Find how many sheets of drywall your project needs. Each dimension has its own unit selector (in/ft/yd/m).
Drywall is estimated by area. Measure each wall and ceiling (length times width, any unit), and add them up. Divide by the sheet coverage - a 4x8 sheet is 32 sq ft, a 4x12 is 48 (use the preset). Add about 10% for waste and cuts. Include both faces of a wall if you finish both sides.