Drywall Calculator

About the Drywall Calculator

Drywall — plasterboard in the UK, gypsum board, Sheetrock — is how most interior walls and ceilings are finished today. It goes up in large sheets screwed to the studs and joists, and the joints are then taped and filled to make a smooth surface ready for paint.

Estimating it is mostly about area: the walls, perhaps the ceiling, less the doors and windows, divided by the size of a sheet. The part that catches people out is waste. Sheets are cut around every opening and trimmed at every corner, so the offcuts add up, and small openings save far less board than their area suggests.

This drywall calculator works out the area for a rectangular room, subtracts openings, adds a waste allowance, and gives the number of sheets in the size you choose — along with an estimate of screws and how many sheets the other common sizes would take.

How to Use the Drywall Calculator

Choose imperial (feet) or metric (metres).

Enter the room's length, width and wall height.

Enter the total area of doors and windows. A standard interior door is about 21 square feet (1.9 m²).

Tick include the ceiling if you are boarding it too.

Pick a sheet size and adjust the waste allowance — 10 to 15% is typical.

How the Sheets Are Worked Out

  walls    =  2 × (length + width) × height
  area     =  walls + ceiling − openings
  sheets   =  area × (1 + waste %) ÷ sheet area, rounded up

Step-by-Step Example

A 12 × 10 ft room with 8 ft walls, a door and a window (40 ft²), ceiling included, 4 × 8 sheets, 12% waste.

  Walls:         2 × (12 + 10) × 8        =  352 ft²
  Ceiling:       12 × 10                  =  120 ft²
  Less openings:                              − 40 ft²
  Area:                                     432 ft²
  With waste:    432 × 1.12               =  484 ft²
  Sheets:        484 ÷ 32                 =  15.1  →  16 sheets

The same room would take 13 sheets of 4 × 10 or 11 sheets of 4 × 12.

Why the Waste Allowance Is Higher for Drywall

With most materials, subtracting a window means you buy that much less. With drywall it does not work out so neatly. A sheet is not skipped where a window sits; it is cut around the window, and the cut-out is usually too awkward a shape to use elsewhere.

Corners, ceiling edges and the strip above a door all produce offcuts too. That is why 10 to 15% is the usual allowance, and why rooms with many openings or angles can need more. A few spare sheets are also useful for repairs later.

Choosing a Sheet Size

4 × 8 ft (or 1200 × 2400 mm) is the standard. It is manageable for one person to carry, fits most vehicles and stairwells, and suits standard 8 ft (2.4 m) walls.

4 × 10 and 4 × 12 ft sheets cover more wall with each piece, which means fewer joints. Taping and finishing joints is usually the slowest and most skilled part of drywall work, so fewer seams saves real time. But larger sheets are heavy — a 4 × 12 sheet of ½-inch board weighs around 80 pounds — and hard to move through a house.

Smaller sheets such as 900 × 1800 mm suit small rooms, awkward access and working alone.

Hanging Order and Orientation

Ceilings first. The wall sheets then support the edges of the ceiling boards and hide their cut ends.

Horizontal on walls, where the height allows: on an 8 ft wall, two sheets hung horizontally leave one long seam at about waist height, easy to tape. Vertical hanging suits taller walls and some fire-rated assemblies.

Stagger joints so no four corners meet at a point, and avoid joints lining up with the edges of doors and windows, where cracks tend to start.

Screws, Tape and Compound

Allow about one screw per square foot of board — roughly 32 per 4 × 8 sheet — at about 12-inch spacing on ceilings and 16-inch on walls, into every stud or joist. Joint tape and compound depend on the number of seams more than the area, another reason larger sheets can save time and materials.

Choosing the Board

Not all board is the same, and the right type matters as much as the quantity.

Moisture-resistant board, usually green, is used in bathrooms, kitchens and utility rooms. It is not waterproof, so showers still need a proper tile backer board.

Fire-rated board, often pink or red, is required between a garage and the house and in other fire separations.

Acoustic board is denser and reduces sound between rooms.

If a room needs more than one type, calculate each area separately.

Understanding Your Result

Sheets needed is the number of sheets in your chosen size, including waste.

Area shows the wall and ceiling areas and the openings subtracted.

Screws gives an approximate screw count.

Other sheet sizes shows how many sheets the other sizes would take.

Worth knowing covers hanging order and orientation.

When Should You Use This Calculator?

Renovating a room, to order board in one delivery.

Finishing a basement or loft, where ceilings add a lot of area.

Comparing sheet sizes for cost and effort.

Pricing a job alongside insulation and studs.

Common Mistakes

Too little waste. Openings save less than their area.

Forgetting the ceiling. It can be a third of the total.

Buying sheets too big to get into the room. Check stairs and doorways.

Lining joints up with door edges. They crack.

Starting with the walls. Ceilings go first.