How to use the plywood calculator
- Choose what you are sheathing. Floors use length × width; walls use total wall length × wall height; roofs use the plan length × width and a pitch.
- Enter the dimensions in feet. For walls, add up every wall you will sheathe.
- Subtract large openings such as garage doors, big windows or stair openings. Small windows are usually sheathed over and cut out, so leave them in.
- Pick the sheet size. 4 × 8 is standard; 4 × 9 and 4 × 10 sheets are used on taller walls to cover the rim and plates in one piece.
- Set waste: 10% for simple rectangles, 15% for hips, valleys, dormers or many angles.
The plywood formula
Sheets = area (sq ft) × (1 + waste %) ÷ sheet area (sq ft)
Roof area = plan length × plan width × √(1 + (pitch ÷ 12)²)
The square root term is the pitch factor: 1.054 for 4/12, 1.118 for 6/12, 1.202 for 8/12 and 1.414 for 12/12. It converts the flat plan area into the sloped surface you actually cover. The roof pitch calculator lists more.
Worked examples
Subfloor. A 24 × 16 ft room is 384 sq ft. 384 ÷ 32 = 12 sheets; with 10% waste 422 ÷ 32 = 13.2, so 14 sheets.
Roof deck. A 30 × 24 ft plan (with overhangs) at 6/12: 720 × 1.118 = 805 sq ft. With 10% waste 885.5 ÷ 32 = 27.7, so 28 sheets.
Walls. 40 ft of wall, 8 ft tall, minus a 36 sq ft opening: 320 − 36 = 284 sq ft; with 10% waste 312 ÷ 32 = 9.8, so 10 sheets.
Common thicknesses
- Roof sheathing: 7/16 in OSB or 1/2 in plywood on rafters or trusses at 24 in on center is common for residential roofs; check the span rating stamped on the panel.
- Wall sheathing: 7/16 in OSB or 15/32 in plywood is typical.
- Subfloor: 23/32 in (sold as 3/4 in) tongue-and-groove plywood or OSB over joists at 16 or 24 in on center.
APA, The Engineered Wood Association, prints a span rating such as 24/16 or 48/24 on every rated panel: the first number is the maximum roof support spacing in inches, the second the maximum floor spacing.
Plywood vs. OSB
Both are accepted structural panels when they carry the same span rating. OSB is usually cheaper and comes in long sizes; plywood handles repeated wetting better and holds fasteners near edges well. The sheet count is the same for both.
Installation tips
- Run the long side across the framing and stagger end joints so they do not line up on the same rafter or joist.
- Leave a 1/8 inch gap at panel ends and edges, as APA recommends, so panels can expand without buckling.
- Glue and screw subfloors to reduce squeaks.
- For roofing on top of the deck, use the roof calculator to get squares, shingle bundles and drip edge.
Frequently asked questions
How many square feet does a sheet of plywood cover?
A 4 × 8 sheet covers 32 sq ft, a 4 × 9 covers 36 sq ft and a 4 × 10 covers 40 sq ft.
How many sheets of plywood do I need for a roof?
Multiply the plan area by the pitch factor, add 10% waste and divide by 32. A 30 × 24 ft plan at 6/12 pitch needs about 28 sheets of 4 × 8.
How many sheets of plywood for a 12x12 room?
A 12 × 12 floor is 144 sq ft, which is 4.5 sheets of 4 × 8; with 10% waste, buy 5 sheets, or 6 if the room is out of square.
How do I calculate plywood for walls?
Multiply the total wall length by the wall height, subtract large openings, add 10% and divide by the sheet area. 40 ft of 8 ft wall with a 36 sq ft door opening needs 10 sheets.
Should I add waste when ordering plywood?
Yes. Add about 10% for simple floors and walls and 15% for roofs with hips, valleys or dormers, because offcuts often cannot be reused.