Year 4AC9M4M02

Area and Perimeter

Recognise ways of measuring and approximating the perimeter and area of shapes and enclosed spaces, using appropriate formal and informal units.

Read this first

Perimeter is the walk around the outside. Area is the space inside.

Count round the edge for perimeter. Count the squares inside for area.

Words you need

Perimeter: The distance all the way round the edge.
Area: How much flat space is inside.
Square units: Area is counted in squares, like cm² or m².

Worked example

e.g. A rectangle is 5 cm long and 3 cm wide. Find its perimeter and its area.

For perimeter, walk round the edge. Add all four sides.
$5 + 3 + 5 + 3 = 16$ . The perimeter is 16 cm.
For area, count the squares inside.
There are 3 rows of 5. $3 \times 5 = 15$ . The area is 15 cm².

Perimeter is a length, so it uses cm. Area is squares, so it uses cm².

Practice

The pictures fade as you go. By the last few you are on your own.

1Find the perimeter of this rectangle.with a picture

Answer16 cm.

6 + 2 + 6 + 2 = 16

2Find the area of this rectangle.with a picture

Answer12 cm². There are 3 rows of 4 squares.

3A rectangle is 7 cm by 2 cm. Find the perimeter.with a hint

Need a hint?

Add all four sides. There are two of each.

Answer18 cm.

7 + 2 + 7 + 2 = 18

4A rectangle is 8 cm by 5 cm. Find the area.with a hint

Need a hint?

Area of a rectangle is length times width.

Answer40 cm².

8 \times 5 = 40

5A square has sides of 6 cm. Find its perimeter.on your own

Answer24 cm.

6A square has sides of 6 cm. Find its area.on your own

Answer36 cm².

7A garden is 9 m long and 4 m wide. How much fence goes round it?on your own

Answer26 m.

9+4+9+4 = 26

Think about it

Why does area use cm² but perimeter only uses cm?

One good answerPerimeter is a length, so it is measured in a straight line. Area counts squares, and each square is one cm by one cm, so we write cm².

Two rectangles both have a perimeter of 12 cm. Must they have the same area?

One good answerNo. A 5 by 1 rectangle has perimeter 12 and area 5. A 4 by 2 rectangle also has perimeter 12 but area 8. Same walk round, different space inside.

What is a quick way to find the perimeter of a rectangle?

One good answerAdd the length and the width, then double it. You have two of each side.

Common mistakes

Adding only two sides for perimeter.

A rectangle has four sides. You need all of them.

Mixing up area and perimeter.

Perimeter walks the edge. Area fills the middle.

Forgetting the little 2 on cm².

Area is always in square units. Leaving it off says you found a length instead.