How much paint do I need, anyway?

The amount of paint or stain varies somewhat, depending on how dry the surface is, and whether it has been sealed with a primer.

A primer will generally cover from 400 to 500 square feet per gallon, depending on how porous the surface is, and how dry it is.

Paint can cover anywhere from 300 to 450 square feet per gallon, also depending on the surface you are painting. A drier surface will absorb more paint, much like a sponge, and so will cover less area. You will obviously also get less coverage if you spread the paint in thicker coats, but you may also get better coverage. Thinner coats will cover more area, but may not cover as completely.