If you're finding diameter or radius based on circumference, use the formula C = πd. You would divide the circumference by pi to get the diameter, then divide the diameter by 2 to get the radius.
If you're finding diameter or radius based on area, use the formula A = πr^2. You would divide the area by pi, then find the square root of that value which is the radius. Just multiply the radius by 2 to find diameter.
