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Maths Adventures

Mermaid maths quest: perimeter

A story-led junior maths adventure using pearl patterns, shell money and underwater symmetry to measure fences, walls and safe zones around places.

Quick answer

Perimeter is the total distance around the edge of a shape. You measure it by adding up all the sides.

Learning outcome

By the end of this page, you will be able to measure the perimeter of any shape by counting or adding up all its sides. Success signs:
  • You can find the perimeter of a rectangle by adding all four sides
  • You can measure the perimeter of a shape using a ruler or by counting
  • You can solve a problem about how much fencing you need for a garden
  • You can explain what "perimeter" means in your own words

Why this matters

Perimeter helps us know how much material we need to go around something. Think of it like wrapping a present – you need to know how much ribbon you need to go all the way around it. In real life, this helps when you're building a fence, planning a garden, or even decorating a room. It's also useful when you're playing games like Mermaid Maths Quest, where you need to find safe zones or measure how far you can swim around a coral reef.

How it works

To find the perimeter of a shape, you simply add up all the sides around the outside. Here's how:
  1. Look at the shape and count how many sides it has
  2. Measure or count the length of each side
  3. Add all the lengths together
  4. Write your answer with the correct unit (like metres or centimetres)
For example, if a rectangle has sides that are 5cm, 3cm, 5cm, and 3cm, you add them all together: 5 + 3 + 5 + 3 = 16cm

Examples you can see

Let's imagine you're on a Mermaid Maths Quest in the ocean. You need to build a safe zone around your underwater home using shells. Each shell is 1cm long. You build a square shape with 4 sides, each side made of 3 shells.
  • Side 1: 3 shells
  • Side 2: 3 shells
  • Side 3: 3 shells
  • Side 4: 3 shells
To find the perimeter, you add all the sides: 3 + 3 + 3 + 3 = 12 shells So the perimeter is 12 shells. Another example: You're planning a garden for your mermaid friend. The garden is a rectangle that is 6 metres long and 4 metres wide.
  • Length (top and bottom): 6m + 6m = 12m
  • Width (sides): 4m + 4m = 8m
Total perimeter = 12m + 8m = 20m You need 20 metres of fencing to go around the garden.

Try it as a mini adventure

Mermaid Maths Quest: Building a Safe Zone You're a mermaid explorer and you've found a beautiful coral reef. You want to build a safe zone around it using seashells. You decide to make a rectangular shape with 4 sides.
  1. First side (top): 5 shells long
  2. Second side (right): 3 shells long
  3. Third side (bottom): 5 shells long
  4. Fourth side (left): 3 shells long
Now, let's find the perimeter together: 5 + 3 + 5 + 3 = ?
  1. First, add the top and bottom: 5 + 5 = 10 shells
  2. Then, add the sides: 3 + 3 = 6 shells
  3. Finally, add those two answers together: 10 + 6 = 16 shells
You need 16 shells to build the perimeter of your safe zone!

Common mistakes

  • Mistake: Only adding two sides of a rectangle, like 6 + 4 = 10cm.
    Why it happens: Not remembering that a rectangle has 4 sides.
    Fix: Always count all 4 sides and add them all together.
  • Mistake: Forgetting to write the unit (like cm or m).
    Why it happens: Thinking the number is enough.
    Fix: Always write the unit after your answer.
  • Mistake: Counting the same side twice when measuring.
    Why it happens: Getting confused about which side you're measuring.
    Fix: Label each side clearly and count carefully.

Parent and carer guide

To help your child with perimeter, try these simple steps:
  1. Use everyday objects like books, boxes, or toys to make shapes
  2. Ask your child to measure the sides with a ruler and add them up
  3. Play a game where you measure around the room (like the perimeter of a table or a book)
  4. Use string or ribbon to trace around shapes and then measure the length
  5. Ask questions like "How much fencing do we need to go around our garden?"
Remember: It's okay if your child makes mistakes – they're learning! Encourage them to explain their thinking and help them find the right answer together.

Practice questions

  1. What is the perimeter of a square with sides of 4cm?
    Answer: 4 + 4 + 4 + 4 = 16cm
  2. Find the perimeter of a rectangle with sides 7m and 3m.
    Answer: 7 + 3 + 7 + 3 = 20m
  3. A triangle has sides of 5cm, 5cm, and 8cm. What is its perimeter?
    Answer: 5 + 5 + 8 = 18cm
  4. How much rope do you need to go around a square pool with sides of 6m?
    Answer: 6 + 6 + 6 + 6 = 24m
  5. What is the perimeter of a shape with sides 2cm, 3cm, 2cm, and 3cm?
    Answer: 2 + 3 + 2 + 3 = 10cm

FAQ

What is perimeter?

Perimeter is the total distance around the edge of a shape. It's like walking all the way around a shape and seeing how far you've walked.

How do you find the perimeter of a rectangle?

For a rectangle, add up all four sides. If the sides are 5cm and 3cm, the perimeter is 5 + 3 + 5 + 3 = 16cm.

Can you find the perimeter of any shape?

Yes! You can find the perimeter of any shape by adding up all the sides around the outside.

What's the difference between perimeter and area?

Perimeter is the distance around the edge of a shape, while area is the space inside the shape. Think of it like a fence (perimeter) versus the grass inside the fence (area).

How can I practice perimeter at home?

Measure the sides of objects around your house, like a book, a table, or a window. Add up the lengths to find the perimeter. You can also draw shapes and ask a friend or family member to help you add up the sides!