How to calculate the area of a semicircle (half circle)

To work out the area of a semicircle you can use the formula A = (Pi*r^2)/2. This is because the area of a full circle is A = Pi*r^2 and a semicircle is half the area of a full circle so you divide this by 2. Just be careful to substitute the radius into the formula and not the diameter! The radius is the distance from the centre of the circle to the outside, so if you are given the diameter then half this number to get the radius.

In the example shown you are asked to calculate the area of a semicircle of diameter 8cm. First half the diameter to give a radius of 4cm. Next, substitute r = 4 into the formula A = (Pi*r^2)/2. So on your calculator type in (Pi*4^2)/2 This will give an answer of 25.1 squared centimetres. If you dont have a scientific calculator then use 3.142 for the value of Pi.