Plane figure · Quadrilaterals
Rectangle
Opposite sides equal and parallel, four right angles.
Dimensions
Results
area A
perimeter P
diagonal d
Formulas
Area counts unit squares: b rows of a squares each. This is where the idea of area as length times length comes from.
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.