How many sheets of drywall do I need?

Drywall is bought in sheets and installed in whole pieces wherever possible, so the count is area divided by sheet size, rounded up — plus a cut allowance for the pieces that get trimmed around openings, corners and ceilings.

Total wall length (ft)

Perimeter of the room: 2 x (length + width).

Wall height (ft)

Doors and windows (count)

Deducted at 21 sq ft each.

Sheet size 4 x 8 ft (32 sq ft)4 x 10 ft (40 sq ft)4 x 12 ft (48 sq ft)

Cut allowance (%)

Sheets needed

12sheets

Wall area 342 sq ftArea to buy 376 sq ft

Assumes full-height flat walls. Bulkheads, stair walls and ceilings add area that is not counted here, and irregular rooms should be split into rectangles.

Add the perimeter of the room rather than a single wall: a 12 × 12 ft room has 48 linear feet of wall and, at 8 ft ceilings, 384 sq ft of board before openings. Two doors and two windows remove about 84 sq ft, which is roughly two and a half sheets — worth deducting, since the offcuts rarely fit anywhere useful.

Sheet size changes the arithmetic more than most people expect. Long sheets mean fewer joints to finish, and finishing joints is the expensive, slow part of the job. A 4 × 12 ft sheet needs two people to hang but covers 50% more wall than a 4 × 8 with the same number of joints.

How the calculation works

Wall area = total wall length × height, minus 21 sq ft per door or window. Sheets = ceil(area × (1 + waste ÷ 100) ÷ sheet area), where sheet area is 32, 40 or 48 sq ft for the three standard sizes.

The 21 sq ft deduction is a standard 3 × 7 ft door; a window is usually smaller, so deducting the same figure is slightly conservative, which is the safe direction.

• Walls are flat and full height, with no bulkheads, soffits or half-walls, which need their own calculation.
• Openings are deducted at 21 sq ft each regardless of type; measure large openings separately.
• The cut allowance covers offcuts that cannot be reused, plus breakage in transport, which is real with 12 ft sheets.
• Ceilings are not included, and ceiling sheets are usually counted separately because they are hung before the walls.

Sources: Standard sheet sizes: 4 × 8 = 32 sq ft, 4 × 10 = 40 sq ft, 4 × 12 = 48 sq ft; Standard door opening deduction: 3 × 7 ft = 21 sq ft

Worked examples

Questions

How many sheets of drywall for a 12 × 12 room?

Twelve 4 × 8 sheets for the walls at 8 ft, allowing 10% for cuts. Add three or four sheets if you are doing the ceiling too.

Should I deduct doors and windows?

Yes, at about 21 sq ft each. Two doors and two windows is roughly 84 sq ft, or two and a half sheets.

What thickness of drywall should I use?

Half-inch is standard for walls and ceilings on 16 in centres; five-eighths is used on ceilings on 24 in centres and for better sound and fire performance. This calculator counts sheets, not thickness.

How much waste should I allow?

Ten percent for a simple room, fifteen where there are many openings or a vaulted ceiling. Twelve-foot sheets break more often in transit, which is a different kind of waste.

Do I need screws, tape and mud as well?

Yes. Roughly one pound of screws per 300 sq ft, and one 4.5 gallon bucket of joint compound per 500 sq ft of board for a level 4 finish.

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Written and maintained by Sam Winters. Last updated 2026-11-04. Every figure is computed in your browser; nothing you enter is sent anywhere. Found an error? Tell me — accuracy reports get fixed first.