Deck & Fence
Plywood Calculator
Find out how many sheets of plywood or OSB you need for a subfloor, wall sheathing or roof deck. Enter the areas and sheet size, and this plywood calculator gives the exact sheets, whole sheets to buy with an allowance, leftover and cost.
Result
Enter your measurements and press Calculate. What to buy, the working and the cost will appear here.
How to use the plywood calculator
- Add each area to cover: floor sections, walls or roof planes. Subtract large openings if they are not sheathed.
- Choose the sheet size: 4 × 8 ft is standard; 9 and 10 ft sheets suit tall walls.
- Set the allowance (10% for floors and walls, 15% for complex roofs), add the price per sheet and press Calculate.
How many sheets of plywood do I need?
sheets = area ÷ sheet area × (1 + allowance), rounded up
A 4 × 8 ft sheet covers 32 ft². A 12 × 16 ft floor (192 ft²) needs exactly 6 sheets, or 7 with 10% for cuts.
Worked example: subfloor for two rooms
A 16 × 12 ft room and a 10 × 8 ft room get a new subfloor of 4 × 8 ft sheets at $42 each:
- Area 1: 16 ft × 12 ft = 192 ft²
- Area 2: 10 ft × 8 ft = 80 ft²
- Sheets: 272 ft² ÷ 32 ft² = 8.5 × 1.1 → 10
Sheets to buy: 10; Area to cover: 272 ft²; Sheets (exact): 8.5; Estimated cost: $420.00. Open this example in the calculator.
Typical uses and thickness
| Use | Common choice |
|---|---|
| Subfloor | 23/32 in (3/4 in nominal) tongue-and-groove plywood or OSB |
| Wall sheathing | 7/16 in OSB or 1/2 in plywood |
| Roof deck | 7/16–5/8 in OSB or plywood, depending on rafter spacing |
| Shelving and cabinets | 3/4 in hardwood plywood |
The right thickness depends on joist or rafter spacing and your local code; the span rating stamped on the sheet tells you where it can be used.
Layout tips
- Run sheets across joists and stagger the end joints so they do not line up.
- Leave the recommended gap (often 1/8 in) between sheets for expansion.
- Plan cuts so offcuts start the next row; the allowance covers typical waste, but awkward shapes may need more.
Assumptions and limitations
- The count is by area, not a cutting diagram; very irregular shapes can need extra sheets.
- Roof areas must be the sloped area; use the roofing calculator to find it.
Plywood questions
How many sheets of plywood do I need for a 12 × 12 room?
144 ft² ÷ 32 ft² = 4.5 sheets. With a 10% allowance that is 4.95, so buy 5.
How many sheets of OSB for a roof?
Divide the sloped roof area by 32 and add 10–15%. A 1,400 ft² roof needs about 44 sheets plus waste, so 49–51.
Formulas, unit conversions and default values are explained on our methodology page. Results are estimates; check manufacturer data and local codes before you buy or build.