Plywood Sheathing Calculator
Enter your floor and roof areas to get the plywood sheets you need, plus an estimated cost. Waste is included.
Sheets needed
Estimated cost
How to calculate plywood sheathing
Sheathing is sold by the sheet, and the standard sheet is 4 ft by 8 ft, which is 32 sq ft. Add the floor area and the roof area, add waste, then divide by 32 and round up.
Formula: sheets = (floor area + roof area) × (1 + waste %) ÷ 32, rounded up
Worked example: 20 ft × 15 ft floor and roof
Floor: 20 × 15 = 300 sq ft. Roof: 20 × 15 = 300 sq ft. Total: 600 sq ft. Add 10% waste: 660 sq ft. Divide by 32: 20.63, rounded up to 21 sheets.
Roof slope
Measure the sloped roof area, not the flat footprint, or you will under-order. A steep roof can be much larger than its footprint.
Common sizes
Cost tips
Prices on this page are editable estimates, not quotes. Get 2 to 3 local quotes before you buy, and ask if delivery is included. Delivery, tax and waste can change the total a lot. Order a little extra for cuts and breakage, and confirm final quantities with your supplier or a qualified professional.
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Frequently asked questions
How many plywood sheets do I need for a 20x15 floor and roof?
A 20 ft by 15 ft floor plus the same roof is 600 sq ft, which needs 21 sheets of 4x8 plywood with 10% waste.
How big is a sheet of plywood?
The standard sheet is 4 ft by 8 ft, which is 32 sq ft.
What if I only need floor or roof?
Enter zero for the part you do not need. The calculator adds floor and roof areas together.
How much waste should I add?
Add 10% for rectangular areas and 15% for roofs with hips, valleys and dormers.
Are the prices accurate?
Prices are editable estimates. The defaults are typical US figures, and your local price can be higher or lower.