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Maths Adventures

Sailing maths quest: perimeter

A story-led junior maths adventure using maps, bearings and voyage planning to measure fences, walls and safe zones around places.

Quick answer

Perimeter is the total distance around the edge of a shape. To find the perimeter, you add up all the sides of a shape.

Learning outcome

By the end of this page, you will be able to measure the distance around any shape by adding up all its sides. Success signs:
  • You can find the perimeter of a rectangle by adding up all four sides
  • You can use a ruler to measure each side and add them together
  • You can work out the perimeter of a square using multiplication
  • You can explain how to find the perimeter of any shape

Why this matters

Perimeter helps us understand how much space is around a shape. When you're planning a garden, building a fence, or even playing a game, you need to know how much material you'll need or how far you'll walk around the edge of something. In sailing, knowing the perimeter helps you plan safe zones and measure distances around islands or obstacles.

How it works

To find the perimeter of any shape:
  1. Measure each side of the shape
  2. Add up all the measurements
  3. Write your answer with the correct units (like metres or centimetres)
For example, if you have a rectangle that is 5cm long and 3cm wide, you have:
  • Two sides that are 5cm
  • Two sides that are 3cm
So you add: 5 + 5 + 3 + 3 = 16cm

Examples you can see

Let's imagine you're planning a sailing adventure around a small island. You need to know how far you'll sail around the edge to make sure you have enough fuel. Example 1: Rectangle Imagine a rectangular island that is 10m long and 6m wide.
  • Two sides are 10m each
  • Two sides are 6m each
Perimeter = 10 + 10 + 6 + 6 = 32m Example 2: Square A square island that is 5m on each side.
  • All four sides are 5m
Perimeter = 5 + 5 + 5 + 5 = 20m Or you can multiply: 5 × 4 = 20m Example 3: Triangle A triangular island with sides of 7m, 8m, and 9m. Perimeter = 7 + 8 + 9 = 24m

Try it as a mini adventure

Sailing Maths Quest: The Safe Zone You are Captain Maths on a sailing adventure. You need to sail around a safe zone to avoid rocks. Your map shows a rectangular safe zone that is 15m long and 10m wide.
  1. First, you need to measure each side of the safe zone
  2. Then, add up all the sides to find the total distance you'll sail
  3. Finally, you'll know how much fuel you need
4th time: You measure one side of the safe zone and it is 15m. What is the opposite side? 5th time: You measure one side of the safe zone and it is 10m. What is the opposite side? 6th time: Add all the sides together to find the perimeter 7th time: How much fuel do you think you'll need if you use 1 litre per 5m of sailing?

Common mistakes

  • Mistake: Forgetting to measure all sides
    • Why it happens: You might think one side is the same as another and skip measuring it
    • Fix: Always measure each side and double-check that you've counted them all
  • Mistake: Adding only two sides instead of all sides
    • Why it happens: You might think you only need to add the length and width
    • Fix: Remember, perimeter means the total distance around the shape, so you need to add all sides
  • Mistake: Mixing up units
    • Why it happens: You might write centimetres when you meant metres
    • Fix: Always write your units at the end of your answer

Parent and carer guide

You can help your child by:
  • Using everyday objects like books or boxes to measure the sides
  • Letting them draw shapes and measure each side
  • Practising with real-life examples like measuring the edge of a table or a garden
  • Encouraging them to explain how they found the perimeter
  • Checking that they include units in their answers
At home, you can also:
  • Use a ruler to measure the sides of a piece of paper or a book
  • Measure the perimeter of a room
  • Draw shapes and calculate their perimeters together
  • Ask your child to find the perimeter of their toys or furniture

Practice questions

  1. What is the perimeter of a rectangle that is 8m long and 4m wide?
  2. Find the perimeter of a square that has sides of 6cm each
  3. A triangle has sides of 5m, 7m, and 9m. What is its perimeter?
  4. What is the perimeter of a rectangle that is 12m long and 5m wide?
  5. Find the perimeter of a square that is 10m on each side
Answers:
  1. 8 + 8 + 4 + 4 = 24m
  2. 6 + 6 + 6 + 6 = 24cm or 6 × 4 = 24cm
  3. 5 + 7 + 9 = 21m
  4. 12 + 12 + 5 + 5 = 34m
  5. 10 + 10 + 10 + 10 = 40m or 10 × 4 = 40m

    6. FAQ

    What is perimeter?

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

    How do I find the perimeter of a rectangle?

    Add up all four sides. If it's 5m long and 3m wide, add 5 + 5 + 3 + 3 = 16m.

    Can I use multiplication to find the perimeter?

    Yes! For a square, multiply one side by 4. For a rectangle, you can add the length and width, then multiply by 2.

    What units do I use for perimeter?

    You use the same units as your measurements, like metres, centimetres, or inches.

    Why is perimeter useful in real life?

    Perimeter helps you know how much fencing you need, how far you'll walk around a park, or how much material is needed for a border.