Perimeter Year 3

Teaching perimeter in Year 3 helps children develop their understanding of length, measurement and the properties of 2D shapes. Pupils learn that perimeter is the total distance around the outside of a shape and begin measuring and calculating the perimeter of simple shapes.

Whether you are introducing perimeter for the first time or looking for Year 3 perimeter worksheets and activities, this guide explains the key concepts, common misconceptions and practical ways to teach perimeter.

What Is Perimeter?

Perimeter is the total distance around the outside edge of a two-dimensional shape. Children can think of perimeter as the length of the boundary that surrounds a shape.

For example, if a rectangle has two sides measuring 6 cm and two sides measuring 3 cm, its perimeter can be calculated by adding all four sides:

6 cm + 3 cm + 6 cm + 3 cm = 18 cm

The perimeter of the rectangle is therefore 18 cm.

Perimeter Year 3 National Curriculum

As part of the Year 3 measurement curriculum, pupils should be taught to measure the perimeter of simple 2D shapes.

Before beginning work on perimeter, children should already have some experience of measuring, comparing, adding and subtracting lengths. Learning about length and perimeter in Year 3 allows pupils to apply these skills to the complete boundary of a shape.

How to Teach Perimeter in Year 3

Perimeter is usually easiest for children to understand when they begin with practical activities. Allow pupils to physically trace, measure and describe the outside edge of shapes before asking them to complete written calculations.

1. Ask children to trace around the outside edge of a shape with their finger.
2. Explain that this outside boundary is the perimeter.
3. Measure each side of the shape using a ruler.
4. Record the length of every side.
5. Add the side lengths together.
6. Include the correct unit in the final answer.

It can help children to mark each side after they have included it in their calculation. This reduces the likelihood of missing or counting a side twice.

Measuring Perimeter Year 3

When children first begin measuring perimeter in Year 3, provide simple shapes with whole-number side lengths. Pupils can use rulers to measure each side in centimetres before adding the measurements together. Alternatively, shapes can be drawn onto centimetre-squared paper and children can physically count the perimeter of the shape. 

If measuring perimeter with a ruler, remind children to begin measuring from the zero mark on the ruler rather than from the physical end. They should also check that they are using the same unit for every side.

Once children are confident measuring drawn shapes, they can measure the perimeter of familiar objects such as:

• books
• tables
• whiteboards
• sheets of paper
• trays
• picture frames

How to Calculate Perimeter in Year 3

To calculate perimeter, children add together the lengths of all the sides of a shape.

Perimeter = the total length of all the sides

For a triangle with sides measuring 4 cm, 5 cm and 6 cm:

4 cm + 5 cm + 6 cm = 15 cm

For a square with sides measuring 5 cm:

5 cm + 5 cm + 5 cm + 5 cm = 20 cm

Some children may notice that the perimeter of a square can also be calculated by multiplying one side length by four. However, they should first understand why this method works.

Length and Perimeter Year 3 Activities

Practical activities can help pupils understand the relationship between length and perimeter before moving on to more formal perimeter questions.

✅ Measure Classroom Objects

Ask children to choose rectangular objects around the classroom. They can measure each side and add the measurements together to find the perimeter.

✅ Make Shapes with Straws

Give pupils straws or craft sticks of known lengths. Challenge them to create different shapes and calculate the perimeter of each one.

✅ Draw Shapes on Squared Paper

Children can draw shapes on squared paper and count the units around the outside edge. This provides a useful introduction to perimeter before pupils use rulers and centimetres.

✅ Find Shapes with the Same Perimeter

Give children a target perimeter, such as 16 units. Ask them to draw as many different shapes as possible with that perimeter.

This activity helps children understand that different shapes can have the same perimeter.

✅ Perimeter Treasure Hunt

Place shapes or measurement challenges around the classroom. Pupils visit each station, calculate the perimeter and record their answers.

Perimeter Worksheets Year 3

Effective Year 3 perimeter worksheets should gradually increase in difficulty. Children may begin by measuring labelled or drawn sides before progressing to calculations, missing lengths and reasoning problems.

