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Maths Facts

Why perimeter uses length units

Show perimeter as a path around a boundary.

Quick answer

Perimeter is the total distance around the edge of a shape, and we measure it using length units like centimetres, metres, or inches because we are adding up lengths.

Learning outcome

By the end of this page, you will understand why we always measure perimeter using length units, not square units or other measurements. Success signs:
  • You can explain that perimeter is the distance around a shape
  • You can tell the difference between perimeter and area
  • You can measure the perimeter of a shape using a ruler
  • You can use length units like cm, m, or in when measuring perimeter

Why this matters

Perimeter helps us solve real-life problems like:
  • How much fencing you need for a garden
  • How far you walk around a playground
  • How much ribbon to put around a gift box
  • How much trim to add to a picture frame
Understanding perimeter is important for building, art, sports, and even cooking. When you measure the edge of something, you are measuring length, so we use length units.

How it works

Let’s break down what perimeter means:
  1. Perimeter is the total distance around a shape
  2. We measure each side of the shape
  3. We add up all the side lengths
  4. We use length units like cm, m, or in to show our answer
Imagine you are walking around the edge of a playground. Each step you take is a length. When you go all the way around, you have walked the perimeter.

Examples you can see

Let’s look at some shapes and their perimeters:

Example 1: A rectangle

A rectangle has 4 sides.
  • Two sides are 5 cm long
  • Two sides are 3 cm long
To find the perimeter:
  • 5 + 5 + 3 + 3 = 16 cm
We add up the lengths of all sides.

Example 2: A square

A square has 4 equal sides.
  • Each side is 4 cm
To find the perimeter:
  • 4 + 4 + 4 + 4 = 16 cm
Or we can multiply: 4 × 4 = 16 cm

Example 3: A triangle

A triangle has 3 sides.
  • One side is 6 cm
  • One side is 5 cm
  • One side is 7 cm
To find the perimeter:
  • 6 + 5 + 7 = 18 cm

Example 4: A pentagon

A pentagon has 5 sides.
  • Each side is 3 cm
To find the perimeter:
  • 3 + 3 + 3 + 3 + 3 = 15 cm
Or we can multiply: 5 × 3 = 15 cm

Try it as a mini adventure

Adventure: The Garden Fence Challenge You are helping your family plan a garden. You want to put a fence around a rectangular garden that is 8 m long and 5 m wide.
  1. First, you need to know the perimeter of the garden
  2. Each side of the garden is a length
  3. You measure the sides: 8 m, 8 m, 5 m, 5 m
  4. Now add them up: 8 + 8 + 5 + 5 = ?
  5. How much fence do you need?
Answer: 8 + 8 + 5 + 5 = 26 m So you need 26 metres of fence. You can act this out with your family:
  • Use a rope or tape measure to mark the garden edges
  • Walk around the edge and count your steps
  • Use a ruler or metre stick to measure each side

Common mistakes

  • Mistake: Using square units like cm² or m² for perimeter
    Why it happens: Confusing perimeter with area
    Fix: Remember perimeter is a distance around the edge, so we use length units like cm or m
  • Mistake: Only measuring one side of a shape
    Why it happens: Not understanding that perimeter is all sides added together
    Fix: Count all sides and measure each one
  • Mistake: Forgetting to write units in the answer
    Why it happens: Thinking the number is enough
    Fix: Always write the unit (cm, m, in) after your answer

Parent and carer guide

As a parent or carer, you can help by:
  • Showing your child how to measure each side of a shape with a ruler
  • Encouraging them to count all sides when finding perimeter
  • Using everyday objects like books, boxes, or tables to practice
  • Asking questions like "How far would you walk if you went around this shape?"
  • Practising with real-world examples like measuring a picture frame or a garden
Remember, children often mix up perimeter and area. Keep explaining that perimeter is the "edge" or "boundary" of a shape. If they get confused, use hands-on activities with real objects.

Practice questions

  1. What is the perimeter of a square with sides of 6 cm?
    Answer: 24 cm
  2. What is the perimeter of a rectangle with sides of 4 cm and 7 cm?
    Answer: 22 cm
  3. What is the perimeter of a triangle with sides of 3 cm, 4 cm, and 5 cm?
    Answer: 12 cm
  4. What is the perimeter of a pentagon with sides of 2 cm?
    Answer: 10 cm
  5. What is the perimeter of a hexagon with sides of 3 cm?
    Answer: 18 cm

FAQ

Why do we use length units for perimeter?

We use length units because perimeter is the total distance around a shape, and distance is measured in length units like cm, m, or in.

What is the difference between perimeter and area?

Perimeter is the distance around the edge of a shape. Area is the space inside the shape. We use different units - perimeter uses length units, and area uses square units like cm² or m².

Can you find the perimeter of a circle?

Yes! The perimeter of a circle is called the circumference. We still measure it in length units like cm or m.

How do I know which sides to measure for perimeter?

You measure all the sides that form the boundary of the shape. Every side that makes up the edge of the shape counts in the perimeter.

Why is it important to write the units?

Writing the units tells us what we are measuring. Without units, we don’t know if we are talking about 10 cm, 10 m, or 10 inches.