2D · curved shape
Circle
Every point on a circle is the same distance from its center. This distance, the radius, connects its boundary, area, and angle measurements.
RectangleChange a dimension to explore
A = w × hDimensions shown in the selected unit
Give your shape a size.
Use the same unit for every dimension. Changing the unit relabels the measurements; it does not convert them.
From dimensions to an answer
Know its properties
| Boundary | One continuous curve |
| Radius | Center to any boundary point |
| Diameter | Twice the radius |
| Symmetry | Infinitely many reflection axes |
Useful formulas
Area: A = πr²
Circumference: C = 2πr
Diameter: d = 2r
A little question. A clearer picture.
Circle: common questions
Wondering about the why? Start here.
Should I enter a circle’s radius or diameter?
Enter the radius: the distance from the center to the circumference. The diameter is twice the radius, so divide a given diameter by two first. A diameter of 10 cm means a radius of 5 cm.
Read the area formulasHow do I calculate a circle’s area and circumference?
Area is πr² and circumference is 2πr. For radius 3, these are 9π square units and 6π length units. The calculator displays rounded decimal results, so keep π in your working when you need an exact answer.
Explore part of a circle