Known radius
For r = 5 cm, d = 10 cm, C ≈ 31.4159 cm, and A ≈ 78.5398 cm².
Enter a circle's radius, diameter, circumference, or area to derive the other three measurements using standard circle relationships.
The calculator normalizes one known measurement to the radius, then calculates diameter, circumference, and area.
The radius r runs from the center to the edge. The diameter d passes through the center between two points on the circle.
The diameter is twice the radius.
d = 2r · r = d / 2Circumference is the distance around the circle.
C = 2πr = πdArea measures the region enclosed by the circle.
A = πr²Divide the known circumference by two times π.
r = C / (2π)Use the positive square root because a circle radius is positive.
r = √(A / π)For r = 5 cm, d = 10 cm, C ≈ 31.4159 cm, and A ≈ 78.5398 cm².
For d = 12 m, r = 6 m, C ≈ 37.6991 m, and A ≈ 113.0973 m².
For C = 20π, r = 10, d = 20, and A = 100π.
For A = 49π, r = 7, d = 14, and C = 14π.
Select the measurement you actually know instead of entering redundant values.
Radius, diameter, and circumference use the selected length unit; area uses its squared form.
π is the ratio of circumference to diameter. Calculations use the full Math.PI value rather than 3.14.
Diameter, circumference, and area are all determined once radius is known. Diameter input is halved, circumference input is divided by 2π, and area input is divided by π before taking the positive square root.
Use a known diameter, circumference, or area: r=d/2, r=C/(2π), or r=√(A/π).
Multiply the radius by two.
Use C=2πr or C=πd.
Use A=πr².
Divide circumference by 2π.
Divide area by π and take the positive square root.
Diameter is twice radius, and radius is half diameter.
π is the constant ratio of a circle's circumference to its diameter.
No. Diameter crosses the circle through its center; circumference measures around it.
Area uses squared units, such as cm², m², in², or ft².
No. This calculator requires a finite measurement greater than zero.
Select Area, enter the squared-unit value, and the calculator derives radius before calculating the remaining values.