Fence coverage is really just a surface-area problem: multiply the linear footage by the height to get the area of one face, then multiply again by however many faces and coats you're applying. That total area gets divided by the product's coverage rate, which depends heavily on the wood — rough-sawn cedar or pine soaks up far more stain per square foot than a smooth, previously-sealed surface, so it's worth checking the coverage rate on the can rather than assuming a standard number.