M3 · Thinker
Geometry
Circle area and circumference
Formula
area = pi r^2 ; circumference = 2 pi r
Rule
Pi is roughly 3.14. The diameter is twice the radius.
Example
r = 5 -> area = 78.5, circumference = 31.4
Tip
Use the radius, not the diameter, in both formulas.
Learn this concept
Perimeter & area