How many sheets of plywood?
Enter your walls or area, get 4×8 sheets of plywood or OSB instantly.
How many sheets of plywood do I need?
Divide the area by the area of one sheet and add about 10% for cuts. A 40 by 30 ft house with 9 ft walls has 2 × (40 + 30) × 9 = 1,260 square feet of wall; with 10% extra that is 1,386, and 1,386 ÷ 32 = 43.3, so 44 sheets of 4 × 8. Set the doors and windows to 0 to count full sheets over the openings.
How many square feet does a sheet of plywood cover?
A 4 × 8 sheet covers 32 square feet, a 4 × 9 sheet 36 and a 4 × 10 sheet 40. Longer sheets let one sheet run from the bottom plate to the top plate on 9 and 10 ft walls. Metric sheets are 1220 × 2440 mm, or 2.98 square meters.
Should I subtract windows and doors?
Many framers do not subtract openings smaller than a sheet, since the cut-out piece is usually waste. Subtracting them gives the smallest count; setting them to 0 gives a safer one. For big openings like a garage door, subtract them.
Is OSB counted the same way as plywood?
Yes. OSB and plywood sheathing come in the same sheet sizes and cover the same area. The span rating stamped on each panel, such as 24/16, tells you how far apart the roof rafters and floor joists can be for that panel.
How do I count sheathing for a roof or a floor?
Use the Area tab and enter the area. For a roof, that is the area of the slopes, not the footprint — the roof calculator turns a footprint and pitch into roof area. For a subfloor, it is the floor area of each room.
How many sheets do I need for a gable end?
A gable is a triangle: ½ × width × rise. For a 24 ft wide building with a 6/12 pitch the rise is 6 ft, so each gable is 72 ft². Two gables are 144 ft², and with 10% extra that takes 5 sheets of 4 × 8.
How the count is made
The calculator adds up the areas you enter (walls, gables, roof or floor), subtracts any openings you choose to deduct, adds the waste percentage, and divides by the area of one sheet: 32 ft² for 4 × 8, 36 ft² for 4 × 9 and 40 ft² for 4 × 10. The answer is rounded up to a whole sheet, since you cannot buy part of one. In metric mode the sheet is 1220 × 2440 mm, which is 2.98 m².
A worked example
A 16 ft by 24 ft subfloor is 384 ft². With 10% extra that is 422.4 ft², which needs 14 sheets of 4 × 8 or 11 sheets of 4 × 10. Because 4 feet is exactly 48 inches, the panel edges of full sheets fall on framing members spaced 16 or 24 inches on center. Choose the panel thickness from the span rating stamped on the sheet, not from the count.
Common mistakes
Using the footprint instead of the roof surface undercounts sheathing, since a sloped roof has more area than the floor under it. Counting only the wall area when buying for walls with gables misses the triangles above the top plate. Choosing the sheet length without looking at the wall height is the third: on a 9 ft wall, a 4 × 9 sheet runs full height, while 4 × 8 sheets need a second row of short pieces. And the 10% extra is a rule of thumb; complex shapes with many angles may need more.
Sources: sheet sizes and panel standards from the APA – The Engineered Wood Association (PS 1 for plywood, PS 2 for OSB); the sheet areas follow directly from the nominal dimensions. Related: roof calculator, roof pitch calculator, drywall calculator and framing calculator.