You may have been thinking about the definition of $\displaystyle\pi\usepackage{cmbright}\Huge\$ , which equals the circumference divided by the diameter. That is, $\displaystyle\pi=\frac{{C}}{{d}}\usepackage{cmbright}\Huge\$ , so the circumference $\displaystyle{C}=\pi{d}\usepackage{cmbright}\Huge\$ .