The formula: sheets of gypsum board
- Wall area = 2 × (length + width) × wall height. The perimeter times the height.
- Ceiling area = length × width, when the ceiling is boarded.
- Net area = wall area + ceiling area − openings. Each opening is count × width × height.
- Sheets = net area × (1 + waste) ÷ sheet area, rounded up. A 4 × 8 ft sheet covers 32 sq ft, a 4 × 10 covers 40 and a 4 × 12 covers 48.
Round up, never to the nearest: board is bought in whole sheets. Walls here are boarded on the room side; a partition boarded on both sides, or a second layer, counts its wall area again.
Worked example 1: walls and ceiling, no openings
A room 30 by 20 ft with 9 ft walls, boarded on the walls and the ceiling, 4 × 12 ft sheets and 10% waste. These are invented study numbers and the calculator's starting example.
- Perimeter = 2 × (30 + 20) = 100 ft. Walls = 100 × 9 = 900 sq ft.
- Ceiling = 30 × 20 = 600 sq ft. Total = 1,500 sq ft.
- With 10% waste: 1,500 × 1.10 = 1,650 sq ft.
- Sheets = 1,650 ÷ 48 = 34.4, rounded up to 35 sheets.
Check: 1,500 ÷ 48 = 31.25 sheets before waste, and 31.25 × 1.1 = 34.4.
Worked example 2: openings and a smaller sheet
A room 14 by 12 ft with 8 ft walls and a boarded ceiling, one door 3 by 7 ft and two windows 4 by 4 ft, 4 × 8 ft sheets and 12% waste.
- Perimeter = 2 × (14 + 12) = 52 ft. Walls = 52 × 8 = 416 sq ft. Ceiling = 14 × 12 = 168 sq ft. Total = 584 sq ft.
- Openings = 1 × 3 × 7 + 2 × 4 × 4 = 21 + 32 = 53 sq ft. Net = 584 − 53 = 531 sq ft.
- With 12% waste: 531 × 1.12 = 594.72 sq ft.
- Sheets = 594.72 ÷ 32 = 18.59, rounded up to 19 sheets.
Check: 531 ÷ 32 = 16.594 sheets before waste, and 16.594 × 1.12 = 18.585. Without the openings the room would take 584 × 1.12 ÷ 32 = 20.44, so 21 sheets. These are original arithmetic examples, not exam questions.
Drywall sheet questions
How do I figure how many sheets of drywall I need?
Add the wall area (perimeter × wall height) and the ceiling area, take off the openings, add your waste, divide by the area of one sheet and round up. A 20 by 30 ft room with 9 ft walls has 900 sq ft of wall and 600 sq ft of ceiling. With 10% waste that is 1,650 sq ft, and 1,650 ÷ 48 = 34.4, so 35 sheets of 4 by 12 ft.
Should I deduct doors and windows?
Do what your estimate or the question says. In the second worked example a door and two windows take 53 sq ft off a 584 sq ft room, and the order drops from 21 to 19 sheets of 4 by 8 ft.
Why round up and not to the nearest sheet?
Board is bought in whole sheets, and rounding down leaves part of the room without board. 34.4 sheets means 35.
Where is this on the NASCLA exam?
Finishes is one of the twelve subject areas on the NASCLA outline, with 5 of the 115 scored questions. The Bid Bond course works this takeoff in its Finishes lesson.
See the NASCLA exam outline for all twelve subject areas and their question counts.