Semicircle Area Calculator

A semicircle is exactly half of a circle, formed when a diameter divides the circle into two equal regions. Its area is therefore half the familiar circle area. If the radius is r, the semicircle area equals pi times r squared divided by two. This calculator applies that formula directly and can begin with either a radius or a diameter. The radius measures from the center of the original circle to its curved edge. The diameter crosses the entire circle through the center and is twice the radius. When you choose the diameter option, the calculator divides the entered diameter by two before finding the area. For example, a diameter of 10 gives a radius of 5, and the semicircle area is one half times pi times 25, or approximately 39.2699 square units. Area always uses squared units. A radius measured in centimeters produces square centimeters, while a radius measured in feet produces square feet. The calculator leaves the unit generic so it works with any consistent system. If you change a length scale, remember that area does not change linearly: doubling the radius creates four times the area because the radius is squared in the formula. A semicircle is also a 180-degree sector and a circular segment whose chord is the diameter. Those descriptions lead to the same area. However, area should not be confused with perimeter. The boundary length of a semicircle includes the curved half-circumference plus the straight diameter. If a problem asks how much material covers the shape, use area; if it asks for edging, fencing, or trim around the boundary, use perimeter instead. Semicircular shapes occur in arched windows, garden beds, running tracks, tunnels, architectural openings, and classroom geometry problems. The formula can help estimate surface material, paint, flooring, glass, or land coverage, provided the shape is truly half of a circle. Measure along the straight edge to get the diameter or from its midpoint to the arc to get the radius. The calculator accepts decimal measurements and uses double-precision arithmetic before rounding the displayed answer. Enter a positive value, select the measurement type carefully, and preserve enough decimal places when transferring the result into a later calculation. The displayed radius is especially useful when starting from diameter because it makes the conversion transparent and easier to verify.