Short answer: divide the wall and ceiling areas by the area of one sheet, add 10–15% waste, and round up. Walls and ceilings are counted separately because offcuts rarely cross over.
Step 1 — Measure walls and ceiling
walls = 2 × (length + width) × height − doors − windows
ceiling = length × width
ceiling = length × width
A 12 × 12 ft room with 8 ft walls: 2 × 24 × 8 = 384 ft² of wall and 144 ft² of ceiling.
Step 2 — Sheets
sheets = ⌈walls × (1 + waste) ÷ sheet area⌉ + ⌈ceiling × (1 + waste) ÷ sheet area⌉
With 4 × 8 ft sheets (32 ft²) and 10% waste: walls 384 × 1.1 ÷ 32 = 13.2 → 14; ceiling 144 × 1.1 ÷ 32 = 4.95 → 5. Total 19 sheets.
Sheet sizes
| Sheet | Area | Good for |
|---|---|---|
| 4 × 8 ft | 32 ft² | Easy to carry, standard walls |
| 4 × 10 ft | 40 ft² | 9 ft walls and ceilings |
| 4 × 12 ft | 48 ft² | Fewer joints, two people |
Which board?
| Use | Board |
|---|---|
| Standard walls | 1/2 in (12.5 mm) |
| Ceilings, 24 in joists | 1/2 in sag-resistant or 5/8 in |
| Garage / fire-rated walls | 5/8 in Type X |
| Bathrooms (outside showers) | 1/2 in moisture-resistant |
Screws, compound and tape
- Screws: about 1 per ft² of board (10.8 per m²).
- Joint compound: about 0.14 lb per ft² for a three-coat finish.
- Tape: about 0.37 ft per ft² of board.
For the example room (528 ft² of board): roughly 530 screws, about 73 lb of compound (two 4.5-gallon buckets) and about 195 ft of tape.
Common mistakes
- Counting walls and ceiling together. Offcuts rarely cross over, so count them separately.
- Skipping waste. Cut-outs and damaged corners add up.
- Short sheets on tall walls. Horizontal joints at eye level are the hardest to finish.