Useful perimeter worksheet activities include:

• measuring the sides of simple 2D shapes
• adding labelled side lengths
• calculating the perimeter of squares and rectangles
• finding missing side lengths
• drawing shapes with a given perimeter
• comparing shapes with equal or different perimeters
• explaining and correcting mistakes

The free lesson above includes differentiated Year 3 perimeter worksheets alongside teaching slides and practical activities.

Perimeter Questions Year 3

Use these perimeter questions to assess whether children can measure, calculate and reason about perimeter:

1. A square has sides measuring 4 cm. What is its perimeter?
2. A rectangle has sides measuring 7 cm and 3 cm. What is its perimeter?
3. A triangle has sides measuring 5 cm, 6 cm and 7 cm. Calculate its perimeter.
4. A shape has sides measuring 2 cm, 4 cm, 3 cm and 5 cm. What is its perimeter?
5. Draw two different shapes with a perimeter of 16 units.
6. A rectangle has a perimeter of 20 cm. Two sides measure 6 cm each. What is the length of each remaining side?

Answers

1. 16 cm
2. 20 cm
3. 18 cm
4. 14 cm
5. Answers will vary.
6. 4 cm

Perimeter Reasoning Questions

Once children can calculate perimeter accurately, reasoning questions can help deepen their understanding.

❓Explain the Mistake

A rectangle has sides measuring 8 cm and 4 cm. A pupil says that its perimeter is 12 cm. What mistake have they made?

Answer: They have added only one length and one width. They need to include all four sides.

❓True or False?

A square with sides measuring 5 cm has a perimeter of 20 cm.

Answer: True, because 5 + 5 + 5 + 5 = 20.

❓Always, Sometimes or Never?

Two rectangles with the same perimeter are always the same shape.

Answer: Never. Rectangles can have different side lengths while still having the same total perimeter.

❓Compare the Shapes

Two shapes have the same perimeter. Does this mean they must have the same side lengths?

Answer: No. Different combinations of side lengths can produce the same total perimeter.

Common Perimeter Misconceptions

Confusing Perimeter with Area

Some children count the squares inside a shape rather than measuring around its outside edge. Ask pupils to trace or highlight the boundary to reinforce the difference.

Missing a Side

Children may include only the labelled sides or forget one side of the shape. Encourage them to mark each side after adding it.

Forgetting the Unit

Perimeter is a measurement of length, so answers should include a unit such as centimetres or metres.

Counting Corners Instead of Sides

Some pupils may focus on the number of corners rather than the length of the boundary. Practical measuring activities can help correct this misunderstanding.

Frequently Asked Questions

What is perimeter in Year 3?

Perimeter is the total distance around the outside edge of a two-dimensional shape. Year 3 children learn to measure the sides of simple shapes and add the lengths together.

How do you calculate perimeter in Year 3?

Measure or identify the length of every side and add all the side lengths together. Remember to include the correct unit in the answer.

What is the difference between area and perimeter?

Perimeter is the distance around the outside of a shape. Area is the amount of space inside the shape.

Do Year 3 pupils calculate the perimeter of rectangles?

Yes. Rectangles and squares are useful shapes for introducing perimeter because children can use their understanding of equal sides to support their calculations.

Are there free Year 3 perimeter worksheets?

Yes. The free PlanBee perimeter lesson includes differentiated worksheets, teaching slides, practical activities and perimeter questions.

How can I make perimeter practical?

Children can measure classroom objects, make shapes using straws, draw shapes on squared paper or use string to trace around larger objects.

Explore More Year 3 Shape Resources

This free perimeter lesson is taken from PlanBee’s complete 2D Shape scheme of work for Year 3.

The complete five-lesson scheme covers:

• recognising and describing 2D shapes
• regular and irregular shapes
• finding perimeter
• sorting shapes according to their properties
• exploring and creating tangrams

👉 View the complete Year 3 2D Shape scheme of work

You can also explore more ready-to-teach lessons and schemes in our Year 3 Maths resources.