It certainly is easier to hit the studs with nails or screws if the sheets are installed horizontally--at the cost of one additional horizontal joint across the wall. It is a lot harder to have to share the 1 1/2 surface of a stud between two panels. Sheetrock, being made of paper and plaster, don't lend much more to the strength of a partition wall, in my humble opinion.
Most sheetrock comes in 4' wide x 8' long sheets. If you are sheetrocking walls, and they are 8ft. High, then just measure the length of the walls in feet and divide by four. If you are sheetrocking a ceiling, calculate by multiplying the length of the ceiling by the width. This will give you the square footage. Then figure the square footage of one piece of sheetrock. 4'x8' =32 square foot. So one piece of sheetrock will cover 32 sq. Ft. If you are rocking the ceiling, be sure to lay the sheetrock horizontally across the beams. If you screw them into the beams in the same direction that the beams are running, then you will lose stability and the sheetrock will start to sag over time.
