Today we are going to learn to calculate perimeters of geometric figures. But first, let’s start with the definition of a perimeter. When we say **perimeter** we are referring to the **length of the polygon’s outline**.

Here you can review the most common types of polygons.

The length of a polygon’s outline can be calculated by adding together all of the segments that form its sides.

### Calculate the perimeter of any polygon

I am going to show you the first strategy for calculating perimeters where the number of sides a polygon has is not important.

**The perimeter of a geometric figure can always be calculated by adding together the length of each side.**

For example, you can calculate the perimeter of this triangle in the following way.

Perimeter = 17cm + 15cm + 11cm = 43cm

You can use this strategy to calculate the perimeter of any polygon.

### Calculate the perimeter of squares

The special characteristic of a square is that it **has four equal sides**. We can take advantage of this by simplifying our calculations.

You can calculate the perimeter of this square by adding together the lengths of the four sides.

Perimeter = 6cm + 6cm + 6cm + 6cm = 24cm

Since **the four sides are equal, multiplying the length of one side by four** will give you the same result.

Perimeter = 4 x 6cm = 24cm

So, you’ve discovered a rule to help you with any square.

**Perimeter of a square = 4 x side length
**

### Calculate the perimeter of rectangles

In every rectangle, the opposite sides are equal to one another. **It has sides that are equal, two by two**.

In order to calculate the perimeter of the rectangle in the example, you can add the length of its sides, two 6cm sides, and two 4 cm sides.

Perimeter = 6cm + 4cm + 6cm + 4cm = 20cm

You can **multiply the sum of the base and height by two** and obtain the same result because all rectangles **repeat the length of their sides two times.**

Perimeter = 2x (6cm + 4cm) = 20cm

So, you have an approach for any rectangle.

**Perimeter of a rectangle = 2 x (base + height)**

### Calculate perimeters of equilateral triangles

The same as with squares, **the sides of an equilateral triangle are equal**. They all measure the same length.

Each side measures 7cm and you can calculate the length of its outline in the following way.

Perimeter = 7cm + 7cm + 7cm = 21cm

Or, there is an easier way. Since the three sides are the same, you can multiply one side by 3 and the result does not change.

Perimeter = 3 x 7cm = 21cm

And this helps you with an equilateral triangle.

**Perimeter of an equilateral triangle = 3 x length of one side**

### Calculate perimeters of any regular polygon

The defining feature of **regular polygons** is that **all of their sides are the same length**.

Since a pentagon has five equal sides, to find its perimeter you multiply the length of one side by five.

Perimeter of a pentagon = 5 x length of one side

And with a hexagon, which has six equal sides, you can multiply one side by six.

Perimeter of a hexagon = 6 x length of one side

From these rules, we are able to extract a rule to calculate the perimeter of any regular polygon in a simple way.

**Multiply the number of the sides of the polygon by the length of one of the sides.**

**Perimeter of a regular polygon = nº of sides x length of one side**

If you would like to learn more about perimeters, geometry, and other topics in primary mathematics, log on to Smartick and try it for free.

Learn More:

- Perimeter: What Is It and How to Find It for Any Polygon
- How to Calculate a Perimeter: Part 2
- How to Calculate the Area?
- Geometric Plane Shapes: Circles, Triangles, Rectangles, Squares, and Trapezoids
- Calculating the Area of Polygons

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