What is perimeter?
The perimeter is the total length around the outside of a 2D shape.
Perimeter Year 3
2D Shape - Lesson 3: Finding Perimeters
Shape up your children's understanding of 2D shapes with this series of 2D Shape lessons for Year 3! Help them identify a variety of 2D shapes and describe their properties, and teach them about pe...
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Perimeter Year 3
How to work out perimeter
To find the perimeter of a 2D shape, you need to add up the lengths of all the sides.
Working out the perimeters of a triangle, square, rectangle and pentagon.
The perimeter of a rectangle
Because the opposite sides of a rectangle are equal in length, to work out the perimeter we only need to know the measurement of the length and the width of the shape. We can write the perimeter of the rectangle as twice the sum of its length and width.
Formula for the perimeter of a rectangle
Perimeter of a rectangle = length + length + width + width
We can write this as:
P = l + l + w + w
This can be simplified to:
P = 2(l + w)
Using the formula to work out the perimeter of a rectangle
The perimeter of a square
Because all of the sides of a square are equal in length, to work out the perimeter we only need to know the measurement of one of the sides. We can write the perimeter of the square as four times the length of one side.
Formula for the perimeter of a square
Perimeter of a square = length + length + length + length
We can write this as:
P = l + l + l + l
This can be simplified to:
P = 4l
Using the formula to work out the perimeter of a square
When should children learn about perimeter?
According to the National Curriculum Maths objectives, children start to learn about perimeter in KS2. It is assigned as compulsory learning under the 'Measurement' strand of learning.
In Year 3, children should learn to:
- measure the perimeter of simple shapes
In Year 4, children should learn to:
- measure and calculate the perimeter of a rectilinear figure (including squares) in centimetres and metres
In Year 5, children should learn to:
- measure and calculate the perimeter of composite rectilinear shapes and centimetres and metres
In Year 6, children should learn to:
- recognise that shapes with the same areas can have different perimeters and vice versa
