Sheets are 4 feet wide, always. The length is the choice, and picking the right one against your ceiling height is the difference between a clean job and a wall full of joints.
Drywall is 4 feet wide in every standard size. Lengths run 8, 10, 12 and up to 16 feet, with 4 by 8, 4 by 10 and 4 by 12 being the common ones.
Because the width is fixed, the length is where the decision is, and it is driven by the room rather than by preference.
Sheets hung horizontally on a wall with an 8 foot ceiling need two courses of 4 foot width, which is exactly 8 feet with no rip. That is why 8 foot ceilings and horizontal hanging go together so neatly.
A 9 foot ceiling leaves a foot to fill, which means either a rip strip along the top or bottom, or hanging vertically with 4 by 10 sheets. The second is usually tidier, because a horizontal rip creates a long joint at eye level.
Longer sheets mean fewer butt joints, and butt joints are the ones that are hard to finish invisibly because there is no tapered edge to bed the tape into. Using 12 foot sheets on a 12 foot wall gives you no butt joints at all on that wall, which is worth real money in finishing time.
It decides the sheet length, the hanging direction and the joint pattern. Assuming 8 feet and discovering 9 after delivery means either a wasted order or a wall with an unnecessary joint running through it.
Total the wall area, add the ceiling area if you are boarding that too, then divide by the area of one sheet.
A 4 by 8 sheet is 32 square feet. A 4 by 10 is 40. A 4 by 12 is 48.
For a room 12 by 15 with an 8 foot ceiling: perimeter 54 feet times 8 is 432 square feet of wall. Divided by 32 is 13.5, so 14 sheets before waste. The 180 square foot ceiling adds another 5.6, so 6 more.
Do not subtract door and window openings from the sheet count on a normal room. Sheets are hung over openings and cut out afterwards, so the opening does not save you a sheet, and the cut-out piece is rarely reusable. Only deduct for very large openings.
Drywall waste comes from cutting rather than from breakage, so it scales with how many awkward features a room has.
A plain rectangular room with two openings wastes very little. A room with several windows, a chimney breast, sloped ceilings, or a lot of small wall sections between openings wastes considerably more, because each one produces offcuts too small to use.
Ordering a few extra sheets is cheap insurance. Drywall is inexpensive relative to the cost of stopping work, and a damaged sheet during handling is common enough to plan for.
Sheet count is one line. These come from the same measurements.
Corner bead in particular is worth counting off a sketch rather than from memory, because external corners are easy to overlook when you are looking at a list of wall lengths.
Drywall is rigid and follows what it is fixed to. A stud wall that is out of plane will show through as a visible dip or bulge once it is boarded and painted, and no amount of jointing hides it.
Shoot the wall from a fixed point at several heights and along its length. Meaningful variation means packing the studs before boarding, which is far easier than fixing it afterwards. The wall measuring method is here, and the paint calculation that follows is here.
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