Circle Formulas
Area
A = pi × r²
where r is the radius
Circumference
C = 2 × pi × r
or C = pi × d (where d is diameter)
Diameter from Radius
d = 2 × r
Radius from Diameter
r = d / 2
About Pi
Pi (pi) is a mathematical constant approximately equal to 3.14159. It represents the ratio of a circle's circumference to its diameter and is the same for all circles.
Pi is an irrational number, meaning its decimal representation never ends or repeats. It has been calculated to trillions of digits, but most calculations only need a few decimal places.
pi = 3.14159265358979323846...
Circle Properties
Radius
The distance from the center to any point on the circle's edge. All radii of a circle are equal.
Diameter
The distance across the circle through its center. The diameter is always twice the radius.
Circumference
The distance around the circle (the perimeter). C = 2pi × r = pi × d.
Area
The space enclosed by the circle. A = pi × r².
Frequently Asked Questions
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