A circle's key parts: the radius ($r$) from centre to edge, and the diameter ($d = 2r$) across the middle.
Formulae
Circumference $= 2\pi r = \pi d$ · Area $= \pi r^2$
Using $\pi \approx \tfrac{22}{7}$, a circle of radius 7 has circumference $2 \times \tfrac{22}{7} \times 7 = 44$ and area $\tfrac{22}{7} \times 7^2 = 154$.
Remember
- Diameter $= 2 \times$ radius.
- Circumference $= \pi d$ · Area $= \pi r^2$.