Enter the size of the wall, floor or roof you’re covering, leave out any openings, and choose the sheet size. The calculator tells you how many sheets of plywood or OSB to buy.
Extra and cost
Works for plywood, OSB and other sheet goods. Sheet thickness depends on the job and your local code.
How many sheets do I need?
- Area to cover = length × width, minus doors, windows and other openings
- Sheets = area × (1 + extra) ÷ area of one sheet, rounded up
- A 4 × 8 ft sheet covers 32 square feet.
Worked example
A 40 ft wall, 8 ft high, sheathed with 4 × 8 sheets:
- 40 × 8 = 320 square feet
- With 10 percent extra: 352 ÷ 32 = 11 sheets
- Leave out 40 square feet of windows and doors: 280 square feet, which is 10 sheets
Sheet sizes
| Sheet | Covers | Typical use |
|---|---|---|
| 4 × 8 ft | 32 sq ft | Almost everything: walls, floors, roofs |
| 4 × 9 ft | 36 sq ft | Wall sheathing on 9 ft walls, fewer joints |
| 4 × 10 ft | 40 sq ft | Taller walls |
Plywood or OSB?
Both are used for sheathing and subfloors. OSB is usually cheaper and comes in larger sizes. Plywood handles moisture a little better and holds fasteners well. The sheet count is the same for both.
Common thicknesses
- Wall sheathing: commonly 7/16 inch OSB or 15/32 inch plywood.
- Roof sheathing: commonly 7/16 to 5/8 inch, depending on rafter or truss spacing.
- Subfloor: commonly 23/32 inch (sold as ¾ inch) tongue-and-groove.
The right thickness depends on the span between framing members and your local code.
Frequently asked questions
How many sheets of plywood do I need?
Divide the area in square feet by 32 for 4 × 8 sheets, add about 10 percent for cuts, and round up.
How many square feet does a sheet of plywood cover?
A 4 × 8 ft sheet covers 32 square feet.
Should I subtract windows and doors?
Yes for large openings. For small ones, many builders don’t bother, because the offcuts from openings are rarely big enough to reuse.
How much extra should I order?
About 10 percent for simple walls and floors. Roofs with hips, valleys and dormers need more.
How do I work out roof area for sheathing?
Measure the length of each roof slope from the ridge to the eave, along the slope, and multiply by the length of the roof. Add the slopes together.
Results are estimates for planning. See How We Calculate for the formulas we use.
