Main content

### Course: 3rd grade > Unit 11

Lesson 1: Perimeter- Perimeter: introduction
- Perimeter of a shape
- Find perimeter by counting unit squares
- Find perimeter by counting units
- Finding perimeter when a side length is missing
- Find perimeter when given side lengths
- Finding missing side length when given perimeter
- Perimeter review

© 2024 Khan AcademyTerms of usePrivacy PolicyCookie Notice

# 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?

- Do 3 dimension objects like cubes have perimeter?(22 votes)
- Perimeter is 2D/2-dimensional shapes ONLY.(4 votes)

- 1:51what is a meaning of gnus(11 votes)
- Gnus: Imaginary shapes made by Sal Khan.(14 votes)

- This is strange but why is sometimes the perimeter bigger than the area(8 votes)
- perimeter in mixed fractions(8 votes)
- To find the perimeter of a shape, just add the lengths of all the sides.(5 votes)

- Also, if you multiply a fraction by a whole number when doing area, it might become smaller than the perimeter(5 votes)
- What does it matter?

Area and perimeter are 2 different types of measurement. You really can't compare them.

Perimeter measures distance (1-dimention)

Area measures units squared (2-dimensions)(9 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.(3 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.)(1 vote)

- Can someone help me with alrealdy knowing the premiter but dont know the length is missing?(4 votes)
- For example, you have a square court and you want to know the length of each side but you want to put your answer in a mixed number or a decimal what would you do?(4 votes)
- If I'm not mistaken, all you got to do is to add the sides together, no matter if they are whole numbers or not.(1 vote)

- according to mr.khan

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.(4 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.