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A drywall calculator does the whole hang-and-finish list, not just the sheets. Enter your room dimensions (or total area) and it returns how many sheets of drywall you need, plus the screws to hang them and the joint compound and tape to finish them.
Most calculators stop at the sheet count, which leaves you guessing on the supplies. This one estimates all four so a single trip covers the job — and the sheets aside, the screws, mud and tape all ship to your door.
Area = the wall perimeter times height, plus the ceiling, then divided by the sheet area (32 sq ft for 4 × 8, 48 for 4 × 12) with 10% waste. Screws are estimated at ~32 per sheet, joint compound at ~1 gallon per 100 sq ft for taping and two coats, and tape at ~40 ft per 100 sq ft.
A 15 × 12 ft room with 8 ft ceilings is about 612 sq ft of wall and ceiling. With 4 × 8 ft sheets that's 22 sheets, roughly 700 screws, 7 gallons of compound and about 245 ft of tape.