Circle Calculator for Radius, Diameter, Circumference and Area

Updated By Unit Conversion Plus

Enter any one circle measurement to calculate the radius, diameter, circumference and area.

Known measurement

The unit labels the measurement; it does not convert the input.

Circle formulas

Every circle measurement can be derived from its radius. The diameter is twice the radius, circumference is 2πr, and area is πr².

d = 2r · C = 2πr = πd · A = πr²

Area and circumference use different units

Radius, diameter and circumference use length units. Area uses square units, such as cm² or m².

Exact and decimal results

Results involving π are irrational. This calculator displays useful decimal approximations while the formulas show where π is used.

Frequently asked questions

Can I calculate radius from area?

Yes. Use r = √(A ÷ π).

What is the difference between diameter and radius?

The diameter crosses the whole circle through its centre and is twice the radius.