I need help..... if the circumference is a low number like 2, doesn't that make the diameter and radius a decimal which also makes the area a decimal? If not, how do you do it? I just don't understand it.

Well, actually it's quite simple! When diameter is 2 cm Then, Radius = 1cm Therefore, A=(pi)r^2 =(pi)*r*r =3.14*1*1 =3.14 (approx.) But, if Radius < 1cm, Then the value of r*r is also a decimal. If, Diameter is 0.5cm The r=2.5cm Therefore, the Area is (pi)r^2 A = (pi)0.25*0.25 = 0.0625(pi) There is no rule that if Radius or Diameter is a decimal, then the Area and the Circumference is also decimal but it ALMOST always shows up that way. Even if it is a decimal no difference. You just have to be careful with all those decimal points!

Area is much simpler than i expected. Once you've remembered the formula it becomes very easy!

agreed. once you remember (pi)*radius^2 it's soo easy especially if you don't try to use 3.14 and just put, for instance, 3(pi)

How could I get the exact circumference? Is it possible

That is a very interesting question. I don't think you can because the circumference of a number is pi times the diameter. Pi is never ending and computers can "only" find like 50 billion digits. I don't think any number multiplies into an infinitely long number perfectly.

im so done my teacher bad

the diameter is the radius but time 2, the radius is half of the diameter, and the circumference is the radius squared times by pi

My teacher would not teach us this yet I have NO idea what this means or what to do

If you dont get it the formula for area is r*r*3.14= a and the circumference is d*3.14= c r= radius d= diameter c= circumference a= area

how do you find the area of a circle when given the circumference?

circumference = 2 x pi x radius. From there we get, radius = circumference / (2 x pi). Now that we have the radius, we can plug it into the expression to find area. (Area = pi x r x r).

This kind of difficult for our grade level so why don’t we just learn it in 8th grade?

Khan Academy wants to give you a head start on proper geometry starting with one of the hardest to understand shapes - the circle.

Either enter an exact answer in terms of \piπpi or use 3.143.143, point, 14 for \piπpi and enter your answer as a decimal.

That is a wrong Pi approximation, brob1. Actual Pi = 3.14159265... The numbers after the decimal never repeat in a pattern because Pi is an irrational number.

Main content

Area of circles review

Q: Area is much simpler than i expected. Once you've remembered the formula it becomes very easy!

agreed. once you remember (pi)*radius^2 it's soo easy especially if you don't try to use 3.14 and just put, for instance, 3(pi)

Q: How could I get the exact circumference? Is it possible

That is a very interesting question. I don't think you can because the circumference of a number is pi times the diameter. Pi is never ending and computers can "only" find like 50 billion digits. I don't think any number multiplies into an infinitely long number perfectly.

Q: im so done my teacher bad

the diameter is the radius but time 2, the radius is half of the diameter, and the circumference is the radius squared times by pi

Q: My teacher would not teach us this yet I have NO idea what this means or what to do

If you dont get it the formula for area is r*r*3.14= a and the circumference is d*3.14= c r= radius d= diameter c= circumference a= area

Q: how do you find the area of a circle when given the circumference?

circumference = 2 x pi x radius. From there we get, radius = circumference / (2 x pi). Now that we have the radius, we can plug it into the expression to find area. (Area = pi x r x r).

Q: This kind of difficult for our grade level so why don’t we just learn it in 8th grade?

Khan Academy wants to give you a head start on proper geometry starting with one of the hardest to understand shapes - the circle.

Q: Either enter an exact answer in terms of \piπpi or use 3.143.143, point, 14 for \piπpi and enter your answer as a decimal.

That is a wrong Pi approximation, brob1. Actual Pi = 3.14159265... The numbers after the decimal never repeat in a pattern because Pi is an irrational number.

CCSS.Math:

7.G.B.4

Google Classroom

Review the basics of area of circles and try some practice problems.

Area of a circle

The area of a circle is the amount of space the circle covers. We can also think of it as the total amount of space inside the circle.

To find the area of a circle, we can use the following formula:

start text, A, r, e, a, space, o, f, space, c, i, r, c, l, e, end text, equals, pi, times, start text, r, a, d, i, u, s, end text, squared

Want a review of circle vocabulary terms (like pi, radius, and diameter)? Check out this article or this video.

