Work out how many sheets of plywood or OSB you need for a subfloor, wall sheathing or roof decking. Enter the size, choose a sheet size and a waste allowance, and get the sheet count, a full-sheet layout check and the material cost.
Last updated:
What are you covering?
Dimensions (feet)
ft
ft
ft
Add up the lengths of every wall you will sheathe.
ft
sq ft
e.g. 3x7 door = 21, 3x4 window = 12. Leave 0 for full sheathing.
ft
ft
/12
0 = flat. A 6/12 roof rises 6 in per foot.
in
Added on every side of the footprint.
sq ft
Sheets
%
10% for simple rectangles, 15% for many cuts, hips and valleys.
$
Optional. Typical CDX plywood or OSB price at your store.
Enter your values to see the result.
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How to use the Plywood Sheet Calculator
Choose the job: floor, walls, roof, or type an area you already know.
Enter the dimensions in feet. For walls add up every wall length; for a roof use the building footprint, pitch and overhang.
Pick the sheet size you plan to buy and a waste allowance (10% is typical, 15% for complex roofs).
Add the price per sheet if you want a cost estimate.
Read the sheet count, then check the comparison table to see whether longer sheets save you money.
How the plywood sheet count is calculated
Sheets = area ÷ sheet area × (1 + waste ÷ 100), always rounded up to a whole sheet.
Floor: area = length × width. Example: 16 × 12 ft = 192 sq ft ÷ 32 = 6 sheets, 6.6 with 10% waste → 7 sheets.
Walls: area = total wall length × height − openings. Example: 88 ft × 8 ft = 704 sq ft − 80 sq ft of doors and windows = 624 sq ft → 19.5 sheets, 21.45 with 10% waste → 22 sheets.
Roof: area = (length + 2 × overhang) × (width + 2 × overhang) × √(1 + (pitch ÷ 12)²). The square root is the pitch factor that turns the flat footprint into the sloped surface.
The full-sheet layout count lays whole sheets edge to edge (rounding each direction up) and ignores reuse of offcuts. If it is higher than the area method, you will only reach the lower number by using cut pieces elsewhere.
Sheet sizes and roof pitch factors
Sheet size
Area
Sheets per 1,000 sq ft (no waste)
4 × 8 ft
32 sq ft (2.97 m²)
31.25
4 × 9 ft
36 sq ft (3.34 m²)
27.8
4 × 10 ft
40 sq ft (3.72 m²)
25
4 × 12 ft
48 sq ft (4.46 m²)
20.8
5 × 5 ft
25 sq ft (2.32 m²)
40
Roof pitch
Angle
Pitch factor
3/12
14.0°
1.031
4/12
18.4°
1.054
5/12
22.6°
1.083
6/12
26.6°
1.118
8/12
33.7°
1.202
10/12
39.8°
1.302
12/12
45.0°
1.414
Tips: stagger the end joints of subfloor and roof sheets by at least one joist or rafter bay, leave a 1/8 in gap at panel edges and ends for expansion, and run the long side of the sheet across the joists or rafters.
Frequently asked questions
How many sheets of plywood do I need?
Divide the area to cover by the area of one sheet, add a waste allowance and round up. A 4×8 sheet covers 32 sq ft, so a 20 × 14 ft floor (280 sq ft) needs 280 ÷ 32 = 8.75 sheets, or 9.6 with 10% waste, which rounds up to 10 sheets.
How many square feet does a 4×8 sheet of plywood cover?
A 4×8 sheet covers 32 square feet (about 2.97 m²). A 4×9 covers 36, a 4×10 covers 40 and a 4×12 covers 48 square feet.
How much waste should I add for plywood or OSB?
About 10% for simple rectangular floors and walls, 12-15% for roofs with hips, valleys, dormers or many openings. Keeping offcuts to fill small gaps lowers the waste you need.
How do I calculate roof sheathing?
Measure the building footprint, add the overhang on every side, and multiply by the pitch factor √(1 + (rise ÷ 12)²). For a 6/12 roof the factor is 1.118, so a 32 × 26 ft footprint with overhang has about 930 sq ft of roof surface, or 30 sheets of 4×8 OSB before waste.
Is OSB or plywood better for subfloors and roofs?
Both are code-approved structural panels when they carry the right span rating. OSB is usually cheaper and very uniform; plywood is lighter, holds fasteners well and recovers better if it gets wet. The sheet count is the same for either.
What thickness of plywood should I use?
Common choices are 23/32 in (3/4 in nominal) tongue-and-groove for subfloors on joists 16 in apart, 7/16 or 15/32 in for wall sheathing, and 7/16 to 5/8 in for roof decking on rafters 16-24 in apart. Always follow the span rating stamped on the panel and your local building code.
How many nails do I need per sheet of plywood?
Typical schedules space fasteners 6 in apart along panel edges and 12 in apart in the field. That works out to roughly 50-60 fasteners per 4×8 sheet, which the calculator uses for a rough count. Your local code or engineer may require tighter spacing.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.