A circle is a rounded figure in which all points on the perimeter are roughly equidistant from the center. A building footprint is classified as circular when its plan resembles a circle; any regular polygon with 9 or more straight sides can be listed as a circle because it appears virtually round.

Circles can be indented by small slots or notches, and may allow small protrusions that do not disturb their overall impression of regularity. Many circular buildings, including stadiums, have open space in their centers.

The round overall impression is the deciding characteristic, so small notches or lobes can remain. A plan with pronounced wings radiating from a common point is radial-lobed; the presence of a central open space alone does not prevent a circular perimeter.