The circumference = pi x r2, and the diameter = 2r.
So if you start with the circumference, you divide that number by pi (3.1416), then take the square root. What's left is the value of the circle's radius; double it to get the diameter.
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Take "pi" times the diameter of the circle. Pi = 3.14159 (ALWAYS) If circle has a diameter of 10cm, circumference 31.41cm If circle has a radius of 2inches, the circumference is 12.57 inches. (radius = 2, diameter = 4)
Assuming you are talking about a circle:* If you know the radius of the circle, multiply that by 2. * If you know the circumference of the circle, divide that by pi. * If you know nothing, you need to measure either the radius, the diameter, or the circumference.
The circumference of a circle with a diameter of 25 is: 78.54
If the diameter of a circle is 9 the circumference is: 28.3
If a circle has a diameter of 2.25, the circumference is: 7.069