Want to learn more about finding area of circles? Check out this video.

Example 1: Finding area when given radius

Find the area of a circle with a radius of start color #11accd, 5, end color #11accd.

The equation for the area of a circle is:

A, equals, pi, r, squared
A, equals, pi, dot, start color #11accd, 5, end color #11accd, squared
A, equals, pi, dot, 25

We can stop here and write our answer as 25, pi. Or we can plug in 3, point, 14 for pi and multiply.

A, equals, 3, point, 14, dot, 25
A, equals, 78, point, 5 square units

The area of the circle is 25, pi square units or 78, point, 5 square units.

Example 2: Finding area when given diameter

Find the area of a circle with a diameter of start color #1fab54, 16, end color #1fab54.

First, let's find the radius:

\begin{aligned} r &= \dfrac d2 \\ \\ r &= \dfrac{\greenD{16}}{2} \\ \\ r &= \blueD{8} \end{aligned}

Now we can find the area.

The equation for the area of a circle is:

A, equals, pi, r, squared
A, equals, pi, dot, start color #11accd, 8, end color #11accd, squared
A, equals, pi, dot, 64

We can stop here and write our answer as 64, pi. Or we can plug in 3, point, 14 for pi and multiply.

A, equals, 3, point, 14, dot, 64
A, equals, 200, point, 96 square units

The area of the circle is 64, pi square units or 200, point, 96 square units.

Practice

Problem 1

Current

Find the area of a circle with a radius of start color #11accd, 7, end color #11accd.
Either enter an exact answer in terms of pi or use 3, point, 14 for pi and enter your answer as a decimal.

unitssquared

Want to try some more area of circle problems? Check out this exercise.

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Maxwell(Max)
Posted 5 years ago. Direct link to Maxwell(Max)'s post “I need help..... if the c...”
I need help..... if the circumference is a low number like 2, doesn't that make the diameter and radius a decimal which also makes the area a decimal? If not, how do you do it? I just don't understand it.
Button navigates to signup pageComment on Maxwell(Max)'s post “I need help..... if the c...”
(27 votes)
Answer
- sasipgis
  Posted 2 years ago. Direct link to sasipgis's post “Well, actually it's quite...”
  Well, actually it's quite simple!
  
  When diameter is 2 cm
  Then, Radius = 1cm
  Therefore, A=(pi)r^2
  =(pi)*r*r
  =3.14*1*1
  =3.14 (approx.)
  
  But, if Radius < 1cm,
  Then the value of r*r is also a decimal.
  
  If, Diameter is 0.5cm
  The r=2.5cm
  Therefore, the Area is (pi)r^2
  A = (pi)0.25*0.25
  = 0.0625(pi)
  
  There is no rule that if Radius or Diameter is a decimal, then the Area and the Circumference is also decimal but it ALMOST always shows up that way.
  
  Even if it is a decimal no difference.
  You just have to be careful with all those decimal points!
  Comment on sasipgis's post “Well, actually it's quite...”
  (22 votes)
Yuyang Liang
Posted 4 years ago. Direct link to Yuyang Liang's post “How could I get the exact...”
How could I get the exact circumference? Is it possible
Button navigates to signup pageComment on Yuyang Liang's post “How could I get the exact...”
(5 votes)
Answer
- Charlie Hu
  Posted 4 years ago. Direct link to Charlie Hu's post “That is a very interestin...”
  That is a very interesting question. I don't think you can because the circumference of a number is pi times the diameter. Pi is never ending and computers can "only" find like 50 billion digits. I don't think any number multiplies into an infinitely long number perfectly.
  Comment on Charlie Hu's post “That is a very interestin...”
  (19 votes)
abigail.2458521
Posted 3 years ago. Direct link to abigail.2458521's post “Area is much simpler than...”
Area is much simpler than i expected. Once you've remembered the formula it becomes very easy!
Button navigates to signup pageComment on abigail.2458521's post “Area is much simpler than...”
(10 votes)
Answer
- Ellie
  Posted 19 days ago. Direct link to Ellie's post “agreed. once you remember...”
  agreed. once you remember (pi)*radius^2 it's soo easy
  especially if you don't try to use 3.14 and just put, for instance, 3(pi)
  Comment on Ellie's post “agreed. once you remember...”
  (2 votes)
espinor004
Posted a year ago. Direct link to espinor004's post “i like math **sobs in th...”
i like math

