If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Basic geometry and measurement

Course: Basic geometry and measurement>Unit 10

Lesson 1: Area and circumference of circles

Area of a circle

The area of a circle is pi times the radius squared (A = π r²). Learn how to use this formula to find the area of a circle when given the diameter. Created by Sal Khan and Monterey Institute for Technology and Education.

Want to join the conversation?

• is there a formula for an oval or is it impossible?
• The term oval is not normally used in geometry because it is not very well defined in common usage.

What you might mean by oval is known in geometry as an ellipse. And, yes, there are equations for it.

There are oval-like shapes that are studied in advanced calculus courses, but that is well beyond this level of study.
• Where did the name pi originate from?
• pi is a greek letter
• No, you do not that one time you square the radius will the only time you will square it.
• question:how many millimeters are in a meter? also, how many meters are in a kilometer?
• There are 1,000 millimeters in a meter, and 1,000 meters in a kilometer. This tells us that there are 1,000,000 millimeters in a kilometer!

Have a blessed, wonderful day!
• Find the area of a circle with a circumference of 12.56
How should I do this problem i have been stuck on it forever and the videos aren't helping
• The circumference of a circle and the area of a circle have one thing in common: the radius.
If I were you I'd do it in two steps:
- first calculate the radius using the circumference formula,
- then calculate the area using the radius you just calculated :-)
Hope that helps!
• How do i type the pi symbol on the keybord?
• I usually use Alt key codes, such as, Alt + 227 = π. Try it, it's easy.
• Why do we use pi to solve the area of circles and circumferences and such (if area and circumference aren't the only one). Is their something else that could possibly help? (other than 22/7 as well). Could tau be used?
• Tau is an other form as well. 1 Tau is equal to 1 complete revolution around a unit circle. So yes, Tau can be used in substitution for pi. Pi is used because you may not see it yet but in Calculus it will be easier to use pi instead of tau. Research and find a definite answer, good luck!
• what is a circumfrence?
(1 vote)
• If you were to put it in other math forms, the perimeter of the circle.
• What's the signficance of squaring mm? Is there a difference between mm and mm2?

Thanks.
• The same difference as betweem m and m2: mm (and m) is a unit of length, and mm2 (and m2) is a unit of area
• if you know the area how do to find the circumference, diameter and radius
(1 vote)
• Area = Pi*R^2

To find the radius (R), divide the area by Pi, then take the square root.

Once you have the radius (R), you can use it to find diameter and circumference.
Diameter = 2*R
Circumference = 2*Pi*R

Hope this helps.