Plywood Calculator
Find how many sheets of plywood you need for a subfloor, wall sheathing, or roof sheathing. It works the same for OSB, which comes in the same sheet sizes. Enter each subfloor, wall, or roof, add the pitch of a roof measured from above, and subtract openings to see the total area and the number of sheets for each common size.
Add a section for each floor, wall, or roof, or subtract openings such as doors and windows. Enter Pitch only for a roof measured from above.
- Choose US or Metric. You can still change the unit of any single field.
- The first section is a Rectangle. Enter the Length and Width of a floor or a roof. For a single wall, enter the wall's length as Length and its height as Width. For the walls all around a building, choose Room walls and enter its length, width, and wall height. For the triangle of a gable wall, choose Triangle and enter its base and height.
- For a roof measured from above, enter its Pitch (optional), as the rise in inches for every 12 inches of run (/12, such as 6 for a 6/12 roof) or as an angle in degrees (°). Leave it blank for floors and walls, and for a roof face measured along the slope, from the eave to the ridge.
- Use Add section for each floor, wall, or roof. To leave out doors, windows, skylights, or a stair opening in a floor, add a rectangle for each one and set it to Subtract. Use Quantity for several of the same size.
- Check the Waste allowance. It starts at 10%.
- Optionally, enter a Custom sheet width and length for a size not in the table, and a price per sheet or per area to estimate the cost.
The results show the number of sheets for every common size, so you can
compare them. Dimension fields accept whole numbers, decimals, fractions,
and mixed numbers, such as 24, 11.75,
3/4, or 47 1/2. The pitch takes whole numbers
and decimals, such as 6 or 22.5.
How many sheets of plywood you need
The calculator adds up the area to cover, adds the waste allowance, and divides by the area of one sheet:
Total area = net area × (1 + waste % ÷ 100)
Sheets = total area ÷ (sheet width × sheet length), rounded up
A sloped roof has more area than the ground it covers. For a section with a pitch, the calculator multiplies the area seen from above by the same pitch factor as the Roofing Calculator:
Roof area = area from above × √(1 + (rise ÷ 12)²)
For a pitch in degrees, the factor is 1 ÷ cos(angle). A 6/12 roof has a factor of 1.118, so its sheathing covers about 12% more than the area from above. A blank pitch or a pitch of 0 counts the area as it is. The pitch also applies to an area you enter directly, and it is not used for room walls.
With several sections, the calculator adds the sections set to Add and subtracts those set to Subtract. The area of each shape is calculated as follows:
| Shape | Area |
|---|---|
| Rectangle | length × width |
| Circle | π × diameter² ÷ 4 (or π × radius²) |
| Triangle (base and height) | base × height ÷ 2 |
| Triangle (three sides) | Heron's formula: √(s(s − a)(s − b)(s − c)), where s = (a + b + c) ÷ 2 |
| Trapezoid | (parallel side A + parallel side B) ÷ 2 × height |
| Ring | π × (outer diameter² − inner diameter²) ÷ 4 |
| Rectangular border | outer length × outer width − (outer length − 2 × border width) × (outer width − 2 × border width) |
| Known area | the area you enter |
| Room walls | 2 × (length + width) × height |
Measuring a roof for sheathing
You can measure a roof in two ways. From above, measure the outside of the building plus the overhang at the eaves and gable ends, and enter the pitch. On the roof itself, measure each face along the eave and from the eave up to the ridge, and leave the pitch blank.
If you measure from above and forget the pitch, the area comes out too small. On a 6/12 roof, the area from above is about 11% less than the roof. The pitch can be up to 80° (68.05/12). For a steeper face, measure the face itself.
