The Circle Calculator finds the radius, diameter, circumference and area of a circle from any one of them.
| Radius (R) | 5.64189584 |
| Diameter (D) | 11.28379167 |
| Circumference (C) | 35.44907702 |
| Area (A) | 100 |
A circle is the set of points at the same distance, the radius, from a centre. The diameter is twice the radius. The circumference is the distance around. The area is the space inside. A chord joins two points on the circle; an arc is part of the circumference.
D = 2R. C = 2πR = πD. A = πR². From the area, R = √(A/π); from the circumference, R = C/(2π).
Worked example (defaults): area 100 gives R = √(100/π) = 5.6419, D = 11.2838 and C = 35.4491.
π ≈ 3.14159265 is the ratio of any circle's circumference to its diameter. It is irrational, so its decimals never end or repeat. Archimedes estimated it between 3 1/7 and 3 10/71 over 2,000 years ago.
How do I find the area of a semicircle? Halve the circle's area.
What is a sector? A pie-slice; see the Area Calculator.
Why does doubling the radius quadruple the area? Area grows with the square of R.