Find a circle's circumference, area and diameter from its radius.
Circumference is the distance around a circle: pi times the diameter, or two pi times the radius. Pi is the ratio of any circle's circumference to its diameter, and it is the same for every circle regardless of size — that constancy is the whole point, and it is why one constant serves every circle ever drawn. Pi is irrational, so its decimal expansion never terminates or repeats; 3.14159 is ample for almost any practical purpose.
The relationship between circumference and area is worth keeping straight, because they scale differently. Circumference grows in direct proportion to the radius, while area grows with the radius squared. Double the radius and you double the distance around but quadruple the space inside. This is why a pizza of twice the diameter gives four times the food for twice the crust, and why larger pipes carry disproportionately more fluid than their circumference suggests. Note also that circumference refers specifically to circles and ellipses; the equivalent for a polygon is its perimeter, simply the sum of its sides.
Pi times the diameter, or equivalently two pi times the radius. Both give the same result, since the diameter is twice the radius.
Yes. The ratio of circumference to diameter is identical for all circles, which is precisely what makes pi a constant and why a single value works for any circle.
Circumference is the distance around a circle or ellipse. Perimeter is the general term for the distance around any shape, and for a polygon it is simply the sum of the side lengths.
Divide the circumference by pi. If you need the radius, divide by two pi instead. This is a practical way to measure a pipe or tree that you cannot measure across directly.