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.