Plane figure · Circles
Circle
All points at the same distance r from the centre.
Dimensions
Results
area A
circumference C
Formulas
Cut the disc into thin slices and lay them head to tail: they form a near-rectangle πr long and r high. π is the same for every circle: circumference divided by diameter.
Checked: the A formula agrees with a Monte Carlo measurement of the shape itself, made from its geometric definition, every time the site is built.
Topology lens
Topologically a disc, like every figure here without a hole: Euler characteristic 1. Stretching turns it into a circle, a square or a star. More on the lens.