Estimating one of them is equivalent to estimating the other.
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You have to calculate the radius - half the diameter. Then square the radius, and multiply by pi (approximately 3.1416). If you want to do a mental estimate, round pi to 3.
Radius = Diameter/2 Diameter = 2* Radius
A radius is one half of a diameter so divide by two to get from a diameter to a radius. Or you can multiply the radius by 2 to get to the diameter.
A radius is half of a diameter. Example Diameter:4 Radius:2
The diameter of a circle is twice its radius.