Balloons are rarely (if ever) spherical, so you need to decide what you mean by its circumference.
On possible way is to make a slip-knot noose with a piece of string. Slide the noose over the balloon, keeping the string tight. The noose will be opened to the extent of the balloon's circumference.
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Eratosthenes
Pi is the number of times the diameter of a circle will fit into the circumference. Pi is not a circle, and does not have a circumference or area. To calculate a circumference USING pi, the circumference is diameter * pi. To find the area using pi, area = pi * radius * radius
The circumference and area of a circle, with radius r is: circumference = 2*pi*r and area = pi*r2 Use the first to calculate r and then the second to calculate the area.
you divide the circumference by pi to get diameter then divide that by 2 to get radius then square the radius and multiply it by pi
it depends on what type of measurement you are using