Age 8 guide to perimeter
Age-appropriate activities, explanations and practice ideas for perimeter.
Quick answer
Perimeter is the total distance around the edge of a shape. You find it by adding up all the sides of a shape.
Learning outcome
By the end of this page, you will be able to measure and calculate the perimeter of simple shapes like rectangles and squares by adding up all the sides.
- You can identify the sides of a shape
- You can add up all the sides to find the perimeter
- You can measure shapes using a ruler
- You can solve real-life problems involving perimeter
Why this matters
Perimeter helps you understand how much space something takes up around the outside. This is useful when you're planning a garden, putting up a fence, or even laying out a football pitch. It's also important for future maths learning like area and geometry. Knowing how to measure perimeter helps you think about how shapes fit together and how much material you need for projects.
How it works
Perimeter is the total distance around the edge of a shape. Here's how to find it:
- Look at all the sides of the shape
- Measure each side (if needed)
- Add up all the measurements
- Write your answer with the right unit (like centimetres or metres)
For example, if you have a rectangle with sides of 5 cm, 3 cm, 5 cm, and 3 cm, you add them up:
5 + 3 + 5 + 3 = 16 cm
Examples you can see
Let's look at some shapes and how to find their perimeter:
Squares
A square has 4 equal sides. If each side is 4 cm:
- Side 1: 4 cm
- Side 2: 4 cm
- Side 3: 4 cm
- Side 4: 4 cm
Add them together: 4 + 4 + 4 + 4 = 16 cm
Rectangles
A rectangle has 2 pairs of equal sides. If one pair is 6 cm and the other is 4 cm:
- Side 1: 6 cm
- Side 2: 4 cm
- Side 3: 6 cm
- Side 4: 4 cm
Add them together: 6 + 4 + 6 + 4 = 20 cm
Triangles
A triangle has 3 sides. If the sides are 3 cm, 4 cm, and 5 cm:
- Side 1: 3 cm
- Side 2: 4 cm
- Side 3: 5 cm
Add them together: 3 + 4 + 5 = 12 cm
Irregular shapes
Some shapes don't have equal sides. You still add up all the sides:
- Side 1: 2 cm
- Side 2: 3 cm
- Side 3: 2 cm
- Side 4: 4 cm
- Side 5: 3 cm
Add them together: 2 + 3 + 2 + 4 + 3 = 14 cm
Try it as a mini adventure
Adventure: The Garden Fence Challenge
You're helping your family plan a new garden. You want to build a fence around it. Your garden is shaped like a rectangle that is 8 metres long and 5 metres wide.
4th time: You need to know how much fence you'll need. What's the perimeter of your garden?
5th time: You decide to add a small gate that is 1 metre wide. How much fence do you really need now?
6th time: You measure your garden and find it's actually 9 metres long and 4 metres wide. What's the new perimeter?
7th time: You want to put a small path around the edge of the garden that is 1 metre wide. How much path will you need?
Common mistakes
- Mistake: Forgetting to measure all sides of a shape
Why it happens: When shapes look similar, children might miss a side or count a side twice.
Fix: Use your finger to trace each side as you count it. You can also draw a line through each side as you measure it.
- Mistake: Adding the length and width instead of all sides
Why it happens: Children often think that for rectangles, just adding length and width is enough.
Fix: Remember that perimeter is all the way around the shape. For a rectangle, you need to add up all four sides.
- Mistake: Not using the correct unit
Why it happens: Children might forget to add "cm" or "m" to their answer.
Fix: Always check that your answer has the right unit at the end. If you're measuring in centimetres, your answer should be in centimetres.
Parent and carer guide
Helping your child learn about perimeter can be fun and practical. Here's how:
- Ask your child to measure the perimeter of everyday objects like books, tables, or their bedroom
- Use a ruler or measuring tape to help them measure each side
- Encourage them to draw shapes and label each side with its measurement
- Play games where you measure the perimeter of different rooms or playgrounds
- Don't worry if they make mistakes - they're part of learning!
- Practice with real-life examples like planning a garden or making a fence
- Use visual aids like drawing shapes or using blocks to help them see all sides
- Ask them to explain how they found the perimeter - this helps them think about it
- Be patient and celebrate small successes
- Remember that it's okay if they need to count on their fingers or use objects to help
Practice questions
- What is the perimeter of a square with sides of 6 cm?
- What is the perimeter of a rectangle that is 7 cm long and 4 cm wide?
- What is the perimeter of a triangle with sides of 5 cm, 6 cm, and 7 cm?
- What is the perimeter of a rectangle that is 10 cm long and 3 cm wide?
- What is the perimeter of a shape with sides of 2 cm, 3 cm, 2 cm, 4 cm, and 3 cm?
Answers:
- 6 + 6 + 6 + 6 = 24 cm
- 7 + 4 + 7 + 4 = 22 cm
- 5 + 6 + 7 = 18 cm
- 10 + 3 + 10 + 3 = 26 cm
- 2 + 3 + 2 + 4 + 3 = 14 cm
FAQ
What is perimeter?
Perimeter is the total distance around the edge of a shape. It's like walking all the way around the outside of a shape.
How do I find the perimeter of a rectangle?
Measure all four sides and add them together. For a rectangle with sides of length l and width w, the perimeter is 2l + 2w.
Can I find the perimeter of any shape?
Yes! You can find the perimeter of any shape by adding up all the sides. Even irregular shapes with sides of different lengths.
What units do I use for perimeter?
You use the same units as your measurements. If you measure in centimetres, your perimeter is in centimetres. If you measure in metres, your perimeter is in metres.
Why do we learn about perimeter?
Perimeter helps us solve real-world problems like planning a garden, building a fence, or designing a playground. It's a useful skill for everyday life and future maths learning.