Semicircle Area Calculator
This semicircle area calculator determines the area of a half circle, as well as the circumference of a semicircle.
Semicircle area157.079633half of πr²
Curved edge31.415927half the circumference
Perimeter51.415927arc plus the diameter
Radius10
Centroid height4.244132above the flat edge
Full circle area314.159265
The formulas
area = ½πr², perimeter = πr + 2r
Everything follows from the radius
A circle has one degree of freedom, so any single measurement fixes all the others:
d = 2rC = 2πr = πdA = πr² = πd²/4 = C²/4π
Going backwards, r = C/2π and r = √(A/π). The calculator above accepts whichever you happen to have.
Area grows with the square
Double the radius and the circumference doubles, but the area quadruples. That is why a 16-inch pizza is more than twice the food of a an 11-inch one, and why pipe capacity rises so sharply with bore.