A 4 × 8 sheet of plywood or OSB covers 32 square feet. Divide the area you are covering by 32, add a waste allowance and round up: a 12 × 16 ft floor is 192 sq ft, 6 sheets without waste and 7 sheets with 10% extra. For cabinets and furniture, area is not enough; switch to Cut list and the calculator lays every part out on real sheets.
How the plywood sheets are calculated
Sheets = area (sq ft) × (1 + waste %) ÷ sheet area (sq ft), rounded up
A 4 × 8 sheet is 32 sq ft, 4 × 9 is 36 and 4 × 10 is 40. These are the three panel sizes in APA's Engineered Wood Construction Guide, with metric soft conversions of 1220 × 2440, 1220 × 2740 and 1220 × 3050 mm; APA calls 4 × 8 feet the standard. The answer shows your chosen size and the other two beside it, so you can compare taller wall sheets with standard ones.
- Floor or subfloor: length × width.
- Walls: total wall length × wall height, minus the area of doors and windows. A 36 × 80 inch door is 20 sq ft and a 36 × 48 inch window 12 sq ft.
- Roof: the building's footprint including overhangs × the pitch factor √(1 + (rise ÷ 12)²). A 6/12 roof has a factor of 1.118, so each square foot of footprint needs 1.118 sq ft of sheathing.
Worked examples
Plywood for a floor. A 12 × 16 ft room is 192 sq ft. With 10% waste that is 211.2 sq ft, or 6.6 sheets of 4 × 8: buy 7 sheets. With 4 × 10 sheets you need 6.
Plywood for walls. A garage with 52 ft of wall, 8 ft high, is 416 sq ft. Leave out one door (20 sq ft) and two windows (24 sq ft) and 372 sq ft remains. Add 10%: 409.2 sq ft ÷ 32 = 12.8, so 13 sheets. Taller walls are where 4 × 9 and 4 × 10 sheets save a row of cut-offs.
Plywood for a roof. A 24 × 40 ft footprint with a 6/12 pitch: 960 sq ft × 1.118 = 1,073 sq ft of roof. With 10% waste that is 1,181 sq ft, 37 sheets of 4 × 8. The calculator counts both slopes of a gable or hip roof; for a single shed slope, enter the width of that slope only.
Plywood calculator by square footage
| Floor | Area | 4 × 8 sheets | 4 × 10 sheets |
|---|---|---|---|
| 10 × 10 ft | 100 sq ft | 4 | 3 |
| 12 × 12 ft | 144 sq ft | 5 | 4 |
| 12 × 16 ft | 192 sq ft | 7 | 6 |
| 16 × 20 ft | 320 sq ft | 11 | 9 |
| 20 × 24 ft | 480 sq ft | 17 | 14 |
| 24 × 24 ft | 576 sq ft | 20 | 16 |
All with a 10% waste allowance.
Laying sheets on floors and roofs
APA's guide asks for sheets installed with the long dimension, the strength axis, across the joists or rafters, with end joints staggered and falling over framing, and a 1/8 inch gap at all panel ends and edges unless the manufacturer says otherwise. The layout drawing follows that: sheets run along the length, each course shifted half a sheet. The gap is too small to change the count.
The span rating stamped on sheathing reads like 32/16: the first number is the largest rafter spacing in inches for roof use, the second the largest joist spacing for subfloor use, both with the long dimension across three or more supports. Check the stamp against your framing before you buy. OSB and plywood with the same span rating do the same job, so the sheet count is the same for both.
Plywood calculator for cabinets and furniture: the cut list
The Cut list tab is a sheet optimizer. Enter each part's length, width and quantity, the saw kerf and the sheet size. The calculator packs the parts with guillotine cuts only, so every layout can be broken down with straight through-cuts on a table saw or a track saw, and it tries fifty packing orders and keeps the one with the fewest sheets.
Turn on Grain runs along the length for parts whose face grain matters: cabinet sides, doors, visible shelves. A locked part keeps its length along the sheet's 8 foot direction; unlocked parts may turn to fill a gap.
The default list is the carcass of a 36 inch base cabinet in ¾ inch plywood: two sides and a bottom of 34½ × 23¼ in, a shelf of 34½ × 22 in and two 34½ × 4 in stretchers, with a 1/8 in kerf. All six parts fit on one 4 × 8 sheet: the two sides stack across the first 34½ inches of the sheet, bottom and shelf take the next 34½, and the unlocked stretchers turn crosswise into the 26¾ inch strip left at the end. The parts cover 23.9 sq ft of the 32, a 74.7% yield. Lock the stretchers too and a second sheet appears, which is the point of the toggle.
The printable cut list gives every part's position from the left and bottom edges of its sheet, and the material list adds the sheets to your project list.
What to work out next
- The framing under the sheathing: the framing calculator and stud calculator count studs and plates.
- The pitch of an existing roof: the roof pitch calculator.
- Solid wood for face frames and edging: the board foot calculator and the lumber calculator.
- Interior walls after the sheathing: the drywall calculator.
All wood and sheet-goods tools are under framing and lumber.