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# Perimeter: introduction

Perimeter is a math concept that measures the total length around the outside of a shape. To find the perimeter, you add together the lengths of all the sides. This works for any shape, including triangles, rectangles, pentagons, and even irregular polygons. Created by Sal Khan.

## Want to join the conversation?

• At , what is the meaning of gnus?
(11 votes)
• Gnus: Imaginary shapes made by Sal Khan.
(16 votes)
• This is strange, but why is sometimes the perimeter bigger than the area?
(10 votes)
• Perimeter and area are measured in two different units. One isn't bigger or smaller than the other because it is like comparing apples and oranges. perimeter measures length, think of steps in a line. Area measure squares of a certain length as in tiles on a floor.
(2 votes)
• Why do you have to add up all the three sides?
(5 votes)
• The perimeter is defined that way! The perimeter is defined as a sum of all the side lengths of a shape.
(4 votes)
• What's the difference between perimeter and area? Is there really a difference? My teacher always says: Area = Length x Width = Area. But what about perimeter?
(2 votes)
• Absolutely, there is a difference!

The perimeter is like a fence around a shape. It's the total distance around the outside.

On the other hand, the area is like the grass inside the fence. It's the total space inside the shape.
(1 vote)
• What figures can you find the perimeter of and what are the formulas?
(0 votes)
• You can find the perimeter of any 2D shape. You can't get the perimeter of 3D shapes for obvious reasons, instead we call the area of the outside of a 3D shape "surface area".

Perimeter doesn't really have a formula, you just need to add up the total sum of the side lengths of a polygon. There are ways to solve for sides of a polygon, however since there are infinite polygons, there are infinite formulas, which means that I can't put them all here. (sorry!) Some famous formulas are the Pythagorean Theorem, for finding the hypotenuse of a triangle given the other two sides, and the laws of sines and cosines, which you will learn more about in Geometry.

Hope this helped!
(1 vote)
• Do 3 dimension objects like cubes have perimeter?
(26 votes)
• Perimeter is 2D/2-dimensional shapes ONLY.
(8 votes)
• Perimeter is the outside length/measurement of the shape. (For example, if a square's singulair side length is 2m, that you would add: 2+2+2+2= 8 to get the perimeter - since perimeter is the the path that surrounds the shape. I'm not sure if this makes it more confusing for you, but I hope it helps.)
(2 votes)
• Why is perimeter important?
(1 vote)
• well say if you want tend build a gate you need perimeter do find how much wood you going to need for the base of the gate
(5 votes)
• Where did the word perimeter com from?
(2 votes)
• Peri is a Greek word that means around meter means measure
(1 vote)
• Question. If you are finding the perimeter of a 2-D polygon, would the unit be squared ?
(Ex: P = 16m squared)
(0 votes)
• The answer would not be squared, that is area
(3 votes)

## Video transcript

When people use the word "perimeter" in everyday language, they're talking about the boundary of some area. And when we talk about perimeter in math, we're talking about a related idea. But now we're not just talking about the boundary. We're actually talking about the length of the boundary. How far do you have to go around the boundary to essentially go completely around the figure, completely go around the area? So let's look at this first triangle right over here. It has three sides. That's why it's a triangle. So what's its perimeter? Well, here, all the sides are the same, so the perimeter for this triangle is going to be 4 plus 4 plus 4, and whatever units this is. If this is 4 feet, 4 feet and 4 feet, then it would be 4 feet plus 4 feet plus 4 feet is equal to 12 feet. Now, I encourage you to now pause the video and figure out the parameters of these three figures. Well, it's the exact same idea. We would just add the lengths of the sides. So let's say that these distances, let's say they're in meters. So let's say this is 3 meters, and this is also 3 meters. This is a rectangle here, so this is 5 meters. This is also 5 meters. So what is the perimeter of this rectangle going to be? What is the distance around the rectangle that bounds this area? Well, it's going to be 3 plus 5 plus 3 plus 5, which is equal to-- let's see, that's 3 plus 3 is 6, plus 5 plus 5 is 10. So that is equal to 16. And if we're saying these are all in meters, these are all in meters, then it's going to be 16 meters. Now, what about this pentagon? Let's say that each of these sides are 2-- and I'll make up a unit here. Let's say they're 2 gnus. That's a new unit of distance that I've just invented-- 2 gnus. So what is the perimeter of this pentagon in gnus? Well, it's 2 plus 2 plus 2 plus 2 plus 2 gnus. Or we're essentially taking 1, 2, 3, 4, 5 sides. Each have a length of 2 gnus. So the perimeter here, we could add 2 repeatedly five times. Or you could just say this is 5 times 2 gnus, which is equal to 10 gnus, where gnu is a completely made-up unit of length that I just made up. Here we have a more irregular polygon, but same exact idea. How would you figure out its perimeter? Well, you just add up the lengths of its sides. And here I'll just do it unitless. I'll just assume that this is some generic units. And here the perimeter will be 1 plus 4 plus 2 plus 2-- let me scroll over to the right a little bit-- plus 4 plus 6. So what is this going to be equal to? 1 plus 4 is 5, plus 2 is 7, plus 2 is 9, plus 4 is 13, plus 6 is 19. So this figure has a perimeter of 19 in whatever units these distances are actually given.