Perimeter is the length around the outside edge of a shape
At its most basic you will have to calculate the perimeter of a simple shape drawn onto squared background
The most reliable way of doing this is to pick a corner (vertice) of the shape and put a start point. Then trace around the outside edge of the shape, counting how many square lengths you cover along the way. You stop when you get back to your start point
e.g calculate the perimeter of the following shape:
1 2 3 4 5
14
6
13
7
12 11 10 9 8
As you can see the perimeter of this shape is 14 units
1
The next step after this is to be able to calculate the perimeter of basic shapes which are not drawn on a square background
To be able to do this you will either have to use your ruler to measure each edge of the shape, or, if it is a question in a test, it is more than likely that each edge that you need to calculate the perimeter will be labelled.
It is important to remember that not EVERY edge may be labelled. You have to have a good enough understanding of the properties of shapes to figure out the lengths of certain edges!
2
Below is an example of a question where you need to figure out some edges yourself
e.g calculate the perimeter of the following:
12cm
5cm
This is a parallelogram and we should know that OPPOSITE SIDES ARE EQUAL on a parallelogram.
So the perimeter is:
Start and finish point
12cm
5cm
5cm
12cm
When calculating the perimeter of any shape (or shapes) we only ADD the lenghts of the edges togather
This gives us a perimeter of: 12cm + 5cm + 12cm + 5cm = 34cm
3
The next step is to be able to calculate the perimeter of a circle.
In order to do this you will need to be aware of some of the key parts and key words associated with circles
Circumference: The length around a circle ­ this is what we call the perimeter of a circle
Radius (r): The distance between the centre of a circle and its circumference
r
Diameter (d): A straight line from one side of a circle to the other, passing through the middle
d
From the above diagrams you can see that the radius is half the length of the diameter (or the diameter is double the length of the radius)
4
The circumference is another word for the perimeter of a circle.
To calculate the circumference of a circle we use the formula:
Circumference = πd ( π x d )
π = 3.14
e.g calculate the circumference of:
17m
From the diagram we can see already that the diameter is 17m
so the circumference = πd
= π x 17m
= 3.14 x 17m
= 53.38m
5
Next we move on to calculating the perimeter of compound shapes A compound shape is what you get when you stick TWO OR MORE simple shapes together.
e.g calculate the perimeter of:
8cm
? 12cm 17cm
7cm
There is a bit of a problem as there is a side length missing.
To figure out the missing side length you look at the long vertical edge on the left. It is 17cm. There is a 5cm vertical edge on the right. To figure out the missing length we need to calculate 5 + ? = 17
5cm
15cm
In order to do this we calculate:
17 ­ 5 = 12
Now we have all sides labelled so we are able to put our start point and begin adding all the side lengths together
This gives us: 17 + 15 + 5 + 7 + 12 + 8 = 64cm
6