Example 1
- Radius: 10
314.16
r = 10 gives 314.16 of area.
Circle area calculator
Enter the radius. The calculator computes area πr² and circumference 2πr. At r = 10 the area is about 314.16 and the circumference is about 62.83.
A = πr². Circumference is on the circumference calculator. Pythagoras is on the Pythagorean theorem page. Sphere: sphere volume. Article: area of a circle.
Result
Radius r (cm). Area shows up here.
How it works
Circle area is π times the radius squared. At r = 10 you get 314.16, because one hundred times π, not ten times π. One gives 3.14. Five gives 78.54. Two gives 12.57. That is paint on the disk, not tape around the rim. Circumference 2πr at the same 10 is 62.83 and is on the neighbouring page.
People type a diameter as r. Diameter 20 is radius 10. If you type 20, the area comes out four times too large.
The calculator does not guess whether ten is r or d. Sphere surface is 4πr², about 1256.64 at r = 10, another page. A cylinder takes the same πr² and multiplies by height; there is no h here.
The calculator does not run when the radius is missing. Switching to inches relabels the result as in² and does not convert a typed 10 into inches. The calculator uses π, not 22/7, so r = 1 is 3.14 to two places, not 3.00.
A half gives 0.79. Seven gives 153.94. Twelve gives 452.39.
A triangle and a rectangle do not insert π. Pythagoras finds a side, not a circle. Treat 314.16 as π arithmetic, not a caliper reading. If you want a hoop, open circumference with the same r.
How to use
- Type the circle radius in centimeters or inches, matching the header switch.
- Do not type a diameter. If you know the diameter, divide it by 2 before you click Calculate.
- Click Calculate. The result uses two decimal places and cm² or in².
- Check that you did not want circumference: the same r = 10 is 314.16 here and 62.83 next door.
- The r = 5 example is 78.54. That is πr², not 2πr and not 4πr² from a sphere.
Formula
A = π × r²
Two decimal places. Not circumference 2πr.
A = πr² at r = 10
π times r squared. r = 10 gives 314.16. r = 1 gives about 3.14. Not circumference 2πr.
- A
- Disk area. r = 10 is 314.16. r = 5 is 78.54. r = 7 is 153.94.
- r
- Radius. 10 gives 314.16. 2 gives 12.57. Circumference at the same r is on the other card.
- π
- Inside πr². At r = 1 the result is about 3.14. At r = 10 that is 314.16, not 62.83 of rim.
Examples
Example 2
- Radius: 1
3.14
r = 1 gives about 3.14.
Example 3
- Radius: 5
78.54
r = 5 gives 78.54.
Example 4
- Radius: 2
12.57
r = 2 gives 12.57.
Example 5
- Radius: 7
153.94
r = 7 gives 153.94.
Example 6
- Radius: 0.5
0.79
r = 0.5 gives 0.79.
Example 7
- Radius: 12
452.39
r = 12 gives 452.39.
Example 8
- Radius: 3
28.27
r = 3 gives 28.27.
Frequently asked questions
Does the circle area calculator also find circumference?
No. Here only π r squared. Circumference 2πr is on its own page. The same r = 10 is 314.16 here and 62.83 there.
Why does r = 10 give 314.16, not 31.4?
Area is π times 100, not π times 10. Ten squared is one hundred. 100 times π is about 314.16.
Should I type a diameter instead of a radius?
No. Area wants r. Diameter 20 is radius 10. If you type 20 as r, the area is four times too large.
Is this sphere surface area?
No. A sphere is 4πr². At r = 10 the wall is about 1256.64. Here the flat circle is 314.16.
What if I leave r empty?
Nothing is computed. Type a radius or load the 10 example. Empty r does not become a point on the plane.
Do inches change 78.54 at r = 5?
Not the digits. A typed 5 still computes the same π r². Only the in² label changes. Convert yourself when 5 cm is not 5 in.
Does a cylinder use the same 314.16?
A cylinder multiplies πr² by height. At r = 10 and h = 1 the volume is in the same ballpark as the area. There is no h here.
Is π 3.14, or the π constant?
The calculator uses π, not 22/7. That is why r = 1 is 3.14 to two places, not 3.00.
Why show r = 0.5 if the result is 0.79?
To show a small radius. A quarter times π is about 0.79. The formula does not need r greater than 1.
Is r = 3 equal to 28.27?
Yes. Nine times π rounds to 28.27. At r = 7 you get 153.94, at r = 12 you get 452.39.
Knowledge sources
The calculator computes the same formula as the definition below.
Page updated in 2026.