Everything hangs off the radius
Whichever measurement you supply, the first move is to recover the radius — d ÷ 2, or C ÷ 2π, or √(A ÷ π) — after which the other three are one formula each. The working shows that conversion explicitly, because it is the step people skip and then wonder why an answer is out by a factor of two or four.
That factor is the usual culprit: doubling the radius doubles the circumference but quadruples the area, since area depends on r². A 16-inch pizza is not twice the food of an 8-inch one; it is four times.
π and how much precision you need
π is irrational, so every answer here is a rounded decimal rather than an exact value. The calculation uses full double precision — around 15 significant digits — which is far beyond any physical measurement you could take.
For mental arithmetic 3.14 is fine, and 22/7 is accurate to about 0.04%. For anything you are going to cut or build, the constraint is your tape measure, not π.
Frequently asked questions
What about a semicircle or a sector?
Halve the area for a semicircle, but note the perimeter is not half the circumference — you also have to add the straight diameter edge across the flat side.
Is anything uploaded?
No. The circle is drawn to scale locally.
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