Perimeter Worksheets, Questions and Revision

Perimeter

The perimeter of a 2D shape is the distance around the edges of the shape. It is found by adding up the lengths of each individual edge of the shape. This can be easy for simple shapes and harder for composite shapes.

Make sure you are happy with the following the following topics:

Perimeter of Simple Shapes

The rectangle shown below has a length of $11$ cm and a width of $3$ cm.

Calculate the perimeter of the rectangle shown.

perimeter of a rectangle example

The opposite sides of a rectangle are equal in length.

So, the two sides that are not labelled must also be $3$ cm and $11$ cm.

Therefore, we get

Perimeter $= 3+3+11+11=28$ cm

Perimeter of Compound Shapes

$ABCDEF$ is a composite shape made up of rectangles.

Calculate the perimeter of shape $ABCDEF$ .

Usually, with compound shapes there will be some missing sides which need to be calculated.

First we need to calculate $ED$ .

We can see that $FA = ED + CB$ , so we can calculate:

$7 = 5 + ED$

$ED = 2$

Next, we do the same with $DC$ .

$AB = FE + DC$

$12 = 3+DC$

$DC = 9$

Finally, we can calculate the perimeter by adding up all the side lengths.

$3 + 2+ 9 + 5+ 12+ 7 = 38$

perimeter compound shapes example semicircle rectangle

Example 1: Compound Shapes

$ABCDE$ is a shape made up of a rectangle and a semi-circle.
$AB = 6,\,BC=13$ .

Calculate the perimeter of this shape to $3$ significant figures.

[2 marks]

With compound shapes we first need to break it up into two parts.

1) Rectangle – We know that rectangles have two pairs of equal sides, so we know that $DC=6$ and $AD=13$ . However, since it is not on the outside of the shape, we won’t be counting $AD$ with the perimeter. Therefore, the perimeter so far (sides $AB$ , $BC$ , and $CD$ ) is

$6+13+6=25$

2) Semi-circle – The formula for the circumference of a circle is $\pi d$ , where $d$ is the diameter. Because it is a semi-circle we’re dealing with, we will half the result. Doing this, we get

$AED=\dfrac{(\pi \times 13)}{2}=20.420...$

Now, adding the values together we get the total perimeter to be

$25+20.420...=45.4$ ( $3$ sf)

Example 2: Perimeter of Triangles

$ABC$ is a triangle. $AB = 8$ cm, $BC = 6$ cm. Angle $ABC$ is a right-angle. Calculate the perimeter of triangle $ABC$ .

[2 marks]

In this case, we are given two sides of the triangle but will have to work out the third if we want to find the perimeter. Since this is a right-angled triangle, we can use Pythagoras! Pythagoras’ theorem says

$a^2+b^2=c^2$ ,

Where $c$ is the hypotenuse and $a$ and $b$ are the other two sides. So, the equation becomes

$8^2+6^2=AC^2$

Evaluating the left-hand side, and then square rooting, we get

$AC^2=64+36=100$

$AC=\sqrt{100}=10\text{ cm}$

Now we have all three sides, simply add them to get the perimeter:

$6+8+10=24\text{ cm}$

Perimeter Example Questions

Question 1: $ABCD$ is square with an area of 64 $\text{m}^2$ .

Calculate the perimeter of $ABCD$ .

[2 marks]

To find the perimeter, we need to find the length of one side (all sides are the same, since it’s a square). If we say that $x$ is the length of one side, then the area is,

$x^2=64$

So, if we square root both sides, we find that $x=8$ . Therefore, the perimeter of the square is,

$8+8+8+8=32\text{ m}$