A circle in taxicab geometry is a square in regular geometry (at an angle of ~45~ degrees if when placed in a Cartesian coordinate system). The area of such a "circle" is thus ~(\sqrt 2 r)^2 = 2r^2~.

The area of a regular circle is ~\pi r^2~. ~\pi~ can be embedded in the source code as a constant or calculated (better) as ~4 \operatorname{atan}(1)~ or ~\operatorname{acos}(-1)~. Some implementations of the C language define the constant M_PI in the header <math.h>, though this is not defined by the C standard.