Pi sign name desc r Radius d Diameter $2r$ C Circumference π Pi $\frac{C}{d}$ τ Tau $\frac{C}{r}$, $2\pi$ π τ Circumference $2\pi r$, $\pi d$ $\tau r$ Area $\pi r^2$ $\frac{1}{2}\tau r^2$ Euler's formula $e^{i\pi} = -1$ $e^{i\tau} = 1$