The circumference of a circle, $C$, is $\pi$ times the diameter, $d$. The diameter is twice the radius, $r$. So if we know any one of these measurements for a particular circle, we can find the others. We can write the relationships between these different measures using equations:

$\displaystyle d = 2r$ $\displaystyle C = \pi d$ $\displaystyle C = 2\pi r$

If the diameter of a car tire is 60 cm, that means the radius is 30 cm, and the circumference is $60 \boldcdot \pi$, or about 188 cm.

If the radius of a clock is 5 in, that means the diameter is 10 in, and the circumference is $10 \boldcdot \pi$, or about 31 in.

If a ring has a circumference of 44 mm, that means the diameter is $44 \div \pi$, which is about 14 mm, and the radius is about 7 mm.