Free Tool
Sheathing and Subfloor Sheet Calculator
Work out how many 4 by 8 foot sheets of plywood or OSB a floor, wall or roof needs. Enter the surface size, the openings and the waste allowance.
Sheathing and Subfloor Sheet Calculator
Result
Preliminary planning only. This tool does not replace the drawings. The designer or engineer of record and the building permit set govern every size, span and detail.
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Paste this link into ChatGPT, Claude, Gemini, Copilot, Perplexity or any AI assistant and ask it to read the result.
Change the numbers in the link to match your job. The full list of tools is at /api/tools/.
How the calculator counts
- Area. Length times width. For a wall, the width is the wall height. For a roof, enter the plan length and the plan width including the overhangs, and the tool applies the slope factor.
- Slope factor. The square root of one plus the square of the pitch divided by 12. A 6/12 roof has a factor of 1.118.
- Openings. The total area you enter is subtracted from the surface area.
- Waste. Your percentage is added to the net area, then divided by the sheet area and rounded up to whole sheets.
- Not counted. Panel thickness, grade and span rating, fasteners, adhesive, edge clips, underlay and house wrap. They come from the drawings and Section 9.23 of the Ontario Building Code.
This is area arithmetic, with no code tables involved. This tool is for preliminary planning. The designer or engineer of record and the building permit set govern.
Frequently asked questions
How many sheets of plywood or OSB do I need?
Divide the surface area by the area of one sheet and round up. A 4 by 8 foot sheet covers 32 square feet. Add a waste allowance for cuts, offcuts and damaged sheets. Ten percent is a common planning figure for simple surfaces.
How does the calculator handle a sloped roof?
Roof sheathing covers the sloped surface, which is larger than the plan area. The calculator multiplies the plan area by the slope factor, the square root of one plus the pitch ratio squared.
Should I subtract the windows and doors?
Enter the total area of openings and the calculator subtracts it. Many builders subtract only openings larger than a sheet, because the cut-outs from small openings are rarely reusable. The waste allowance covers the difference.
What panel thickness do I need?
Thickness depends on the joist or stud spacing, the panel span rating and the use, and is set by the drawings and the Ontario Building Code. This tool counts sheets only.
Does this count fasteners, house wrap or underlay?
No. It counts sheets. Fastening is set by the drawings and Section 9.23 of the Ontario Building Code.
Is this a quote?
No. It is a planning tool. Contact us with your plans and we will arrange a free site visit.
For wrap and panel choices, read Sheathing and House Wrap.
Related: Subfloor and sheathing, Wall framing calculator, Roof pitch and rafter length calculator.
More free tools are on the framing tools page.
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Want a framing quote?
Send your plans, photos or a description of the project. We will arrange a free site visit and return an itemized quote.
