If you have the area of a semicircle with diameter that means that you have to divide the diameter by two to get the radius. R radius If you know the circles diameter you can find the radius by dividing the diameter in half. Use this calculator to easily calculate the area of a circle given its radius in any metric.
The radius is half the diameter.
This is because a semi-circle is just the half of a circle and hence the area of a semi-circle is the area of a circle divided by 2. The area formula is. Area of a circle p d2 2. If you have the area of a semicircle with diameter that means that you have to divide the diameter by two to get the radius.