Reckoning / Shape / Annulus (ring)
Plane figure · Circles
Annulus (ring)
The flat ring between two circles with the same centre, like a washer.
Dimensions
Results
area A
Formulas
The big disc minus the hole. Written as π(R + r)(R − r) it is the mean circumference times the width: unroll the ring into a strip.
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
A region with one hole. Its Euler characteristic is 0: it can be stretched into any other ring, but never into a disc without tearing. More on the lens.