The area of a circle is pi times the radius squared. This is half of a circle with a radius of 1 (no units) such that it would have an area of pi/2.

As we celebrate again in 2018, here are some Pi Day facts you should know. What Is Pi? Pi represents the ratio of a circle’s circumference to its diameter.

How do we know? Cast your mind back to your geometry class, and you’ll remember that you find the area of a circle by squaring the radius and then multiplying all of that by pi.