What this calculator does
Find out how many sheets of plywood or OSB to buy for your walls, floor or roof. Sheathing is the layer of structural panels nailed over the framing of walls, floors and roofs. The panels are plywood or oriented strand board (OSB, made from pressed wood chips). Sheathing stiffens the frame. It also gives siding, flooring and shingles something solid to fasten to.
Ordering it comes down to area. Find the surface to cover, add an allowance for cuts, and divide by the area of one sheet.
This sheathing calculator handles the three common cases. For walls, it multiplies the total wall length by the height and can subtract openings. For a floor or deck, it uses length times width. For a gable roof (two slopes meeting at a center ridge), it adds the overhang on every side. Then it multiplies that flat area by the slope factor for the pitch, the roof's steepness. That turns the footprint into the true area of both roof slopes. Last, it rounds up to whole sheets of 4 × 8, 4 × 9 or 4 × 10 ft.
Where it's used
Why this math matters outside the classroom:
- Roof decks. Roofers find a 6/12 slope factor as √(12² + 6²) ÷ 12 = 1.118, so a 1,092 ft² plan needs about 1,221 ft² of deck.
- Wall sheathing. Framers sheathing 40 ft of 8 ft wall cover 320 ft², which is exactly 10 sheets of 4 × 8 before waste.
- Subfloors. A 24 × 16 ft floor is 384 ft², or 12 sheets, so builders order 14 with a 10% allowance.
- Shed builds. DIY builders price a backyard shed by adding the sheets for the floor, the four walls and the two roof slopes.
- Pythagorean theorem. Students see the rafter for a 12 in run and 9 in rise is √(144 + 81) = 15 in, a 3-4-5 triangle scaled up.
Grade level and standards: Grades 7–8; Common Core State Standards for Mathematics 7.G.B.6, 8.G.B.7. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Choose walls, a floor or a gable roof.
- Enter the sizes. For walls, that's total wall length and height. For a floor, it's length and width. For a roof, it's the building's length, width, overhang and pitch.
- Pick the sheet size and a waste allowance.
- Read the number of sheets to buy.
What the inputs mean
- Total wall length
- Add up every exterior wall, or use the perimeter of the building.
- Door and window area to subtract
- Sheathing is often run straight across, with the openings cut out afterward. The offcuts are seldom reusable. Subtract openings only for large windows or garage doors, where you'll truly use less material.
- Roof pitch: rise per 12 of run
- Pitch is the rise over 12 in of level run. A 4/12 roof is low, 6/12 to 8/12 is common for houses, and 12/12 is a 45° roof. The roof-pitch calculator converts between pitch, angle and percent.
- Sheet size
- 4 × 9 and 4 × 10 ft sheets let you sheath a 9 or 10 ft wall in one upright piece. They reach from the sill to the top plate without a joint.
- Waste allowance
- Covers cuts at edges, gables, hips and openings. 10% suits simple shapes; complex roofs need more.
Formula and method
walls: area = total wall length × height − openings floor: area = length × width roof: area = (length + 2 × overhang) × (width + 2 × overhang) × √(12² + rise²) ÷ 12 sheets = area × (1 + waste ÷ 100) ÷ sheet area, rounded up
In the formulas, rise is the inches the roof climbs for every 12 inches of level run. Waste is your allowance for cuts, as a percent. The slope factor is how much sloped roof there is per foot of level run. It comes from the Pythagorean theorem, the rule for the long side of a right triangle. A run of 12 and a rise of r give a sloped length of √(144 + r²).
For a 6/12 roof the factor is 1.118, so the deck is about 12% larger than the footprint.
Each sheet covers its face area. Rounding up after the waste allowance gives whole sheets.
Worked example
A 40 × 24 ft house with a 6/12 gable roof and 1 ft overhangs, decked with 4 × 8 ft sheets and 10% waste.
- Plan area with overhangs: (40 + 2) × (24 + 2) = 42 × 26 = 1,092 ft².
- Slope factor: √(144 + 36) ÷ 12 = 1.118034.
- Roof deck area: 1,092 × 1.118034 = 1,220.8931 ft².
- Sheets: 1,220.8931 × 1.1 ÷ 32 = 41.9682, rounded up to 42.
Result: Order 42 sheets of 4 × 8 ft roof sheathing for the 1,221 ft² deck.
Understanding the result
The sheet count is your order. If it lands just over a whole number, pause before rounding up. Check whether a slightly smaller waste allowance is realistic for your layout.
For roofs, the deck area also tells you how many roofing squares of underlayment and shingles you'll need. A square is 100 ft².
Assumptions and limitations
- Walls are all one height and floors are rectangles. Roofs are simple gables with equal slopes and the same overhang all around.
- The waste allowance covers cutting losses on average.
- It doesn't handle hip roofs, dormers, valleys or the triangles of wall at the gable ends.
- Sheathing thickness, grade, span rating and nailing are set by the building code and the engineer or designer, not by this calculator.
- Stair openings and bump-outs in floors need to be subtracted or added separately.
Frequently asked questions
Plywood or OSB?
Both are used for wall, floor and roof sheathing, and they come in the same sheet sizes. So the count is the same. The choice depends on cost, how wet the panels may get during building, and local habit. Check the panel's span rating (how far apart its supports can be) for the job.
How much should I add for waste?
About 10% suits simple walls, floors and gable roofs. Roofs with hips, valleys and dormers, or walls with many openings, often need 15% or more.
Do I include the gable-end triangles?
Not automatically. Each gable triangle is half the building width times the height from the top plate to the ridge. Work out that area and add sheets for it. One way is to run the floor mode with sizes that give the same area.
Why is a roof bigger than the footprint?
The roof slopes, so each foot of level run has more than a foot of deck along the slope. The steeper the pitch, the bigger the difference. A 4/12 roof adds about 5%, 6/12 about 12% and 12/12 about 41%.
How this calculator is checked
Method: Rectangular wall and floor areas; gable roof area as plan area including overhangs times the slope factor √(12² + rise²) ÷ 12; whole-sheet rounding after a waste allowance. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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