Work out how many sheets of drywall a wall needs, one or both sides, in 4×8, 4×10 or 4×12 board.
Sheets = wall length × height × sides ÷ sheet area, plus waste. Openings are usually not deducted for small doors and windows, since the cut-outs are rarely reusable.
With 8 ft walls the perimeter is 44 ft, so 352 sq ft of wall: about 11 sheets of 4x8 for the walls, plus 4 for a 120 sq ft ceiling, before waste.
Most estimators don't subtract small openings because the offcuts rarely get reused. Subtract large openings like garage doors or storefronts.
10–15% is typical; more for rooms with many corners, soffits or short walls.
Doing it from plans instead? Read How to estimate drywall sheets from plans, or see all free takeoff calculators.