Enter any single circle measurement - radius, diameter, circumference, or area - and get all the others calculated automatically.
Circle Calculator
LiveChange any one field and the other three update automatically.
The formulas
Diameter = 2 x radius
Circumference = 2 x pi x radius
Area = pi x radius^2
Example
Radius 5: diameter = 10, circumference = 2 x pi x 5 ≈ 31.42, area = pi x 25 ≈ 78.54.
Step-by-step guide
- Enter any one measurement you already know - radius, diameter, circumference, or area.
- The other three fields update automatically based on what you entered.
Common mistakes
Confusing radius and diameter - the diameter is always exactly twice the radius, a very common mix-up.
Mixing up circumference and area - circumference is the distance around the circle (a length), while area is the space inside it (squared units).
Frequently asked questions
What is pi?
Pi (approximately 3.14159) is the ratio of a circle's circumference to its diameter - the same for every circle, regardless of size.
Can I enter a negative number?
A circle's measurements are always positive in the real world, so negative inputs won't give a meaningful result here.
How is this different from the Area Calculator?
The Area Calculator covers several shapes and only solves for area. This tool is circle-specific and solves for all four measurements from just one input.
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