What Does Perimeter Mean?

Welcome to page all about 'What does perimeter mean'?.

We will explain what perimeter is and how you can measure it.

There will also be links to some of our other perimeter pages.

What does Perimeter Mean?

  • Perimeter is the distance around the outside of a shape.
  • It is measured as a length and can be in a range of units such as cm, inches, feet, miles, etc.

In the triangle below, the word 'perimeter' has been written around the outside going all the way round the triangle to show what the perimeter represents visually.

perimeter image 1

One way I like to imagine perimeter is if you dropped a tiny stick-person on the outside of a shape.

The stick-person walks all the way around the outside of the shape until they got back to where they started.

The distance the person walked would be the perimeter of the shape.

perimeter image 2

How do you find the perimeter?

To find the perimeter of a shape you basically just need to add up the length of each of the sides.

If the shape is drawn accurately to scale, you can simply use a ruler and measure the length of each side and then add up all the lengths to find the perimeter.

However you will often be required, particularly at a higher level, to use geomtric properties of shapes and formulas to work out some of the missing lengths of shapes which are not drawn to scale.

To help you further understand what does perimeter mean, we have looked at working out the perimeter of some 2d shapes below.

Perimeter of a Rectangle

How to find the perimeter of a rectangle.

  • To find the perimeter of any shape we just need to add up the length of each side.
  • In any rectangle, however, opposite sides are always equal in length.
  • The perimeter of a rectangle can be found by adding the length of the 2 adjacent sides together and doubling the answer.
  • Or Perimeter of a rectangle = (length + width) x 2
  • Another way to find the perimeter quickly is to double the length of each of the adjacent sides and add them together.
  • Perimeter = (length x 2) + (width x 2).

Examples

Example 1

rectangle perimeter example 1

The perimeter of the rectangle above is 7 + 2 + 7 + 2 = 18cm.

However, we could also just work out (7 + 2) x 2, as we are simply adding 7 plus 2 twice.

Example 2

rectangle perimeter example 2

This rectangle is actually a square.

The perimeter is 6 + 6 + 6 + 6 = 24cm.

As all the sides of a square are the same length, we just need to multiply the length of one side by 4 to find the perimeter.

So 6cm x 4 = 24cm.

Perimeter of a Rectilinear Shape

A rectilinear shape is a closed shape made up of just vertical and horizontal lines.

  • To find the perimeter, we simply need to add up the lengths of the sides.
  • If the length of the sides are not given, we will need to measure them first.
  • If the shape is not drawn to scale and several lengths are missing, we will need to use our logical thinking and calculation skills to find the missing lengths.

Examples

Example 1

rectilinear perimeter example 1

To find the perimeter of the shape, we just need to add up the length of each of the sides.

Since all the sides have been given to us, we just need to add them up.

So the perimeter is 8 + 3 + 5 + 1 + 3 + 2 = 22cm.

Example 2

rectilinear perimeter example 2

In this particular example, there are two sides missing.

However, if we look at the missing side on the left hand side, we can see that it is a long vertical line which is the same length as the two vertical lines on the right hand side put together.

So this missing side must be equal to 5cm + 3cm = 8cm.

rectilinear perimeter example 2a

If we now look at the other missing side, it is a short horizontal line.

You can see that the length of this line added to the 5cm horizontal line is the same length as the 7cm long horizontal line.

So the length of the other missing side must be 7cm - 5cm = 2cm.

rectilinear perimeter example 2b

Now that we have worked out both missing sides, we can finally find the perimeter of the whole shape.

Adding up the sides gives us: 5cm + 3cm + 2cm + 5cm + 7cm + 8cm = 30cm

So the perimeter of the shape is 30cm.

Perimeter of a Regular Shapes

How do we find the perimeter of regular shapes?

Well, a regular shape is a polygon that has all sides and angles equal.

When we are looking at perimeter, it is the 'all sides equal' part that we are interested in.

  • To find the perimeter of a regular shape, we simply need to multiply the length of one side by the number of sides.

Examples

Example 1
Find the perimeter of this equilateral triangle.

equilateral triangle perimeter example

This shape is an equilateral (regular) triangle with 3 equal sides.

So the perimeter is the length of one side multiplied by 3.

5cm x 3 = 15cm

So the perimeter is 15cm.

Example 2
Find the perimeter of this regular pentagon.

pentagon perimeter example

This shape is a regular pentagon with sides length 8cm.

So the perimeter of this shape is 8cm x 5 = 40cm

Example 3:
Find the length of each side of this regular octagon.

octagon perimeter example

This shape is a regular octagon and we are told that the perimeter is 24cm.

We have to find the length of one side.

We know that length of a side x 8 = 24cm

So the length of a side must be 24 ÷ 8 = 3cm

So the length of each side is 3cm.

More Recommended Math Resources

Take a look at some more of our resources similar to these.

Perimeter Worksheets

The perimeter worksheets are all graded in order from easiest to hardest.

Using these sheets will help your child to:

  • work out the perimeter of a range of rectangles;
  • find the perimeter of rectilinear shapes.

All the sheets in this section support Elementary Math Benchmarks.

Area Worksheets

The area sheets are all graded in order from easiest to hardest.

Using these sheets will help your child to:

  • work out the areas of a range of rectangles;
  • find the area of rectilinear shapes.

All the sheets in this section support Elementary Math Benchmarks.

3rd Grade Measurement Worksheets

Here is our selection of measurement worksheets for 3rd graders.

These sheets involve reading scales going up in halves, ones, fives, tens, fifties and hundreds to find the length, weight or liquid capacity.

Using these sheets will help children to consolidate their counting and place value as well as learning to read and estimate using a simple scale.

Online Perimeter Practice

Another way to learn about 'What does perimeter mean' is through exploring shapes.

This online area and perimeter activity by toytheater.com will let you explore 'what does perimeter mean' through creating your own shapes.

The link below will open in a new browser window.

 

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