Unit 9 Section 3 : Area and Perimeter of a Rectangle
In a rectangle we can still find the area by counting squares, but we can also multiply the two lengths together.
For example:
We can see that the area is 10 cm² by counting squares, but if we multiply the lengths:
5 cm × 2 cm = 10 cm²
We have to be careful to use the same units when doing calculations.
 If the lengths are in cm then the answer will be in cm².
 If the lengths are in mm then the answer will be in mm².
 If the lengths are in m then the answer will be in m².
If the lengths are not in the same units we must convert them using the rules in the previous section.
Different Area Units
Although 1 centimetre is equal to 10 millimetres, 1 cm² is not equal to 10 mm².
A square which is 1cm × 1cm has area 1cm².
The same square is 10mm × 10mm, and 10×10=100, so its area is 100 mm².
Similarly, a square which is 1m × 1m has area 1m².
The same square is 100cm × 100cm, and 100×100=10000, so its area is 10000 cm².
Be careful to remember these facts whenever you are converting the units of an area.
Example Question
Look at the rectangle on the grid below. Each square on the grid is 1cm × 1cm.

Practice Questions
Do each of these questions then click on the button marked
to see whether you are correct.
The rectangle measures 3cm × 4cm so the area is 12 cm².
(a) Convert this into an area in mm².
(b) Find the area in mm² by converting the lengths to mm first.

Exercises
Work out the answers to the questions below and fill in the boxes. Click on the
button to find out whether you have answered correctly. If you are right
then will appear and you should move on to the next
question. If appears then your answer is wrong. Click
on to clear your original answer and have another go.
If you can't work out the right answer then click on
to see
the answer.
Question 1
Find the areas (in cm²) and perimeters (in cm) of the rectangles below.
The squares on each grid are 1cm × 1cm.
Question 2
Find the areas and perimeters of the rectangles below.
The diagrams have not been drawn accurately.
Question 3
Look at the rectangle below.
Question 4
Find the area of this rectangle in mm² and cm².
Question 5
A rectangle has an area of 48 cm². The length of one side is 6 cm.
Question 6
A rectangle has a perimeter of 24 cm and an area of 32 cm².
You have now completed Unit 9 Section 3
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Produced by A.J. Reynolds January 2001