Circle Calculator

Calculate radius, diameter, circumference, and area with a labeled diagram

r = 5 | d = 2r | C = 2πr | A = πr²
Radius
5
Diameter
10
Circumference
31.415927
Area
78.539816
r = 5d = 10C = 31.42A = 78.54

About this calculator

The Circle Calculator finds radius, diameter, circumference, and area from any single measurement. Enter one value and instantly see all four results with a labeled diagram. Useful for geometry, engineering, and any everyday problem involving circular shapes.

Common examples

  • Radius 5 cm → diameter 10 cm, circumference 31.42 cm, area 78.54 cm²
  • Diameter 12 in → radius 6 in, circumference 37.70 in, area 113.10 in²
  • Circumference 100 m → radius 15.92 m, diameter 31.83 m, area 795.77 m²
  • Area 50 ft² → radius 3.99 ft, diameter 7.98 ft, circumference 25.07 ft
  • Radius 1 (unit circle) → diameter 2, circumference ≈ 6.2832, area ≈ 3.1416