Plywood and OSB sheet sizes
Plywood and OSB are sold in the same sizes, so the calculator counts them the same way. It lists these sizes, written as width × length, with the area of one sheet in parentheses:
| Where | Common sizes |
|---|---|
| US | 4 × 8 ft (32 ft²), 4 × 9 ft (36 ft²), 4 × 10 ft (40 ft²) |
| UK and Canada | 1220 × 2440 mm (about 2.98 m²) |
| Australia and New Zealand | 1200 × 2400 mm (2.88 m²) |
| New Zealand | 1200 × 2700 mm (3.24 m²) |
4 × 8 ft is the most common size in North America, and the Canadian Wood Council lists 1220 × 2440 mm as the most common metric size, with 1200 × 2400 mm also available. Huber's ZIP System sheathing comes in 8, 9, and 10 ft lengths, and the longer 9 and 10 ft sheets are made for walls, so you can run one sheet up a tall wall. BRANZ in New Zealand gives 2.4 and 2.7 m as the usual lengths of flooring plywood.
UK and Australian products are often listed with the length first, such as 2440mm x 1220mm. For other sizes, such as 2250 × 1200 mm flooring in Australia or narrower tongue and groove flooring sheets in the UK, enter the sheet's width and length as a custom sheet.
Gaps and tongue and groove sheets
Panels are laid with a small gap at the edges and ends. APA, The Engineered Wood Association, explains that panels marked "Sized for Spacing" are made slightly smaller than 48 × 96 inches, and Boise Cascade and Weyerhaeuser call for gaps of 1/8 in. A sheet plus its gap still covers 4 × 8 ft, so the calculator counts each sheet at its full size and does not add the gaps.
Tongue and groove subfloor sheets cover a little less, because the tongue
fits into the groove of the next sheet. Huber gives the face of its
AdvanTech subflooring as about 47 1/2 × 95 7/8 in. The difference is about
1%, which the waste allowance usually covers. To be safe on a large floor,
enter a custom sheet of 47 1/2 in × 8 ft.
Extra plywood for waste
Cutting sheets to fit at the edges, around openings, and along hips and valleys leaves offcuts. The waste allowance starts at 10%. iCEV's lesson on estimating flooring materials adds 10% for waste to the sheets for a subfloor, and the framing credit of the LEED for Homes rating system asks for an estimated waste factor of 10% or less, counting roof decks and wall sheathing as framing. Raise it for a hip roof, a roof with valleys, or walls with many openings. You can change it from 0% to 100%.
Examples
A subfloor
A floor 24 ft × 16 ft is 384 ft². With a 10% waste allowance, the total is 422.4 ft². That is 422.4 ÷ 32 = 13.2, so 14 sheets of 4 × 8 ft. At $32.50 a sheet, that is $455.00.
Roof sheathing
A roof 40 ft × 30 ft from above with a 6/12 pitch has an area of 1,200 × √1.25 = 1,341.64 ft². With 10% added, the total is 1,475.80 ft², so 47 sheets of 4 × 8 ft. Without the pitch, the count would be 42 sheets, 5 too few.
Wall sheathing
The walls of a building 30 ft × 20 ft with 8 ft walls are 2 × (30 + 20) × 8 = 800 ft². Taking out four windows of 3 × 4 ft and a door of 3 ft × 6 ft 8 in leaves 732 ft², or 805.2 ft² with 10% added. That is 26 sheets of 4 × 8 ft, or 23 sheets of 4 × 9 ft.
Metric dimensions
A floor 5 m × 4 m is 20 m², or 22 m² with 10% added. That is 22 ÷ 2.9768 = 7.39, so 8 sheets of 1220 × 2440 mm, or 8 sheets of 1200 × 2400 mm.
FAQ
A 12 × 12 ft floor is 144 ft². With 10% extra, that is 158.4 ft², so 5 sheets of 4 × 8 ft.
A 4 × 8 ft sheet covers 32 ft², a 4 × 9 ft sheet 36 ft², and a 4 × 10 ft sheet 40 ft². A 1220 × 2440 mm sheet covers about 2.98 m², and a 1200 × 2400 mm sheet 2.88 m².
A roofing square is 100 ft², so it takes 100 ÷ 32 = 3 1/8 sheets of 4 × 8 ft, before any waste.
Yes. OSB sheathing and subflooring come in the same sizes as plywood, so the number of sheets is the same.
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