This article is from the Puzzles FAQ, by Chris Cole firstname.lastname@example.org and Matthew Daly email@example.com with numerous contributions by others.
How much will a band around the equator rise above the surface if it is
made one meter longer? Assume the equator is a circle.
The formula for the circumference of a circle is 2 * pi * radius. Therefore,
if you increase the circumference by 1 meter, you increase the radius by
1/(2 * pi) meters, or about 0.16 meters.