Measure the circle’s radius (r)
Use (C = 2pi r) to find the circumference
Measure the circle’s diameter (d)
Use (C = pi d) to find the circumference
If you know the area (A), compute (r = sqrt{frac{A}{pi}})
Use (C = 2pi r) after finding (r)
Use (pi approx 3.14159) if a decimal value is needed