Plywood Calculator: How Many Sheets of Plywood Do I Need?

How to measure for plywood or OSB

Measure in feet and round up to the next half foot; divide inches by 12 (8 ft 6 in is 8.5 ft). If you already know the total square footage, enter it as the length and 1 as the width.

Floors and subfloors

1. Measure the framing edge to edge. For a shed or platform floor, measure the length and width to the outside of the rim joists.
2. Split odd shapes into rectangles. Add the pieces of an L-shaped floor with our square footage calculator, then enter the total as the length with a width of 1.
3. Subtract large openings. A stair opening or floor hatch goes in the "Openings to subtract" field, in square feet.
4. Note the joist spacing. It doesn't change the count, but the panel's span rating must suit it (see the tips below).

Walls

1. Measure the wall length along the outside of the framing. For a whole building, add all the walls together. Framing the walls too? The wall stud calculator counts studs and plates.
2. Measure the height. Enter the wall length as the calculator's length and the height as its width. For drywall on the inside, the drywall calculator counts sheets from the room size.
3. Add gable ends. A gable triangle is its width times its height, divided by 2. Add it to the wall area and enter the total as the length with a width of 1.
4. Subtract doors and windows (width times height of each). The cutout from a small window is often too small to reuse, so you may choose to subtract only doors and large windows.

Roofs

1. Use the roof surface, not the floor plan. A sloped roof has more area than the footprint under it. Our roofing calculator takes the house length, width, eave overhang, and pitch, and shows the roof surface in square feet. Not sure of the pitch? The roof pitch calculator finds it from a rise and run.
2. Or measure each plane. On a simple gable roof, multiply each plane's length along the eave by its length up the slope to the ridge, then add the planes.
3. Enter the roof surface as the length and 1 as the width. The roofing calculator's default house (40 x 30 ft, 1 ft overhang, 6/12 pitch) has 1,503 sq ft of roof. Here that is 1,503 ÷ 32 = 47.0 sheets, or 51.7 with 10 percent extra: 52 sheets of 4 x 8.
4. Adjust for openings and cuts. Subtract skylight openings. Hips, valleys, and dormers mean more angled cuts, so consider raising the extra above 10 percent.

How many sheets of plywood do I need? Chart by size

Every row uses the calculator's formula with 4 x 8 sheets (32 sq ft) and 10 percent extra, with no openings unless noted.

The 16 x 20 ft floor lands exactly on 11.0, so its 11 sheets carry no spare beyond the 10 percent.

Sheets of plywood or OSB by square footage

35 sheets of 4 x 8 plywood or OSB cover 1,000 sq ft with 10 percent extra, and 69 cover 2,000 sq ft. On a roof, one square is 100 sq ft, or 3.125 sheets of 4 x 8 before the extra. Look up the floor, wall, or roof surface, not the footprint:

Every cell is the calculator's formula: area ÷ sheet area × (1 + extra ÷ 100), rounded up to a whole sheet. The 15 percent column shows what a larger extra does on a roof with hips, valleys, or dormers; type 15 in the "Extra for cuts" field to get the same count.

Plywood for a shed: floor, walls, and roof

25 sheets of 4 x 8 plywood or OSB sheathe a 10 x 12 ft shed: 5 for the floor, 13 for the walls, and 7 for the roof. The table assumes 8 ft walls with gable ends, a 4/12 gable roof with 1 ft overhangs, no openings, and 10 percent extra on each part:

The walls are the perimeter times 8 ft, plus two gable triangles on the short ends: width times rise, divided by 2. A 4/12 roof rises 4 in per foot of half-width, so 20 in on a 10 ft wide shed. The roof is the surface our roofing calculator gives for the shed's size at a 4/12 pitch with 1 ft overhangs: 177 sq ft for the 10 x 12 shed. Run each part as its own estimate, since the floor, walls, and roof often take different thicknesses, and put the door and windows in "Openings to subtract."

