Exactly how round is the Earth?

The shape of the Geode, as it is called, is nearly a perfect sphere, but because the earth is spinning, it is about 21.5 kilometers flatter at the poles, and bulged-out at the equator by about the same amount. There are also other 'higher-order' shape deviations which make the Earth slightly pear- shaped with a larger southern hemisphere surface area than in the northern hemisphere, but at a level of a kilometer or so in radial girth. The biggest effect, though, is its polar flattening. If you had a basketball to represent the Earth's spherical average shape, the flattening would be 21/6500 = about 1/300 the radius of the basketball or 1/32 of an inch...give or take.