The area of a circle is πr², where r is the radius. The circumference (perimeter) is 2πr.
A circle with radius 3 has area π × 3² = 9π ≈ 28.27. The circumference is 2π × 3 = 6π ≈ 18.85.
If you have the diameter d instead of the radius, remember r = d / 2. Area becomes π(d/2)² = πd²/4.