Circle calculator
Circumference is 2πr, so adding one metre of length to a rope wrapped around the Earth raises it by 1 ÷ 2π — about 16 centimetres — all the way round. The result is independent of the original radius, which is why the same 16 centimetres would apply to a rope around a tennis ball. It is the clearest demonstration there is that circumference scales linearly with radius while area does not.
Diameter is twice the radius, circumference is 2πr, and area is πr². A circle of radius 5 has a diameter of 10, a circumference of 31.42 and an area of 78.54. Any one of the four gives the other three.
How to work with circles
Doubling the radius doubles the circumference and quadruples the area, because one is linear in r and the other goes with r². That single fact explains why a 16-inch pizza is more than twice the pizza of a 12-inch, why a wider pipe carries disproportionately more water, and why doubling the diameter of a telescope collects four times the light. It is the most useful piece of intuition in the whole of plane geometry.
Questions
2πr, or πd if you have the diameter.