Plywood Calculator
Work out how many sheets of plywood or OSB you need for floors, walls, or roofs. Enter the area to cover and your sheet size to get sheet count with a waste allowance. Covers sheathing, subfloor, and underlayment jobs.
Plywood Sheet Layout Diagram
How the Formula Works
Calculate area
Area (ft²) = Length (ft) × Width (ft) This is the floor, wall, or roof surface to be covered
Apply waste factor
Area with waste (ft²) = Area × (1 + Waste %) Accounts for offcuts at edges, openings, and irregular shapes
Calculate sheets required
Sheets = ⌈Area with waste ÷ Sheet size (ft²)⌉ Always round up — you cannot buy a fraction of a sheet. A 4×8 sheet covers 32 ft², 4×10 covers 40 ft², 4×12 covers 48 ft²
Calculate total coverage
Coverage (ft²) = Sheets × Sheet size (ft²) The area all purchased sheets would cover if laid without cuts — excess above your project area is the expected offcut material
Verify overage
Overage = Coverage − Area A healthy overage is 10–15% of the project area. If overage exceeds 20%, check whether a larger sheet size reduces waste
Frequently Asked Questions
A 12×10 ft room is 120 sq ft. With standard 4×8 sheets (32 sq ft each): 120 ÷ 32 = 3.75, rounded up to 4 sheets net. With 10% waste: 120 × 1.10 = 132 sq ft ÷ 32 = 4.13, rounded up to 5 sheets. The calculator handles all rounding and waste automatically for any room size.
4×8 ft (1220×2440 mm) is the standard and most widely available size — use it for subfloors, wall sheathing, and roof sheathing. 4×10 and 4×12 sheets reduce the number of seams on long runs (like roof rafters or floor joists spaced at 10-foot intervals) but cost more per sheet and are harder to handle solo. Use longer sheets when minimizing seams matters more than ease of transport.
3/4 inch (18–19 mm) tongue-and-groove plywood is the standard for subfloors over joists at 16-inch spacing. Use 1-1/8 inch (28 mm) for joists at 19.2 or 24-inch spacing. For roof sheathing, 7/16 inch (11 mm) OSB or 1/2 inch plywood is common at 24-inch rafter spacing; 5/8 inch for heavier loads. Wall sheathing is typically 7/16 to 1/2 inch.
10% is standard for rectangular rooms with few openings. Increase to 15% for rooms with angled walls, bay windows, or many doors. Use 15–20% for diagonal subfloor or sheathing layouts, which produce significantly more offcut waste at every edge. Never go below 5% — even perfect layouts lose material to saw kerfs and edge trimming.
Measure the roof slope length (not the horizontal span) for each roof plane, multiply by the ridge length to get the plane area, then add all planes together. For a gable roof: each side = rafter length × ridge length. Add 10% waste for standard gable roofs; 15% for hip roofs with diagonal cuts at all four corners. Use this calculator for each rectangular plane and sum the sheet counts.
Both are code-compliant for subfloors. OSB is 10–15% cheaper per sheet and dimensionally uniform. Plywood is stiffer, holds fasteners better over time, and handles moisture exposure better during construction. Most builders use OSB for cost savings; some prefer plywood in wet climates or for premium builds. The sheet count calculation is identical — this calculator works for both materials.
3/4 inch plywood costs $55–$80 per 4×8 sheet at hardware stores (2025 pricing). For a 12×10 ft room needing 5 sheets: $275–$400 in material. OSB is $30–$50 per sheet for the same size. Prices vary significantly by region and fluctuate with lumber markets — always check current local pricing before budgeting.
Yes — always stagger seams by at least half a sheet (24 inches for 4×8 sheets) between rows. Aligned seams create a continuous weak line across the floor or wall. Staggering distributes loads across multiple joists or studs and is required by most building codes. Staggering does not increase material usage but does affect how you lay out cuts — plan your sheet arrangement before cutting.