*sobs in the corner while trying to find the answer*
Button navigates to signup pageButton navigates to signup page
(7 votes)
Answer
- Arjun patil
  Posted 3 months ago. Direct link to Arjun patil's post “no cap”
  no cap
  Button navigates to signup page
  (3 votes)
paytongroenewold
Posted 5 years ago. Direct link to paytongroenewold's post “My teacher would not teac...”
My teacher would not teach us this yet I have NO idea what this means or what to do
Button navigates to signup pageComment on paytongroenewold's post “My teacher would not teac...”
(3 votes)
Answer
- Yuyang Liang
  Posted 4 years ago. Direct link to Yuyang Liang's post “If you dont get it the fo...”
  If you dont get it the formula for area is r*r*3.14= a and the circumference is d*3.14= c
  r= radius d= diameter c= circumference a= area
  Button navigates to signup page
  (7 votes)
brob1
Posted 4 years ago. Direct link to brob1's post “Either enter an exact ans...”
Either enter an exact answer in terms of \piπpi or use 3.143.143, point, 14 for \piπpi and enter your answer as a decimal.
Button navigates to signup pageComment on brob1's post “Either enter an exact ans...”
(1 vote)
Answer
- Denis Orlov
  Posted 3 years ago. Direct link to Denis Orlov's post “That is a wrong Pi approx...”
  That is a wrong Pi approximation, brob1.
  
  Actual Pi = 3.14159265... The numbers after the decimal never repeat in a pattern because Pi is an irrational number.
  Comment on Denis Orlov's post “That is a wrong Pi approx...”
  (6 votes)
Iris C
Posted a year ago. Direct link to Iris C's post “im so done my teacher bad”
im so done my teacher bad
Button navigates to signup pageComment on Iris C's post “im so done my teacher bad”
(4 votes)
Answer
- 🥇😀🥇connormaines🥇😀🥇
  Posted a year ago. Direct link to 🥇😀🥇connormaines🥇😀🥇's post “the diameter is the radiu...”
  the diameter is the radius but time 2, the radius is half of the diameter, and the circumference is the radius squared times by pi
  Button navigates to signup page
  (3 votes)
Elaia Wilson
Posted a year ago. Direct link to Elaia Wilson's post “how do you find the area ...”
how do you find the area of a circle when given the circumference?
Button navigates to signup pageComment on Elaia Wilson's post “how do you find the area ...”
(3 votes)
Answer
- Amulya M
  Posted a year ago. Direct link to Amulya M's post “circumference = 2 x pi x ...”
  circumference = 2 x pi x radius.
  From there we get, radius = circumference / (2 x pi).
  Now that we have the radius, we can plug it into the expression to find area.
  (Area = pi x r x r).
  Button navigates to signup page
  (5 votes)
MORTON,KYLEE
Posted 4 years ago. Direct link to MORTON,KYLEE's post “This kind of difficult fo...”
This kind of difficult for our grade level so why don’t we just learn it in 8th grade?
Button navigates to signup pageComment on MORTON,KYLEE's post “This kind of difficult fo...”
(2 votes)
Answer
- .:Rory:.
  Posted 4 years ago. Direct link to .:Rory:.'s post “Khan Academy wants to giv...”
  Khan Academy wants to give you a head start on proper geometry starting with one of the hardest to understand shapes - the circle.
  Comment on .:Rory:.'s post “Khan Academy wants to giv...”
  (6 votes)
aditikilambi2
Posted 3 months ago. Direct link to aditikilambi2's post “Hey! So I need help with ...”
Hey! So I need help with finding the area of parts of a circle. I noticed that most of these comments were five years ago...but if someone is here who can help me, then please do! Tysm!
Button navigates to signup pageComment on aditikilambi2's post “Hey! So I need help with ...”
(3 votes)
Answer
- superaa723
  Posted 3 days ago. Direct link to superaa723's post “lets say your trying to f...”
  lets say your trying to find 3/4 of a circle's area. you would find the area including the missing 1/4. lets say the r is 2. Doing the formula you should get 4pi. Then multiply by the fraction your trying to find. In this case, it would be (3/4)*4pi which would be 3pi.
  Button navigates to signup page
  (1 vote)

Basic geometry and measurement

Course: Basic geometry and measurement > Unit 10