How to Calculate Plywood Requirements

Sheet coverage

A standard structural panel is 4 ft × 8 ft, which covers 32 ft². APA – The Engineered Wood Association lists 4 × 8 ft as the standard OSB panel, and its plywood page calls 48 × 96 in the traditional nominal size of APA Rated Sheathing, so the sheet count is the same for either. Longer 4 × 9 ft (36 ft²) and 4 × 10 ft (40 ft²) panels are also made; divide by the area of the panel you will actually buy.

Sheets = area to cover ÷ panel area × (1 + waste allowance), rounded up to whole sheets. The Plywood Calculator uses the same formula and offers 4 × 8, 4 × 9 and 4 × 10 panels.

Measuring the area to cover

Measure the surface the panels will actually cover. The Plywood Calculator asks you to subtract large openings, such as a garage door or a stairwell, before it divides by the panel area.

A pitched roof has more surface than the building's footprint, so roof sheathing counted from the footprint comes up short. Get the sloped area, including overhangs, from the Roof Area Calculator or the roof area guide first.

Layout waste

Real layouts rarely use every sheet perfectly. Sheets are cut at edges and around openings, and end joints need to land on a support. The Plywood Calculator starts at 10% for a simple rectangle and suggests 15% or more for a cut-up roof or a wall with many openings. These are estimating allowances, not code values.

Panel gaps don't change the count

APA recommends leaving a 1/8 in space at panel edge and end joints, because plywood and OSB expand slightly as their moisture content changes and tightly butted panels can buckle. APA describes the gap as a recommendation, not a code requirement, and says to follow the panel manufacturer's instructions. A 1/8 in gap on a 48 in panel is about 0.3% of its width, so you still count sheets from the full panel size.

Sheet thickness depends on application, not on this calculation

Sheet count from area covers how many sheets you need — it says nothing about what thickness to use. Subfloor, wall sheathing and roof decking each have their own thickness and span-rating requirements set by the framing spacing and application, which should come from the structural design or local code, not from a coverage calculation.

APA Rated Sheathing is stamped with a span rating such as 32/16. The left number is the maximum recommended spacing of supports, in inches, when the panel is used as roof sheathing; the right number is the maximum for subflooring. Check the stamp against your rafter, truss or joist spacing and against your plans or code.