The relationship between $A$, the area of a circle, and $r$, its radius, is $A=\pi r^2$. We can use this to find the area of a circle if we know the radius. For example, if a circle has a radius of 10 cm, then the area is $\pi \boldcdot 10^2$ or $100\pi$ cm^{2}. We can also use the formula to find the radius of a circle if we know the area. For example, if a circle has an area of $49 \pi$ m^{2} then its radius is 7 m and its diameter is 14 m.

Sometimes instead of leaving $\pi$ in expressions for the area, a numerical approximation can be helpful. For the examples above, a circle of radius 10 cm has area about 314 cm^{2}. In a similar way, a circle with area 154 m^{2} has radius about 7 m.

We can also figure out the area of a fraction of a circle. For example, the figure shows a circle divided into 3 pieces of equal area. The shaded part has an area of $\frac13 \pi r^2$.