Plywood and OSB sheet sizes

APA, The Engineered Wood Association, lists plywood panel sizes of 4 x 8, 4 x 9, and 4 x 10 ft. It says standard OSB panels are 4 x 8 ft, with oversized panels up to 8 x 24 ft for panelized systems, SIP facers, and modular floors. The calculator starts at 4 x 8, so by default it works as a 4x8 plywood calculator; pick 4 x 9 or 4 x 10 from the Sheet size list to divide by 36 or 40 sq ft instead. Solid lumber is measured in board feet; see the board foot calculator.

Plywood and OSB weight per sheet by thickness

48 lb for a 4 x 8 sheet of 1/2 in plywood and 70 lb for 23/32 in, from APA's approximate panel weights. APA says to add 10 percent for OSB, which makes those 53 and 77 lb. Each sheet is the weight per square foot times 32, rounded to the pound:

A 4 x 10 sheet weighs 1.25 times as much: about 88 lb in 23/32 in plywood. APA gives these weights for calculating dead loads, and Boise Cascade's weights table lists 2.2 to 2.5 lb per sq ft for 23/32 or 3/4 in plywood or OSB, so treat them as approximate and check the panel maker's data. The 7 sheets of 23/32 in OSB for the 16 x 12 ft floor come to about 540 lb.

Tips for subfloors, wall sheathing, and roof decks

• Plywood and OSB count the same. The count depends only on sheet size. APA lists subflooring, single-layer floors, and wall and roof sheathing among the uses of both, and its construction guide says APA Rated Sheathing "can be manufactured as OSB, plywood, or other wood-based panel."
• Leave a 1/8 in gap. APA recommends 1/8 in spacing at all panel ends and edges, unless the panel maker says otherwise. Panels marked "Sized for Spacing" are made slightly smaller than the nominal length and width, APA explains. The gaps do not change how many sheets you buy.
• Run the long side across the supports. For roofs and floors, APA says to install panels with the long dimension or strength axis across the supports, continuous over two or more spans.
• Land the ends on framing, and stagger them on floors. APA's roof and single-layer floor instructions put panel end joints over framing, and the floor instructions add "Stagger panel end joints." That trimming is what the extra for cuts and offsets is for.
• Plan for edge support. APA calls for tongue-and-groove edges or 2 in blocking on single-layer floors, and panel clips, tongue-and-groove edges, or blocking on roofs where the plans or APA call for edge support. Add clips or blocking to your list.
• Read the grade stamp for the span rating, not just the thickness. In APA's example, a 32/16 Rated Sheathing panel may be used for roof decking over supports up to 32 in on center or for subflooring over supports up to 16 in on center. Sturd-I-Floor panels for single-layer floors come with Span Ratings of 16, 20, 24, 32, or 48. APA notes the same rating can appear on panels of different Performance Categories (its thickness designation), so match the stamp to your framing, plans, and local code.
• Keep panels dry. APA advises storing panels under a roof when possible, never in direct contact with the ground, with their edges and ends protected.

Plywood and OSB thickness by joist and rafter spacing

23/32 in is APA's minimum for a single-layer floor over joists 24 in on center, and 19/32 in over joists 16 in on center. Roofs need less: 3/8 in panels rated 24/0 span rafters or trusses 24 in on center when the panel edges are supported. Match the Span Rating on the stamp; the thickness next to it is the Performance Category APA lists with it:

The floor columns come from Tables 14 and 16 of APA's Engineered Wood Construction Guide and the roof columns from its Table 33. Edge support on a roof means tongue-and-groove edges, panel edge clips (one midway between each support, two between supports 48 in on center or more), or lumber blocking. The 20 oc Sturd-I-Floor rating is for joists 19.2 in on center, and APA allows a panel of the same Performance Category with a higher span rating than your joist spacing. These are minimums: APA notes that heavy traffic and concentrated loads may require more, and your plans and local code have the final word.

Common mistakes to avoid

• Using the floor plan for a roof. The roofing calculator's default house has a 1,344 sq ft footprint with overhangs but 1,503 sq ft of roof. Sheathing the footprint would leave you 5 sheets short.
• Counting on every window to save a sheet. In the wall example, 45 sq ft of openings saved one sheet: 6 instead of 7.
• Skipping the extra. At 0 percent, the default floor needs exactly 6 sheets, with nothing left for trimming ends to land on joists.