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# Radius, diameter, circumference & π

Learn how the number Pi allows us to relate the radius, diameter, and circumference of a circle. Created by Sal Khan.

## Want to join the conversation?

• Are there any tips or tricks for remembering the equations for finding the diameter and the circumference?
• The saying for circumference and area that sticks with me is "Cherry pies delicious, apple pies are too." That is, C = πd and A = πr²
• Can someone give me a problem (NOT FORMULA!) I can keep calculating to keep accurately getting digits of pi? This is just out of curiosity. I know that circumference divided by diameter should equal pi, but I can never be exact when measuring it.
Also, can this problem preferably involve whole numbers?
(Sorry if I come off as picky and demanding)
• Pi actually goes on forever, so most people just use 3.14. Here are the first 100 digits of pi, according to math.com: 3.1415926535 8979323846 2643383279 5028841971 6939937510 5820974944 5923078164 0628620899 8628034825 3421170679. It also tells you the first 1000 digits if you really want to know.
• How can I prove my math ?
I want to be a pilot but I'm not good enough in math
• well all you have to do is study math alot and keep working harder. and do your best
• how do you find area if all you have is circumference?
• divide the circumference by 3.14 or pie then divide the answer by 2 then times that answer by itself then times it by pie or 3.14 and you have the area
• Why does every radius in a circle has the same length? I don't get that and its really confusing.
• same what if i have 27/9
• pi is a infinite number right?
• Yes it is proven it goes on forever
• this stuff is confusing
• I'm saying 😭
• do you ever have to use the entire pi or just 3.14? Also what is pi?
• It depends on what the question is asking... 3.14 is what a lot of questions use, especially if you want to get a rounded answer. Regular pi goes on forever, and is more useful in exact answering.
Also, pi is a number that goes on forever and ever without stopping. It is irrational, and it describes the attributes of circles.
Hope this helps! If you need more explanation, please tell me and I will come! :D
#YouKhanLearnAnything
• PI (approximately 3.14159...) is commonly known as Archimedes's constant, representing the ratio of a circle's circumference to its diameter. It has various applications, such as finding the area of a circle and solving the volume of a sphere. In a video, Sal explains how pi (π) is used to relate the radius, diameter, and circumference of a circle. (PS. To type π on Windows, hold ALT and type 227 on the numeric keypad).

The radius of a circle is the distance from the center to the edge (or circumference). Sal explains at that all radii are equal in a given circle. The diameter is the distance from one end of the circle to the other and is equal to 2 times the radius (r times 2).

The diameter is the widest point on a circle and has various applications, although not mentioned here. It is particularly useful in calculating the circumference of a circle.

As Sal explains at , the ratio of the circumference to the diameter (C:D) is indeed pi (π).

Pi (π) can be used to calculate the area of a circle by multiplying it by the square of the radius (πr^2). It is also worth noting that the circumference is equal to π times the diameter.

I hope this information is helpful to anyone seeking to understand the concept of pi (π) and its applications in geometry.
(PS. this was surprisingly hard to write)