$${\mathtt{c}} = {\mathtt{\pi}}{\mathtt{\,\times\,}}{\mathtt{d}}$$
c= circumference (distance around a circle)
$${\mathtt{\pi}}$$= about 3.14 (um... it just "is" - to keep it simple)
d=diameter ($${\mathtt{2}}{\mathtt{\,\times\,}}{\mathtt{r}}$$) (distance across the circle, through the center)
r=radius (distance from the center to the edge of a circle)
swapping d for r (and re-arranging) gets: $${\mathtt{c}} = {\mathtt{2}}{\mathtt{\,\times\,}}{\mathtt{\pi}}{\mathtt{\,\times\,}}{\mathtt{r}}$$
.$${\mathtt{c}} = {\mathtt{\pi}}{\mathtt{\,\times\,}}{\mathtt{d}}$$
c= circumference (distance around a circle)
$${\mathtt{\pi}}$$= about 3.14 (um... it just "is" - to keep it simple)
d=diameter ($${\mathtt{2}}{\mathtt{\,\times\,}}{\mathtt{r}}$$) (distance across the circle, through the center)
r=radius (distance from the center to the edge of a circle)
swapping d for r (and re-arranging) gets: $${\mathtt{c}} = {\mathtt{2}}{\mathtt{\,\times\,}}{\mathtt{\pi}}{\mathtt{\,\times\,}}{\mathtt{r}}$$
