The radius of a circle is the length of the line from the center to any point on its edge. The plural form is radii (pronounced "ray-dee-eye"). In the figure above, drag the orange dot around and see that the radius is always constant at any point on the circle.
Sometimes the word 'radius' is used to refer to the line itself. In that sense you may see "draw a radius of the circle". In the more recent sense, it is the length of the line, and so is referred to as "the radius of the circle is 1.7 centimeters"
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Use the calculator above to calculate the properties of a circle.
Enter any single value and the other three will be calculated. For example: enter the radius and press 'Calculate'. The area, diameter and circumference will be calculated.
Similarly, if you enter the area, the radius needed to get that area will be calculated, along with the diameter and circumference.
Diameter Which is the circle's 'width'. The diameter is two times the radius.
See diameter of a circle
Circumference
The circumference is the distance around the edge of the circle. See
Circumference of a Circle for more.