How do Icalculate the area of a circle from its diameter?

$A_{circle}=\pi r^2\\ d_{circle}=2r$

$d_{circle}=2r\\ r=\frac d2\\ A_{circle}=\pi \times (\frac{d}{2})^2\\ A_{circle}=\pi \times \frac{d^2}{4}$