How to work out how many sheets of plywood you need
The math is area ÷ sheet area, rounded up. Multiply your length by your width to get the surface area in square feet, add a waste allowance, then divide by the coverage of one sheet: a standard 4 × 8 sheet covers 32 ft², a 4 × 10 covers 40 ft², and a 5 × 5 Baltic birch sheet covers 25 ft². Because you can only buy whole sheets, the result is always rounded up to the next sheet.
The waste allowance is what separates a usable number from a second trip to the yard. Straight, uninterrupted runs such as a subfloor or a shed wall lose very little, so 5–10% is usually enough. Add more when the layout works against you: irregular rooms, lots of openings to cut around, a stagger pattern that forces you to start each row with a cut piece, or sheathing that must break on framing members. Also check the direction you plan to run the sheets before ordering, since running them the other way can change how the offcuts fall and either save or waste a full sheet on a narrow job. If the plywood is a finished surface where grain direction and face matching matter, buy an extra sheet outright rather than relying on a percentage.
Sheet sizes, actual thicknesses and prices vary by supplier and product; nominal 4 × 8 sheets are sometimes sold slightly oversized. Structural sheathing thickness and fastening are governed by local building code.