CalculatorEnter your measurements below and it works the answer out.
Area divided by coverage, plus 10% waste, rounded up to whole sheets. Enter what you measured and pick how it is sold.
The arithmetic. Work out the area, divide by what one sheet covers, add 10% waste, round up. Nothing more than that — the value of the page is the coverage figures and the waste allowance, both of which are stated here rather than hidden inside the tool.
Why the waste allowance is 10%. Plywood and OSB are both sold as 4 × 8 sheets covering 32 square feet, so the count is the same whichever you use. Thickness changes the price and the span, not the sheet count.
How it is sold. Full sheets are 4 × 8 ft. Most stores also cut handy panels at 2 × 4 and 2 × 8, which cost more per square foot but fit in a car.
Measure the openings honestly. Deduct large openings — a garage door, a picture window, a stairwell — and ignore small ones. Cutting a sheet or a board around a normal door wastes more material than the opening saves, which is why trades quote gross area and let the waste allowance absorb it.
Round up to how it is sold. A calculator that tells you 6.3 sheets is not finished. Everything below is rounded up to whole units, because that is what you can actually buy.
Divide your area in square feet by the coverage of one sheet, then add 10% and round up. At 32 sq ft each, a 200 sq ft job needs 7.
10% is what this calculator adds. Plywood and OSB are both sold as 4 × 8 sheets covering 32 square feet, so the count is the same whichever you use. Thickness changes the price and the span, not the sheet count.
144 sq ft divided by 32 sq ft a sheet is 4.5 sheets, so 5 with 10% waste. Subfloor is usually laid staggered, which is where the waste goes.