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A plywood calculator works out how many sheets you need to cover an area — flooring, sheathing, subfloor, a wall or a project — with a waste allowance for the cuts. It works the same for OSB, MDF, drywall and any 4 × 8 sheet good.
A sheet of plywood covers 32 square feet, but real coverage is lower once you cut around openings and match grain, so this rounds up to whole sheets and adds waste so you do not come up one short.
Sheets = area × waste multiplier ÷ sheet coverage (32 sq ft for a 4 × 8). The result is whole sheets rounded up. Cutting around openings, grain direction and staggered joints all waste material, so 10% is a sensible minimum; raise it for cut-up layouts.
A 20 × 20 ft area (400 sq ft) in 4 × 8 sheets needs about 14 sheets with 10% waste.