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Maths Adventures

Ocean maths quest: perimeter

A story-led junior maths adventure using reefs, ships and underwater maps 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 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. Success signs:
  • You can identify all the sides of a shape
  • You can add up all the sides to find the total distance around
  • You can measure shapes using a ruler or counting
  • You can solve problems about how much fencing you need for a garden

Why this matters

Perimeter is like a journey around the edge of a shape. It's useful for:
  • Building a fence around a garden
  • Planning how much rope you need to surround a boat
  • Measuring a swimming pool for safety
  • Creating a safe zone for a ship in the ocean
  • Designing a reef sanctuary for sea creatures
In your daily life, you already use perimeter when you:
  • Walk around a playground
  • Measure your bedroom to know how much carpet you need
  • Plan where to place a new fish tank
  • Calculate how far you swim in a pool

How it works

To find the perimeter of any shape:
  1. Look at all the sides of the shape
  2. Measure each side (or count the units)
  3. Add up all the sides
  4. Write your answer with the correct units (like metres or centimetres)
For example, if you have a rectangle with sides 5m, 3m, 5m, and 3m:
  • 5 + 3 + 5 + 3 = 16 metres

Examples you can see

Let's go on an ocean adventure to find out how to measure perimeters! Adventure 1: The Coral Reef Fence Imagine you're a marine scientist building a fence around a coral reef to protect it from pollution. The reef is shaped like a rectangle that is 8 metres long and 6 metres wide.
  • Side 1: 8 metres
  • Side 2: 6 metres
  • Side 3: 8 metres
  • Side 4: 6 metres
To find the perimeter, add all sides:
  • 8 + 6 + 8 + 6 = 28 metres
So you need 28 metres of fencing to protect the reef. Adventure 2: The Pirate Ship's Safe Zone A pirate ship is sailing in the ocean. The captain wants to create a safe zone around the ship using a rope. The ship is shaped like a square with each side measuring 10 metres.
  • Side 1: 10 metres
  • Side 2: 10 metres
  • Side 3: 10 metres
  • Side 4: 10 metres
Perimeter = 10 + 10 + 10 + 10 = 40 metres The captain needs 40 metres of rope to make the safe zone. Adventure 3: The Underwater Garden You're designing a garden underwater for sea creatures. You want to build a rectangular garden that is 7 metres long and 4 metres wide.
  • Side 1: 7 metres
  • Side 2: 4 metres
  • Side 3: 7 metres
  • Side 4: 4 metres
Perimeter = 7 + 4 + 7 + 4 = 22 metres You need 22 metres of underwater fencing.

Try it as a mini adventure

The Ocean Maths Quest: Building a Sea Sanctuary You are the ocean's guardian and need to build a safe zone for baby sea turtles. Your sanctuary is shaped like a rectangle that is 12 metres long and 8 metres wide.
  1. First, draw a rectangle that is 12 metres long and 8 metres wide
  2. Measure each side and write down the measurements
  3. Add up all the sides to find the perimeter
  4. Write your answer: Perimeter = ? metres
4th time: Perimeter = 40 metres Now you can tell the baby turtles how much safe zone they have!

Common mistakes

  • Mistake: Only adding two sides instead of all four sides of a rectangle.
    Why it happens: Children might think a rectangle only has two different sides.
    Fix: Always count all four sides, even if some are the same length.
  • Mistake: Forgetting to write the units (metres or centimetres).
    Why it happens: Children focus on the numbers and forget to label their answer.
    Fix: Always write the unit after your answer, like 24 metres.
  • Mistake: Measuring incorrectly with a ruler.
    Why it happens: Not starting from the zero mark or not lining up the edge properly.
    Fix: Start from the zero mark and make sure the edge lines up with the ruler.

Parent and carer guide

To help your child with perimeter:
  1. Use everyday objects like books, boxes, or toys to measure their edges
  2. Draw shapes on paper and count the units along each side
  3. Ask your child to find the perimeter of their bedroom or garden
  4. Play a game where you measure different items around the house
  5. Encourage your child to explain how they found the perimeter
When helping:
  • Ask questions like "How many sides are there?" or "What do you add together?"
  • Don't correct every mistake immediately, let them think through it
  • Use real-life examples like measuring a garden or a swimming pool
  • Keep it fun and exploratory, not like a test
What to watch for:
  • Does your child count all sides of a shape?
  • Do they remember to add the units to their answer?
  • Can they measure accurately with a ruler?
  • Do they understand that perimeter is the distance around?

Practice questions

  1. What is the perimeter of a square with sides of 5 metres?
  2. What is the perimeter of a rectangle that is 6 metres long and 4 metres wide?
  3. A garden is 10 metres long and 5 metres wide. What is the perimeter?
  4. What is the perimeter of a triangle with sides 3 metres, 4 metres, and 5 metres?
  5. How much rope do you need to surround a square with sides of 7 metres?
Answers:
  1. 20 metres (5 + 5 + 5 + 5)
  2. 20 metres (6 + 4 + 6 + 4)
  3. 30 metres (10 + 5 + 10 + 5)
  4. 12 metres (3 + 4 + 5)
  5. 28 metres (7 + 7 + 7 + 7)

FAQ

What is perimeter?

Perimeter is the total distance around the edge of a shape. You add up all the sides to find it.

How do I find the perimeter of a circle?

For circles, we use a different measurement called circumference, which is not covered in this adventure. But for shapes with straight sides, you add up all the sides.

Can I find the perimeter of any shape?

Yes! As long as the shape has straight sides, you can find the perimeter by adding all the sides together.

Why do I need to know about perimeter?

Perimeter helps you plan things like fencing, rope, or borders. It's useful for building, decorating, and protecting spaces.

How can I practice perimeter at home?

Measure your bedroom, garden, or any room and add up the sides. You can also draw shapes and count the units around the edge.