How many sheets of drywall do I need? The complete method

The short answer, and why it is not enough

The quick rule is to measure the area of the walls and ceiling in square feet, divide by 32 for 4 × 8 ft sheets, and add 10%. For a plain room it lands close. It goes wrong in three predictable ways: it counts the area of doors and windows that will never be covered, it ignores how sheets fall on walls whose length is not a multiple of the sheet, and it mixes walls and ceilings, which are cut from separate sheets and should be rounded up separately.

The method below fixes all three. It takes ten minutes with a tape measure and gives a count you can order without a second trip to the yard. The drywall sheet calculator follows it exactly, so you can check each step against it.

Step 1: measure the walls

Measure each wall’s length along the floor and the height from floor to ceiling. Round lengths up to the next half foot; the offcut is cheaper than a short sheet. In a rectangular room the walls come in two pairs, so the gross wall area is the perimeter times the height:

For rooms that are not rectangles, add the walls one by one. A wall under a sloped ceiling is a trapezoid, its length times the average of its two heights; a gable end is a triangle, half the width times the rise. The wall area calculator takes up to six walls with sloped tops, and the vaulted ceiling calculator handles the sloped ceiling itself.

Step 2: take out the openings

Subtract each door and window at its rough opening, not the glass or the slab. Typical figures are 20 sq ft for an interior door and 15 sq ft for a window; a patio door or a large picture window is worth measuring. Do not subtract outlets, switches, vents or small cut-outs: the board is cut around them and the scrap is waste anyway.

Some estimators never subtract openings and call the difference waste. That works on a house with few windows and over-orders on a sunroom. Subtracting openings and then adding an honest waste allowance gives a figure you can defend.

Step 3: measure the ceiling

The ceiling is length times width, from wall to wall. Measure it separately from the walls because it is hung separately, often with longer sheets and sometimes a different thickness. An L-shaped ceiling is two rectangles; hatches and skylight wells come out. The ceiling area calculator adds the pieces, and the ceiling drywall calculator turns the area into sheets with the right board for the joist spacing.

Step 4: choose the sheet

Four by eight is the default because one person can carry it and every store stocks it. Longer sheets, 4 × 10, 4 × 12 and 4 × 14, cover a wall with fewer butt joints, the end-to-end joints that have no taper and take the most work to hide. On an 8 ft wall hung horizontally, two rows of 4 ft sheets cover the height exactly, and the only question is how many sheets fit along each row. On a 9 ft wall, two rows fall a foot short; 54 in wide board or a vertical layout solves it. The guide to sheet sizes and orientation goes through the choices, and the sheet size optimizer compares every size on one wall.

Step 5: add waste and round up

Waste covers the cuts the room forces: offcuts too small to use, mistakes, and sheets broken in handling. The usual allowances are:

The waste factor calculator builds the percentage from the room’s shape, its openings and its height. Then divide each area by the sheet area, multiply by the waste factor and round up, walls and ceiling separately:

If you only have an area, the square feet to sheets converter does the same in one step.

A worked room

The waste rule gave 10% here: a simple room with four openings. The waste factor calculator arrives at the same figure from the room’s shape.

Rooms that need a different calculator

Some rooms have their own rules, and each has a calculator that applies them:

• Basements, several rooms at once, with or without ceilings: the basement drywall calculator.
• Garages attached to the house, where the wall and ceiling next to living space need fire-rated board: the garage drywall calculator and the garage fire separation guide.
• Bathrooms, where moisture-resistant board goes on the walls and cement board behind the tub: the moisture-resistant board calculator.
• One wall at a time, for a partition or a repair: the single wall calculator.
• Fire-rated walls and ceilings, in one or two layers of type X: the fire-rated drywall calculator.
• Curved walls, in two layers of 1/4 in board: the curved wall calculator.

From sheets to everything else

The sheet count drives the rest of the order. Screws are counted per sheet from the framing spacing: 28 for a 4 × 8 sheet on a wall with studs at 16 in, 35 on a ceiling, as the screw calculator shows and the screws per sheet guide explains. Joint compound follows the area and the finish level, about 9.4 gallons per 1,000 sq ft at Level 4, which the joint compound calculator turns into pails. Tape follows the joints, about 12 ft per 4 × 8 sheet plus the inside corners, in the tape calculator.

Board thickness matters too: 1/2 in on walls, and 5/8 in or sag-resistant 1/2 in board on ceilings with joists 24 in apart. The drywall thickness guide sets out where each goes.

Common mistakes

• Counting floor area. Drywall covers walls and ceilings; a 168 sq ft room needs about 514 sq ft of board.
• Rounding the whole job instead of each surface. Round walls and ceiling up separately.
• Ignoring the height. A 9 ft wall in 4 ft sheets needs a third row.
• No spare. One extra sheet costs less than a trip back for one broken corner.
• Buying by the bundle price alone. The sheet that suits the room saves joints, and joints are where the labour goes.

Put the calculator on your own site

Contractors, supply houses and DIY writers can put the sheet calculator on their own pages, free and without sign-up, from the embed page. It updates whenever the site’s figures are reviewed.