Circle circumference calculator
π ≈ 3.14159Enter one value and unit—everything else updates in real time.
📏Radius (r)
r = d/2
ⓘ
Choose a length unit. The calculator will convert automatically between mm, cm, m, km, in, ft, yd, and miles.
Enter a non-negative number.
🎯Diameter (d)
d = 2r
ⓘ
Length unit—same set as radius. You can type here instead of radius if you prefer.
Enter a non-negative number.
🧭Circumference (C)
C = 2πr = πd
ⓘ
Length around the circle. Pick any length unit; results convert automatically.
Enter a non-negative number.
🟦Area (A)
A = πr²
ⓘ
Choose a square unit (e.g., cm², m²). Area converts automatically when you change units.
Enter a non-negative number.
Step-by-step derivation
References & formulae
| Quantity | Formula |
|---|---|
| Circumference | C = 2πr = πd |
| Area | A = πr² |
| Diameter | d = 2r |
| Radius | r = d/2 = C/(2π) = √(A/π) |
Sources: Wikipedia: Circumference, Wolfram MathWorld: Circle
Tips: Press Tab to move between inputs. Change any one field (radius, diameter, circumference, or area) and the rest update instantly.