Alien planet maths quest: perimeter

Quick answer

Perimeter is the total distance around the edge of a shape. You can 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 the perimeter of any shape by adding up all its sides, and understand how to use perimeter in real life.

• You can count all the sides of a shape and add them together.
• You can find the perimeter of rectangles, squares, triangles, and other shapes.
• You can use perimeter to solve problems about fencing, walls, and safe zones.
• You can apply your knowledge to a space adventure!

Why this matters

Perimeter helps us know how much fencing we need to surround a garden, how far we have to walk around a playground, or how big a safe zone should be on an alien planet. In space, knowing the perimeter helps astronauts build safe walls and plan their journeys.

How it works

Perimeter is like walking all the way around a shape. If you walk around a shape and come back to where you started, the distance you walked is the perimeter.

1. Look at all the sides of the shape.
2. Measure each side (or count the units if the shape is drawn on grid paper).
3. Add all the side lengths together.
4. Write your answer with the correct unit (like metres or centimetres).

Examples you can see

Imagine you are exploring an alien planet. You need to build a fence around a safe zone to keep your spaceship safe from space creatures.

Example 1: Square safe zone

Your safe zone is a square with each side measuring 5 metres.

• Side 1 = 5m
• Side 2 = 5m
• Side 3 = 5m
• Side 4 = 5m

Perimeter = 5 + 5 + 5 + 5 = 20 metres

Example 2: Rectangular safe zone

Your second safe zone is a rectangle that is 6 metres long and 4 metres wide.

• Long side = 6m
• Short side = 4m
• Long side = 6m
• Short side = 4m

Perimeter = 6 + 4 + 6 + 4 = 20 metres

Example 3: Triangular safe zone

Your third safe zone is a triangle with sides of 3m, 4m, and 5m.

• Side 1 = 3m
• Side 2 = 4m
• Side 3 = 5m

Perimeter = 3 + 4 + 5 = 12 metres

Try it as a mini adventure

You are on a space mission to an alien planet. You need to build a safe zone around your landing site using a rope. You have 24 metres of rope.

Adventure Step 1: Draw a rectangle with a perimeter of 24 metres.

Adventure Step 2: Find a square that uses exactly 24 metres of rope.

Adventure Step 3: Build a triangle with a perimeter of 24 metres.

Adventure Step 4: Compare your three shapes. Which one gives you the most space inside?

Common mistakes

• Mistake: Forgetting to count all the sides of a shape.
  Why it happens: When shapes look different, children may not notice all sides.
  Fix: Use a finger to trace around the shape and count each side carefully.
• Mistake: Adding the lengths but forgetting to write the unit (like metres).
  Why it happens: Children focus on the numbers and forget to complete the answer.
  Fix: Always ask, "What are we measuring?" and write the unit at the end.
• Mistake: Confusing perimeter with area.
  Why it happens: Both are measurements of shapes, but one is distance around and one is space inside.
  Fix: Remember, perimeter is the edge of a shape, like a fence. Area is the inside, like grass in a garden.

Parent and carer guide

Help your child by:

• Playing with shapes at home using string or rope to measure around objects like books, boxes, or toys.
• Looking at real-world examples: How far would you walk around your garden? How much fencing do you need for a pet area?
• Encouraging them to draw shapes and measure each side before adding.
• Using the adventure challenge to explore different shapes with the same perimeter.
• Noticing when they mix up perimeter and area – ask them to show you the edge of a shape vs. the inside.

Practice questions

1. What is the perimeter of a square with sides of 7m?
2. A rectangle has sides of 5m, 3m, 5m, and 3m. What is its perimeter?
3. What is the perimeter of a triangle with sides of 4m, 5m, and 6m?
4. If a pentagon has 5 sides of 4m each, what is its perimeter?
5. What is the perimeter of a shape with sides of 10m, 8m, 10m, and 8m?

Answers:

1. 28m
2. 16m
3. 15m
4. 20m
5. 36m

FAQ

What is perimeter?

Perimeter is the total distance around the edge of a shape.

How do I find the perimeter of a shape?

Add up the length of all the sides of the shape.

Can shapes have the same perimeter but different areas?

Yes! A square and a rectangle can have the same perimeter but one will have more space inside than the other.

Why is perimeter important?

Perimeter helps us work out how much fencing we need, how far we walk around a playground, or how big a safe zone should be.

How can I practice perimeter at home?

Measure around objects using a ruler or tape measure, or draw shapes and calculate their perimeters.