Beside

Perimeter: The Distance All the Way Around a Shape

In one line: The perimeter of a shape is the total length of its outside edge, which you find by adding up the lengths of all its sides.

Why it works

Perimeter helps us solve real problems, like working out how much fencing goes around a garden, how much ribbon you need for the edge of a card, or how far you run around a field. It also builds strong adding and measuring skills, and it prepares you for later ideas such as area (the space inside a shape) and circumference (the perimeter of a circle).

Think of it like: Imagine an ant walking along the very edge of a rectangular sandwich, starting at one corner and walking all the way around until it gets back to where it started. The total distance the ant walks is the perimeter of the sandwich. (Where this breaks: A real ant might cut corners or wander off the edge, but perimeter follows the edge exactly with no shortcuts. Also, the ant's walk tells you nothing about how much sandwich there is inside; that is a different measurement called area.)

Worked example

A rectangular garden bed is 6 metres long and 4 metres wide. How much fencing is needed to go all the way around it?

  1. Remember that a rectangle has 4 sides, and opposite sides are the same length.
  2. So the sides are 6 m, 4 m, 6 m and 4 m.
  3. Add all four sides: 6 + 4 + 6 + 4.
  4. 6 + 4 = 10, then 10 + 6 = 16, then 16 + 4 = 20.
  5. Write the unit: the answer is in metres, because the sides were measured in metres.

Answer: The perimeter is 20 metres, so 20 metres of fencing is needed.

Common mistakes

  • Adding only the two side lengths you can see written down (like 6 + 4 = 10) instead of all four sides; fix it by labelling every side before you add.
  • Multiplying length by width, which gives the area, not the perimeter; fix it by remembering that perimeter means adding the edges.
  • Leaving out the units or writing square units like cm²; fix it by writing plain length units such as cm or m, because perimeter is a length.
  • Mixing units, such as adding centimetres to metres; fix it by changing all sides into the same unit before adding.

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A square has sides that are each 5 cm long. What is its perimeter?

Questions parents ask

What year do kids learn perimeter?

In the UK, children start measuring perimeter in Year 3 and calculate it for rectangles and other straight-sided shapes in Year 4. In Australia it is usually taught across Years 3 to 5, and in the US it is introduced in Grade 3. Older students later extend it to circles and harder shapes.

What is the difference between perimeter and area?

Perimeter is the distance around the outside of a shape, measured in units like cm or m. Area is the amount of flat space inside a shape, measured in square units like cm² or m². A quick way to remember: perimeter is the fence, area is the grass inside it.

How can I help my child with perimeter at home?

Give your child a tape measure or ruler and ask them to find the perimeter of a book, a table or a window. You can also walk around a room together, counting steps along each wall and adding them up. Talking about real jobs, like fencing a garden or framing a picture, helps the idea stick.

Is there a formula for the perimeter of a rectangle?

Yes: perimeter = 2 × (length + width), because a rectangle has two lengths and two widths. For a square you can use 4 × side length. It is still a good idea for younger children to add all the sides first, so they understand why the formula works.