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

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To figure out how many sheets of plywood you need, divide the area you are covering by 32 sq ft, the size of one 4 x 8 sheet, then add about 10 percent for cuts and round up to whole sheets. Our plywood calculator does exactly that: a 16 x 12 ft floor is 192 sq ft, or 6.0 sheets, which becomes 7 sheets of 4 x 8 plywood or OSB with the extra.

Enter a floor, a wall, or a roof below. The tool works as a subfloor plywood calculator and as an OSB calculator, because a 4 x 8 sheet of OSB covers the same 32 sq ft as a 4 x 8 sheet of plywood. You can also switch to 4 x 9 or 4 x 10 sheets and subtract large openings.

How many sheets of plywood do you need?

Enter the area to cover and your sheet size. Works for subfloors, walls, and roof sheathing; OSB counts the same.

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Standard panels are 4 x 8 ft; some mills make larger sizes. APA recommends a 1/8 in gap at all panel edges and ends.

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.
  2. Measure the height. Enter the wall length as the calculator's length and the height as its width.
  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.
  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.

The plywood formula

The calculator runs three lines of math:

  • Area = length (ft) × width (ft) − openings (sq ft)
  • Sheets before extra = area ÷ sheet area (32 sq ft for 4 x 8, 36 for 4 x 9, 40 for 4 x 10)
  • Sheets to buy = sheets before extra × (1 + extra ÷ 100), rounded up to a whole sheet

The extra, 10 percent by default, covers cuts and offsets: sheets trimmed so their ends land on framing, pieces cut around openings, and offcuts too small to use.

Worked example (the calculator's default values)

Say you are decking a 16 x 12 ft shed floor with 4 x 8 sheets, no openings, and 10 percent extra.

  1. Area: 16 × 12 − 0 = 192 sq ft
  2. Sheets before extra: 192 ÷ 32 = 6.0 sheets
  3. With 10 percent extra: 6.0 × 1.10 = 6.6 sheets
  4. Rounded up: 7 sheets

The calculator shows the same result: "You need 7 sheets of 4 x 8 ft plywood or OSB. 192 sq ft to cover at 32 sq ft per sheet is 6.0 sheets, plus 10% for cuts and offsets."

Switch to 4 x 10 sheets and the same floor is 192 ÷ 40 = 4.8 sheets, or 5.28 with the extra, so you buy 6. Bigger panels save one sheet here, not more, because every partial sheet rounds up.

Second example: a wall with a door and two windows

Now take a wall 24 ft long and 8 ft high with one 3 x 7 ft door and two 3 x 4 ft windows.

  1. Gross wall: 24 × 8 = 192 sq ft
  2. Openings: 3 × 7 = 21 sq ft for the door, plus 2 × (3 × 4) = 24 sq ft for the windows, 45 sq ft in all
  3. Area to cover: 192 − 45 = 147 sq ft
  4. Sheets before extra: 147 ÷ 32 = 4.59, shown as 4.6 sheets
  5. With 10 percent extra: 4.59 × 1.10 = 5.05 sheets
  6. Rounded up: 6 sheets

The calculator shows: "You need 6 sheets of 4 x 8 ft plywood or OSB. 147 sq ft to cover at 32 sq ft per sheet is 4.6 sheets, plus 10% for cuts and offsets."

How many sheets of plywood for a 16 by 12 foot floorTop view of a 16 by 12 foot floor, 192 square feet, covered with 4 by 8 foot sheets laid with the long side across the joists in three rows: two full sheets in the first and last rows, and one full sheet with a half sheet at each end in the middle row so the end joints are staggered. That is exactly 6 sheets: 192 divided by 32 square feet per sheet is 6.0. Adding 10 percent for cuts gives 6.6, rounded up to 7 sheets, so the seventh sheet is the spare for cuts and offsets.4 × 8 SHEETS FOR A 16 × 12 FT FLOOR14 × 8 ft23a43b56Length 16 ftWidth12 ftLong side across the joists (dashed), ends staggered192 sq ft16 ft × 12 ft floor6.0 sheets192 ÷ 32 sq ft per sheet7 sheets6.0 × 1.10 = 6.6 → 77spare for cuts16 ft × 12 ft = 192 sq ft ÷ 32 sq ft per sheet = 6.0 sheetsWith 10% for cuts: 6.0 × 1.10 = 6.6 → 7 sheets of plywood or OSB
Divide the area by 32 sq ft per 4 x 8 sheet and add 10% for cuts: 16 × 12 ft = 192 sq ft ÷ 32 = 6.0 sheets, × 1.10 = 6.6, so buy 7 sheets of plywood or OSB.

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.

Floor or wallArea to coverArea ÷ 32× 1.104 x 8 sheets
8 x 10 ft floor80 sq ft2.52.753
10 x 12 ft floor120 sq ft3.754.1255
12 x 12 ft floor144 sq ft4.54.955
12 x 16 ft floor192 sq ft6.06.67
16 x 20 ft floor320 sq ft10.011.011
20 x 24 ft floor480 sq ft15.016.517
24 x 8 ft wall192 sq ft6.06.67
24 x 8 ft wall, 1 door and 2 windows (45 sq ft)147 sq ft4.595.056

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

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.

Sheet sizeCoversSheets for 1,000 sq ft with 10% extra
4 x 8 ft32 sq ft35
4 x 9 ft36 sq ft31
4 x 10 ft40 sq ft28

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.

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.

Frequently asked questions

How many sheets of plywood do I need for a 12 x 16 floor?

A 12 x 16 ft floor is 192 sq ft. Divided by 32 sq ft per 4 x 8 sheet, that is 6.0 sheets, or 6.6 with 10 percent extra, so buy 7 sheets of plywood or OSB.

How many square feet does a sheet of plywood cover?

Multiply width by length: a 4 x 8 sheet covers 32 sq ft, a 4 x 9 sheet 36 sq ft, and a 4 x 10 sheet 40 sq ft.

Is an OSB calculator different from a plywood calculator?

No. A plywood sheets calculator only needs the area and the sheet size, and a 4 x 8 sheet of OSB covers the same 32 sq ft as a 4 x 8 sheet of plywood, so the count is identical.

How many sheets of plywood do I need for a roof?

Start from the roof surface, not the footprint. A 40 x 30 ft house with 1 ft overhangs and a 6/12 pitch has about 1,503 sq ft of roof: 47.0 sheets of 4 x 8, or 52 with 10 percent extra.

How much extra plywood should I buy?

The calculator adds 10 percent by default. A plain rectangle can get by on that; a roof with hips, valleys, and dormers, or a floor with many angles, makes more offcuts, so raise the extra to suit.

What do the numbers on a plywood stamp mean?

On APA Rated Sheathing, two numbers such as 32/16 are the Span Rating: the maximum recommended support spacing in inches for roof sheathing (left) and subflooring (right). APA says Exposure 1 panels suit uses without long-term exposure to weather; Exterior panels have bonds made to withstand it.

How many sheets of plywood do I need for a 10 x 12 shed floor?

A 10 x 12 ft floor is 120 sq ft. Divided by 32 sq ft per 4 x 8 sheet, that is 3.75 sheets, or 4.125 with 10 percent extra, so buy 5 sheets of plywood or OSB.

Should I leave a gap between plywood sheets?

Yes. 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 their nominal size, and the gaps do not change how many sheets you buy.

Which way should plywood run on joists or rafters?

APA says to install roof and floor panels with the long dimension or strength axis across the supports, continuous over two or more spans. Land panel ends on framing, and stagger the end joints on floors.