How to use the plywood calculator
- Choose roof sheathing or subfloor. For walls, use the stud calculator, which already counts wall sheathing with the studs and plates.
- Enter the plan length and width. On a roof, measure the eaves length and the span including overhangs, or enter several planes as extra square feet.
- For a roof, enter the pitch as inches of rise per 12 inches of run. Subtract large openings (stairs, chimneys, skylights) if you want them out of the count; small vents are usually left in the waste.
- Set waste. Ten percent is a usual starting point for a simple rectangle. Hips, valleys and many cuts need more.
Sheets, pitch factor and waste
Plan area = length × width, plus extras (also entered as plan area). For a roof, sloped area = plan area × √(1 + r²), where r is rise ÷ run (a 6-in-12 roof has r = 0.5 and a factor of 1.118). Openings are subtracted after that multiply, because they are measured on the roof surface. Net area = sloped area − openings. Sheets = ceil(net area × (1 + waste) ÷ 32). The same 32 sq ft coverage applies to plywood and to OSB in a 4 × 8 sheet. Thickness and span rating are a code choice, not a sheet-count choice. The pitch math is the same factor used on the roof pitch calculator.
Worked examples: a 30 × 40 roof and a 20 × 16 subfloor
A 30 × 40 ft roof is 1,200 sq ft in plan. At a 6-in-12 pitch the surface is 1,200 × 1.118 = 1,341.6 sq ft. With 10% waste that is 1,475.8 sq ft, or 47 sheets of 4 × 8. A 20 × 16 ft subfloor is 320 sq ft; subtract a 40 sq ft stair opening and you have 280 sq ft net, 308 sq ft with 10% waste, or 10 sheets.
Sources & assumptions
How this calculator gets its numbers. Full method: our methodology.
- A 4 × 8 sheet covers 32 sq ft, plywood or OSB.
- Sloped roof surface = plan area × √(1 + r²); openings are subtracted after that multiply. r = rise/run, the same pitch factor as the roof pitch calculator.
- Sheets = ceil(net area × (1 + waste) ÷ 32). Default waste is 10%.
- Wall sheathing is counted on the stud calculator, not here.
Sources
- APA — The Engineered Wood Association, APA Engineered Wood Construction Guide, Form E30. Standard 4 × 8 ft panel coverage and the role of span rating and thickness, which this page does not select.
- CalcTheJob roof pitch calculator, Roof pitch factor (common-rafter / sloped-area multiplier). The √(1 + r²) factor applied to roof plan area on this page.
Last reviewed . Product figures vary by brand; the label on what you buy wins.
Frequently asked questions
How many 4 × 8 plywood sheets for a 30 × 40 roof at 6-in-12?
47 sheets, including 10% waste. The sloped surface is about 1,342 sq ft because of the 1.118 pitch factor, and 1,476 sq ft after waste.
Does a 4 × 8 sheet of OSB cover the same area as plywood?
Yes — 32 sq ft either way. Grade, exposure rating and thickness still have to match the span and the code for the job.
Why is wall sheathing not on this page?
The stud calculator already counts wall sheathing with the studs, plates and headers. This page is for roof sheathing and subfloor, so the two takeoffs do not double-count a wall.
Should I subtract every roof vent from the sheet count?
Usually no. Small vents and pipe jacks are covered by the waste allowance. Subtract only large openings such as chimneys and skylights if you want them out of the net area.