A rectangle
A rectangle with sides 6 cm and 4 cm has a perimeter of 20 cm.
Add every outer side once.
Year 3
Measure the perimeter of simple 2-D shapes
Find the distance around simple 2-D shapes by adding all the side lengths.
Lesson overview
Measurement
Perimeter means the total distance around the outside of a shape. To find it, add the lengths of all the sides.
A square
All four sides are equal on a square.
4 equal sidesperimeter adds all 4 sides
If one side is 5 cm, the perimeter is 5 + 5 + 5 + 5.
Perimeter is around the edge
We measure around the outside, not across the middle.
The whole outline counts.
Worked example
Find the perimeter of a square with side length 7 cm.
- A square has 4 equal sides.
- Each side is 7 cm.
- Add the 4 sides together.
- The perimeter is 28 cm.
Try it
Trace around the outside of the shape in your mind, then add all the side lengths.
Question 1
What is the perimeter of a rectangle with sides 5 cm and 3 cm?
Question 2
What is the perimeter of a square with side length 4 cm?
Question 3
A triangle has sides 3 cm, 4 cm and 5 cm. What is its perimeter?
Question 4
Which word means the distance around a shape?
Question 5
What must you add to find perimeter?
all outside sides
corners only
Common mistakes
Watch for these when working through the lesson.
- Missing one side when adding.
- Adding only two sides of a rectangle.
- Measuring across the shape instead of around the outside.
Related topics
These ideas fit closely with this lesson.
- Measure, compare, add and subtract lengths, mass and volume or capacity.
- Draw 2-D shapes and make 3-D shapes using modelling materials.