Circle Calculator (Radius • Diameter • Circumference • Area)
Relations: C = 2πr, d = 2r, A = πr², and inverses r = d/2 = C/(2π) = √(A/π).
Radius (r)
Use mixed units if that is easier to enter.
Diameter (d)
d = 2r
Circumference (C)
C = 2πr
Area (A)
A = πr²
Step-by-step derivation
Enter any one field, then click Calculate.
References & Notes
- Circle relations: C = 2πr, d = 2r, A = πr².
- Inverse relations: r = d/2 = C/(2π) = √(A/π).
- Mixed units are supported for convenience: ft/in and m/cm.
- After the first calculation, changing values or units recalculates automatically from the stored source value.
Use this Circumference Calculator to find the radius, diameter, circumference, and area of a circle from any one known measurement.
Calculation Method
Diameter = 2 × radius
Circumference = 2πr
Circumference = πd
Area = πr²
To find radius from circumference:
Radius = circumference ÷ (2π)
Example
For a circle with a radius of 5 cm:
Diameter = 10 cm
Circumference = 2 × π × 5 ≈ 31.42 cm
Area = π × 5² ≈ 78.54 cm²
Important Note
These formulas assume a true circle. Radius is measured from the center to the edge, while diameter passes through the center and is twice the radius.