   SEARCH HOME Math Central Quandaries & Queries  Question from Tracey: Hello, I own a custom workroom, I am figuring out fabric quantities or an estimate. When at the job site, I forgot to measure the diameter of the semi circle shape that I have to make a cushion for!!! If the circumference of the semi circle is 165" what would the diameter be, maybe half this measurement???? I appreciate any help...I basically need the width from left to right to see if I can fit the cushion on one width of fabric that is 52" wide! Hi Tracey,

The relationship between the circumference $C$ and the diameter $D$ of a circle is

$C = \pi \; D$

where $\pi$ is approximately 3.1516. Thus if you know the diameter of a circle you can find the circumference by multiplying the diameter by $\pi.$

If you divide both sides of the equation above by $\pi$ it becomes

$D = \frac{C}{\pi}$

and hence if you know the circumference of a circle you can find its diameter by dividing the circumference by $\pi.$

I hope this helps,
Penny      Math Central is supported by the University of Regina and the Imperial Oil Foundation.