How do you find the area of a circle given its diameter?

The general equation for the area of a circle is A = [Pi]r2. Since the radius, r, is one half the diameter, divide the diameter by two and substitute that result for rin the equation. Alternatively, you could use this equation instead: A = [Pi]d2 / 4. The value of Pi is 3.14. If you need greater precision, you could use 355/113, which is accurate to six -- count 'em, six! -- decimal places.