Circle Calculator

All four circle measures interconvert through the radius: d = 2r, C = 2πr, A = πr². Given any one, the rest follow — from area, r = √(A/π); from circumference, r = C/2π. π ≈ 3.14159 is the one constant tying them together.

π is the ratio of any circle's circumference to its diameter — the same for a coin and a planet. For estimation, C ≈ 3d is within 5%: a tree trunk you can just wrap arms around (~1.5 m) is about half a meter thick.