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Perimeter: Formula, Definition, Examples, Math Steps, Shapes

Welcome to the wonderful world of geometry! Today, we’re diving into the concept of perimeter and the types of Perimeter Formula. Picture a fence around a yard or a frame enclosing a painting – that’s perimeter! In mathematical terms, perimeter is the total distance around a shape. It’s like taking a stroll along the edges of a figure, measuring how far you’ve walked.

Why does perimeter matter? Well, it’s incredibly useful in real life! Let’s explore a few examples:

Fencing a yard

Imagine you want to build a fence around your backyard. To determine how much fencing material you need, you’d calculate the perimeter of your yard. Exciting, right?

Picture frames

When choosing the perfect frame for a treasured photo or artwork, knowing the perimeter ensures you get a frame that fits like a glove. It’s all about the right fit!

Running tracks

Athletes training on a running track cover the perimeter with each lap. Understanding perimeter helps them track their distance and set goals. Ready, set, go!

Here is a set of classic math problem worksheet PDF compiled by WuKong Math, which contains relevant exercises on perimeter calculation, with answers and detailed analysis. Parents who are interested can download it for free for your children.

What is the Perimeter Formula? Perimeter Formulas for Different Shapes

Now that we’ve grasped the concept of perimeter, let’s dive into the world of perimeter formulas. Each shape has its own unique formula to find its perimeter. It’s like a secret code waiting to be cracked!

What is the Perimeter Formula? Perimeter Formulas for Different Shapes

Square or rectangle perimeter formula

Squares and rectangles are the easiest shapes to find the perimeter for. Just add up all four sides! For squares, where all sides are equal, multiply one side by 4. For rectangles, add twice the length and twice the width. Here’s the formula:

Square: A = 4s (s = side length)
Rectangle: A = 2(l + w) or P = 2l + 2w (l = length, w = width)

Square or rectangle perimeter formula

Circle perimeter formula

To find the perimeter of a circle, we use the term circumference. It’s like wrapping a string around the circle and measuring its length. The formula is:

Circle: C = 2πr (r = radius, π ≈ 3.14)

Circle perimeter formula

Triangle perimeter formula

Triangles have three sides, and finding the perimeter is as simple as adding them together:

Triangle: P = a + b + c (a, b, c = side lengths)

Triangle perimeter formula

Parallelogram perimeter formula

Parallelograms have opposite sides that are parallel and equal. To find the perimeter, add twice the length of one side and twice the length of the adjacent side:

Parallelogram: P = 2(a + b) (a, b = lengths of adjacent sides), P = a x h (h is the height)

Parallelogram perimeter formula

Trapezoid perimeter formula

Trapezoids have four sides, and to find the perimeter, simply add them all together:

Trapezoid: P = a + b + c + d (a, b, c, d = side lengths) , P = (a + b)*h/2 (h is the height)

Trapezoid perimeter formula

Table summarizing perimeter formulas

SquareP = 4s
RectangleP = 2(l + w) or P = 2l + 2w
CircleC = 2πr
TriangleP = a + b + c
ParallelogramP = 2(a + b)
TrapezoidP = a + b + c + d

How to Find the Perimeter

Armed with our perimeter formulas, let’s explore how to find the perimeter of various shapes. It’s like being a geometry detective!

Finding perimeter of regular shapes

Square perimeter

To find the perimeter of a square, multiply the length of one side by 4. For example, if each side of a square is 5 units long, the perimeter would be: P = 4 × 5 = 20 units.

Circle perimeter

For circles, we use the formula C = 2πr. If a circle has a radius of 3 units, the perimeter (circumference) would be: C = 2 × π × 3 ≈ 18.85 units.

Rectangle perimeter

To calculate the perimeter of a rectangle, add twice the length and twice the width. If a rectangle has a length of 6 units and a width of 4 units, the perimeter would be: P = 2(6 + 4) = 2 × 10 = 20 units.

Finding perimeter of irregular shapes

Break shape into smaller shapes

For irregular shapes, break them down into familiar shapes like triangles, rectangles, or squares. Find the perimeter of each smaller shape and add them together.

Measure each part

Measure the length of each side of the smaller shapes using a ruler or given dimensions.

Add all parts together

Add the perimeters of all the smaller shapes to find the total perimeter of the irregular shape.

Solved Examples Using the Perimeter Formula

Let’s put our perimeter knowledge to the test with some examples!

Example 1: Perimeter of an equilateral triangle
If an equilateral triangle has sides measuring 7 units each, find its perimeter.

P = a + b + c
P = 7 + 7 + 7 = 21 units

Example 2: Perimeter of a parallelogram
A parallelogram has a base of 12 units and a height of 5 units. Find its perimeter.

P = 2(a + b)
P = 2(12 + 5) = 2 × 17 = 34 units

Example 3: Perimeter of a circle
Find the circumference of a circle with a diameter of 10 units. (Use π ≈ 3.14)

Radius = diameter ÷ 2 = 10 ÷ 2 = 5 units
C = 2πr
C = 2 × 3.14 × 5 ≈ 31.4 units

Example 4: Perimeter of a regular pentagon
A regular pentagon has sides measuring 4 units each. Calculate its perimeter.

P = 5s
P = 5 × 4 = 20 units

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Frequently Asked Questions (FAQs)

Q1. What is the difference between perimeter and area?

Perimeter measures the distance around a shape, while area measures the space inside a shape.

Q2. Can the perimeter formula be used for 3D shapes?

No, the perimeter formula is only for 2D shapes. For 3D shapes, we use the term surface area.

Q3. How do you find the perimeter of a shape with missing side lengths?

If you know the lengths of some sides and the relationship between the missing sides and the known sides, you can set up an equation to solve for the missing lengths.


In conclusion, the perimeter formula is an essential tool for measuring the distance around any shape. By understanding the formulas for different shapes like squares, circles, triangles, parallelograms, and trapezoids, students can confidently calculate perimeter in various real-life applications. WuKong Math offers a comprehensive online learning experience that helps students master these concepts and excel in math.

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