Step 1: Measure ceiling and walls separately
Always calculate ceiling drywall first, as it's installed before the walls. For walls, measure total linear feet and multiply by ceiling height.
Step 2: Choose the right sheet size
Standard sheets are 4x8 feet, but using 4x12 sheets means fewer joints to tape and sand. If you can maneuver larger sheets into the room, use them.
Step 3: Ignore doors and windows
When buying drywall, do not subtract for doors and windows unless they are massive. You will run the drywall right over the openings and cut them out.