Circle Calculator
Enter either a radius or diameter to calculate the main properties of a circle.
How to Use the Circle Calculator
Enter a radius, diameter or circumference value to calculate the other circle measurements.
Practical Example
A circle with a radius of 5 units has an area of about 78.54 square units and a circumference of about 31.42 units.
Factors That Affect the Result
Measurement accuracy, rounding and whether you use radius or diameter as the input can affect the displayed result.
Frequently Asked Questions
What is the area formula?
Area equals π multiplied by radius squared.
What is circumference?
It is the distance around the circle.
Is diameter twice the radius?
Yes. Diameter equals two times the radius.
Related Tools
Using Circle Measurements
Circle calculations connect radius, diameter, circumference and area. Diameter equals 2r, circumference equals 2πr, and area equals πr². If one measurement is known, the others can be derived consistently.
These formulas assume a true circle. For physical parts, measure across the widest point when using diameter and avoid mixing radius with diameter. Because area depends on the square of the radius, even a small measurement error can have a larger effect on area than on circumference.