Detective maths quest: perimeter
A story-led junior maths adventure using evidence, charts and mystery patterns to measure fences, walls and safe zones around places.
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.
- You can identify all the sides of a shape
- You can add up the sides to find the total distance around the shape
- You can use a ruler or measuring tape to measure each side
- You can solve detective-style problems about fencing, walls, and safe zones
Why this matters
Perimeter is like a detective's clue! When you're solving mysteries, you need to know how far something goes around. This helps you figure out how much fencing you need for a garden, how far you've walked around a park, or how much rope you need to make a safe zone.
Perimeter is used in real life for:
- Building fences around your garden
- Measuring how far you run around a sports field
- Planning where to put a new wall in your house
- Designing safe zones for animals or children
- Creating treasure maps for games
How it works
Perimeter means "the distance around". Here's how to find it:
- Look at the shape and find all its sides
- Measure each side with a ruler or measuring tape
- Add up all the measurements
- Write your answer with the right unit (like metres or centimetres)
For example, if you have a rectangle with sides of 5 cm, 3 cm, 5 cm, and 3 cm:
5 + 3 + 5 + 3 = 16 cm
Examples you can see
Imagine you're a detective solving a mystery about a garden. The garden is shaped like a rectangle. Let's measure it!
| Side | Length |
|---|---|
| Top side | 6 m |
| Right side | 4 m |
| Bottom side | 6 m |
| Left side | 4 m |
Now add them all up:
6 + 4 + 6 + 4 = 20 m
The perimeter of the garden is 20 metres.
Here's another example with a square:
A square has 4 equal sides of 3 cm each.
3 + 3 + 3 + 3 = 12 cm
The perimeter is 12 cm.
Try it as a mini adventure
Let's go on a detective adventure! You're a detective investigating a mystery at the local park.
Step 1: Look at the park entrance gate. It's a rectangle with sides of 2 m, 1 m, 2 m, and 1 m.
Step 2: Find the perimeter of the gate.
Step 3: The park has a square sandbox with sides of 2 m each.
Step 4: Calculate the perimeter of the sandbox.
Step 5: The playground has a triangular slide with sides of 3 m, 4 m, and 5 m.
Step 6: Find the perimeter of the slide.
Step 7: Add up all the perimeters to find how much fencing was used around the whole park.
Common mistakes
- Mistake: Only measuring two sides of a rectangle and forgetting the other two sides.
Why it happens: Detective work requires checking all clues!
Fix: Always look at all sides of a shape and measure them all.
- Mistake: Forgetting to write the unit (m, cm) when giving the answer.
Why it happens: We sometimes forget to be neat!
Fix: Always write your answer with the correct unit.
- Mistake: Adding measurements in the wrong order.
Why it happens: It's easy to get confused when counting!
Fix: Count each side carefully and write down the numbers in order.
- Mistake: Measuring a curved side as a straight line.
Why it happens: Curved lines can be tricky to measure.
Fix: For curved sides, you might need to use a flexible measuring tape or count small straight parts.
Parent and carer guide
Help your child understand perimeter by:
- Using everyday objects like books, boxes, or rooms to measure
- Walking around the house and measuring different edges
- Playing detective games with shapes and measurements
- Using rulers or measuring tapes together to practice
- Encouraging them to write down their measurements
- Checking that they include units in their answers
- Reinforcing that all sides must be counted
- Letting them make up their own detective mysteries
- Using visual aids like drawing shapes and labeling sides
- Being patient when they make mistakes - they're learning!
Practice questions
- What is the perimeter of a rectangle with sides of 5 cm, 3 cm, 5 cm, and 3 cm?
Answer: 16 cm
- Find the perimeter of a square with sides of 4 cm each.
Answer: 16 cm
- A triangle has sides of 6 m, 8 m, and 10 m. What is its perimeter?
Answer: 24 m
- What is the perimeter of a rectangle with sides of 7 m, 2 m, 7 m, and 2 m?
Answer: 18 m
- Calculate the perimeter of a square with sides of 9 cm each.
Answer: 36 cm
- A pentagon has sides of 3 cm, 4 cm, 5 cm, 4 cm, and 3 cm. What is its perimeter?
Answer: 19 cm
- Find the perimeter of a rectangle with sides of 10 m, 5 m, 10 m, and 5 m.
Answer: 30 m
- What is the perimeter of a square with sides of 6 cm each?
Answer: 24 cm
- A triangle has sides of 2 m, 3 m, and 4 m. What is its perimeter?
Answer: 9 m
- Calculate the perimeter of a rectangle with sides of 8 cm, 3 cm, 8 cm, and 3 cm.
Answer: 22 cm
FAQ
What is perimeter?
Perimeter is the total distance around the edge of a shape. You measure it by adding up all the sides.
How do I find the perimeter of any shape?
Measure each side of the shape, then add all the measurements together. The answer will have a unit like cm or m.
Can I find the perimeter of a circle?
Yes! The perimeter of a circle is called the circumference. It's calculated using a special formula, but for now, we'll focus on shapes with straight sides.
Why do I need to know perimeter?
Perimeter helps you solve real problems like knowing how much fencing you need for a garden, how far you've walked around a park, or how much rope you need for a safe area.
What shapes can I find perimeter for?
You can find the perimeter of any shape with straight sides, like squares, rectangles, triangles, pentagons, hexagons, and more!