want a personalised version of this lesson?
Use Staffroom to generate a complete lesson plan tailored to your class — add context about ability, recent learning, or specific pupils and get a plan ready to teach. Free plan, no card required.