Perimeter of a circle

Area A = 2 x π x r

with π (number pi) approximately equal to 3.14
Radius = AO = OB = r
Diameter = AB = 2 r

Example

Take a circle with radius r = 3 cm
Perimeter P of the circle = 2 x π x r ≈ 2 x 3.14 x 3 ≈ 18.84 cm²