what's the formula to find circumference?

d is diameter
r is radius
C is circumference
C = π d
C = 2 π r

If it's for the circle: C the circumference of that circle of Diameter D and Radius r is:
C=D*pi, or C=2*r*pi
Else for an ellipse of semi-major and semi-minor a,b respectively
C=pi*(a+b)

Circumference = pi*diameter

Circumference = 2*pi*radius

Formula Circumference

Circumference Formula

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RE:
what's the formula to find circumference?

Circumference = diameter * pi (2 * radius * pi)

circumference is 2*pi*radius or simply pi*diameter

= 2*pi*radius

2*pi*radius = pi*diameter