What Is the Drywall Calculator?
The Drywall Calculator estimates how many sheets of gypsum board are needed to cover the walls and ceiling of a room. It also estimates the finishing materials that go with the board: joint compound, paper or mesh tape, and screws.
Drywall, also called sheetrock, gypsum board or plasterboard, is sold in sheets 4 feet wide and 8, 10 or 12 feet long. The calculator works from the room’s dimensions, subtracts standard allowances for doors and windows, adds a waste factor, and divides by the area of the sheet size you choose.
A sound estimate prevents two familiar problems. Too few sheets stops the job while you fetch more. Too many leaves heavy, fragile sheets to store or return.
How the Calculator Works
The wall area of a rectangular room is its perimeter multiplied by its height. The calculator finds the perimeter from the length and width, multiplies by wall height, and subtracts 20 square feet per door and 15 square feet per window.
If you choose to include the ceiling, it adds the floor area of the room, since a flat ceiling is the same size as the floor.
The total is increased by the waste percentage and divided by the sheet area. The result is rounded up to whole sheets. Joint compound is estimated at one gallon per 100 square feet, tape at about 0.37 feet per square foot, and screws at roughly one per square foot of board.
The Drywall Formula
Wall area = 2 × (Length + Width) × Height − (Doors × 20) − (Windows × 15)
Total area = Wall area + Ceiling area (Length × Width)
Sheets = Total area × (1 + Waste % ÷ 100) ÷ Sheet area, rounded up
Sheet areas
- 4 × 8 feet = 32 square feet
- 4 × 10 feet = 40 square feet
- 4 × 12 feet = 48 square feet
Step-by-Step Explanation
- Add the room length and width and multiply by 2 for the perimeter.
- Multiply the perimeter by the wall height.
- Subtract 20 square feet for each door and 15 for each window.
- Add length times width if the ceiling is being boarded.
- Multiply by 1.10 to add 10 percent waste.
- Divide by the sheet area and round up.
Worked Example
A bedroom is 14 feet by 12 feet with 8 foot walls, one door and two windows. Walls and ceiling will both be boarded with 4 × 8 sheets.
- Perimeter: 2 × (14 + 12) = 52 feet.
- Gross wall area: 52 × 8 = 416 square feet.
- Openings: 20 + 30 = 50 square feet.
- Net wall area: 366 square feet.
- Ceiling: 14 × 12 = 168 square feet.
- Total: 534 square feet.
- With 10 percent waste: 587 square feet.
- Sheets: 587 ÷ 32 = 18.4, so 19 sheets.
Finishing needs about 5.3 gallons of joint compound, 198 feet of tape and around 600 screws, which is 2 pounds. Using 4 × 12 sheets instead would need 13 sheets and leave fewer joints to finish.
How to Use the Calculator
- Enter the room length, width and wall height.
- Choose whether to include the ceiling.
- Enter the number of doors and windows.
- Pick the sheet size you plan to buy.
- Set the waste allowance.
- Press Calculate and read sheets and finishing materials.
For rooms with sloped ceilings or irregular walls, measure each surface with the Square Footage Calculator and divide the total by the sheet area. Insulate exterior walls before boarding, using the Insulation Calculator to size the order. New partition framing can be priced with the Lumber Calculator. Once the board is finished and sanded, the Paint Calculator estimates primer and paint, or use the Wallpaper Calculator for papered walls.
Practical Tips
- Use the longest sheets you can handle and get into the room. Fewer joints mean less taping and a flatter wall.
- Hang ceilings first, then walls. Wall sheets help support the ceiling edges.
- Hang wall sheets horizontally in rooms with 8 foot ceilings. The long tapered seam sits at a comfortable working height.
- Stagger end joints so they do not line up between rows.
- Keep sheets flat and dry before use. Stored on edge, they bow.
- Use 1/2 inch board for walls, 5/8 inch or sag-resistant 1/2 inch for ceilings, and moisture-resistant board in bathrooms.
- Drive screws so they dimple the paper without breaking it.
Common Mistakes
Subtracting every opening exactly. Sheets are cut around doors and windows, and the cut-outs are rarely reusable. Many installers subtract nothing for small openings.
Forgetting the ceiling. It can be a third of the total area.
Using the wrong thickness. Thin board on ceilings sags between joists over time.
Underestimating compound. A smooth Level 5 finish uses noticeably more than a standard finish.
Too many small pieces. Using offcuts everywhere to save a sheet creates extra joints, which cost more in finishing time than the sheet would have.
Not planning for access. A 12 foot sheet will not turn in a narrow stairwell. Check before ordering.
Board Types
Standard white or grey board suits most rooms. Moisture-resistant board, often green or purple, is used in bathrooms, laundries and kitchens. Type X fire-rated board is 5/8 inch thick and is required in garages attached to houses and around some utility rooms. Cement backer board, not drywall, should go behind tile in showers. Sound-damping board is available for bedrooms and media rooms.
Finishing Materials Explained
Joint compound comes premixed in pails or as a setting powder. All-purpose premix is the easiest to use. Setting compound hardens chemically and is useful for filling and first coats. Paper tape is strongest for flat joints and inside corners. Mesh tape is self-adhesive and quicker but should be used with setting compound. Outside corners need metal or vinyl corner bead, measured in linear feet.
Screw Spacing
On walls, screws are typically placed every 16 inches along each stud. On ceilings, spacing tightens to 12 inches. A 4 × 8 sheet on studs at 16 inch centres takes about 28 to 32 screws. Use 1 1/4 inch coarse-thread screws for 1/2 inch board on wood framing and fine-thread screws for steel studs.
Horizontal or Vertical Hanging?
In a room with 8 foot walls, two horizontal rows of 4 foot wide sheets cover the full height. The single long seam runs at about waist height, and 12 foot sheets can span many walls without any butt joints. Vertical hanging places every seam on a stud and is common in commercial work with steel framing and in rooms with ceilings above 9 feet. For most houses, horizontal hanging gives fewer linear feet of joint to finish.
Planning Delivery and Handling
A 4 × 8 sheet of 1/2 inch drywall weighs about 51 pounds, and a 4 × 12 sheet about 77 pounds. Arrange delivery into the room where the sheets will be used if you can. Stack them flat on the floor, across the joists, and spread the load in older houses. A rented panel lift makes ceiling work safer and is well worth the cost for anything more than a small room. Cut sheets by scoring the face paper with a sharp knife, snapping the core, and cutting the back paper.
Estimating Cost
Drywall itself is one of the cheaper materials in a renovation. The larger cost is finishing. When you budget, list the sheets, compound, tape, corner bead and screws from this calculator, then add primer. If you are hiring out the work, installers usually quote per square foot of board, hung and finished to a stated level. Multiply the area with waste by that rate to check a quote. Ask whether the price includes delivery to the room, sanding, clean-up and disposal of offcuts, because those are common extras. For small repairs, a setting-type compound and a single sheet cut into patches is often all that is needed.