Formula
A rectangle is length times width. A triangle, such as a gable, is base times height divided by two. Net area is what you add minus what you subtract. A roof plan is multiplied by the square root of one plus the rise-over-run squared. Waste multiplies that surface. Sheets are the wasted area divided by the sheet face, rounded up, then sheets you already have are subtracted. Thickness does not change the count. APA says floor and roof panels generally run with the strength axis across the supports, and wall panels may go either way. This calculator does not lay the sheets out.
surface = plan × √(1 + (rise/run)²)
sheets = ceil(surface × (1 + waste) / sheet area)
A 4×8 face is 32 square feet. Some panels are a little smaller when they are marked sized for spacing. Order by the face you will buy, and do not assume the offcuts from one wall fill the next opening perfectly.
Worked examples
20 × 8 ft, 4×8, 10 percent. 6 sheets. Minus a 3×7 door and a 4×3 window, 5 sheets. 24 × 16 floor. 14 sheets. 30 × 20 plan at 6/12, 15 percent. 25 sheets. Two 24 × 6 gables, 5 sheets. A 4×9 sheet on 160 square feet with 10 percent waste is 5 sheets. Fourteen sheets with 4 on hand means 10 to buy. Six sheets at an example $45 plus 6 percent tax is $286.20.
Reference
4×8 is 2.9729 m². A 5 m by 2.4 m wall is 12 m², 13.2 m² with 10 percent waste, 5 sheets.
Questions
How many sheets of plywood do I need?
Divide the area, after waste, by the face area of one sheet, and round up. A 20 by 8 foot wall is 160 square feet. At 10 percent waste and a 4×8 sheet, that is 6 sheets.
How many square feet are in a 4×8 sheet?
32 square feet, 2.9729 square metres, using the nominal 4 by 8 face. A sheet marked sized-for-spacing can be a fraction under that. Measure the sheet if the cut is tight. This takeoff uses the nominal face.
How much waste should I allow?
The opening figures are 10 percent for a wall or a floor and 15 percent for a roof. They are planning allowances you can edit. Cuts, edges, and a damaged corner are why the allowance exists. The page warns above 40 percent.
How do I calculate plywood for a roof?
Enter the horizontal plan and a pitch, or enter the sloped area directly with the pitch left at zero. A 6/12 pitch multiplies the plan by 1.118. A 30 by 20 plan at 15 percent waste is 25 sheets of 4×8.
Plywood or OSB for sheathing?
APA notes that OSB and plywood are generally interchangeable for structural sheathing, and that some plans call for plywood or for Structural I. The grade stamp decides. This page only changes the words on the shopping line.
What thickness do I need?
This tool does not select thickness, span rating, or grade. Those are on the APA trademark and in the code tables. The thickness field here is a label on the shopping line.
How do I subtract doors and windows?
Add the wall, then subtract each opening as its own rectangle. A 3 by 7 door and a 4 by 3 window take the example wall from 6 sheets to 5.
Sources
- APA – The Engineered Wood Association. Floor and roof sheathing is installed with the strength axis across the supports. Wall sheathing may run either way; edges of a shear wall still need framing or blocking. Span ratings on the trademark are the support spacing, and this page does not read or copy them.
- APA’s plywood overview: span ratings such as 32/16 are the maximum recommended support spacing when the strength axis crosses the supports. Grade and thickness come from that stamp and from the code, not from this count.
- Nominal sheet faces used for takeoff: 4×8 = 32 ft², 4×9 = 36, 4×10 = 40, 5×5 = 25. 1 ft² = 0.09290304 m². The pitch multiplier is the same geometry as the roofing calculator.