This uses the formula : Area = π d2/4. When dividing by the constant “square root of 4/p” in the example above, we are multiplying by “square root of π/4”. When we use the B scale for our answer, we are squaring this value and thus obtain the value π/4 and the square of the diameter, completing the formula.

Another method:

Another method ... a direct use of the formula: Area = π r2