Plywood & OSB Sheathing Calculator

Enter the floor, wall or roof areas to sheathe to see how many sheets of plywood or OSB to buy.

How to use the plywood calculator

1. Enter the length and width of each area in feet; use How many for repeats like identical walls.
2. Pick the sheet size and a waste allowance.
3. Subtract the area of doors and windows if you are sheathing walls.

The formula

Sheets = area (sq ft) ÷ sheet area × (1 + waste), rounded up

The drawing shows the first area laid with staggered joints, as sheathing should be. For roof planes, find the slope length with the rafter calculator.

Walls: worked example

Enter each wall as length × height. Walls the same size go on one row with How many; a gable end can be entered as a rectangle and the offcut counted as waste.

A 24 × 16 ft shed with 8 ft walls has two walls of 24 × 8 ft and two of 16 × 8 ft: 2 × 192 + 2 × 128 = 640 sq ft. Take off a 3 × 7 ft door (21 sq ft) and two 3 × 4 ft windows (24 sq ft) and 595 sq ft is left. 595 ÷ 32 = 18.59 sheets, × 1.10 for waste = 20.45, so buy 21 sheets of 4 × 8.

Roofs: worked example

Measure each roof plane along the eave and up the slope from eave to ridge, not across the floor plan. The rafter calculator gives the slope length from the run and pitch.

A gable roof with two planes, each 30 ft along the eave and 14 ft up the slope: enter 30 × 14, how many 2. That is 840 sq ft; 840 ÷ 32 = 26.25 sheets, × 1.10 = 28.88, so 29 sheets. With hips and valleys, allow 15%: 26.25 × 1.15 = 30.19, so 31 sheets.

Floors: worked example

Enter the floor as length × width, one row per rectangle for an L-shaped room. A 24 × 20 ft floor is 480 sq ft: 480 ÷ 32 = exactly 15 sheets, × 1.10 = 16.5, so 17 sheets. Lay the long side of each sheet across the joists and stagger the end joints, as the drawing shows.

Waste factor

Waste covers the offcuts you can't reuse: cuts at the ends of walls and floors, around openings, and the angled cuts at hips, valleys and gable ends. There is no code figure for it; as a rule of thumb allow about 10% for floors and plain walls and 15% or more for roofs with hips and valleys or walls with many openings. Allow more if you are new to it, and less for a plain rectangle laid out on the 4 ft grid.

Sheet sizes: 4 × 8, 4 × 9 or 4 × 10

• 4 × 8 ft (32 sq ft) — the standard sheet for floors, roofs and 8 ft walls.
• 4 × 9 ft (36 sq ft) — for 9 ft walls: stood on end, one sheet covers the wall top to bottom with no horizontal joint to block.
• 4 × 10 ft (40 sq ft) — the same for 10 ft walls.

The shed above with 9 ft walls is 720 sq ft, 675 after openings: 675 ÷ 36 = 18.75 sheets of 4 × 9, × 1.10 = 20.63, so 21 sheets. Pick the size in the calculator and it divides by the right area.

Subtracting door and window openings

Add up width × height of each opening and enter the total under Subtract doors and windows; the calculator takes it off before dividing. A 20 × 8 ft wall (160 sq ft) with a 6 × 4 ft window (24 sq ft) and a 3 × 7 ft door (21 sq ft) leaves 115 sq ft: 115 ÷ 32 = 3.59, × 1.10 = 3.95, so 4 sheets. Without subtracting it would be 6.

Many crews sheathe straight over small windows and cut them out afterwards, because the piece that comes out is too small to use elsewhere. Subtract only the openings big enough to leave a usable offcut.

OSB or plywood, and what thickness

The IRC (R503.2.1) accepts wood structural panels made to DOC PS 1 or PS 2, and uses the Performance Category on the grade stamp as the panel thickness. So OSB and plywood with the same span rating are allowed the same spans; choose on price and availability. OSB edges can swell if left wet, so keep panels covered until the roof is on.

The span rating on the stamp is two numbers: the left is the maximum spacing of roof supports in inches, the right the maximum spacing of floor joists (0 = not for floors). The figures below are the minimums in the code tables for panels continuous over two or more spans with the long side across the supports. Your local code or engineer has the final say — check before you buy.

Roofs and subfloors — IRC Table R503.2.1.1(1)

Maximum spacing of rafters, trusses or joists. *24" for 15/32" and 1/2" panels. Edge support means blocking, panel edge clips (one midway between supports) or tongue-and-groove edges.

So on rafters or trusses 24" apart, 24/0 (3/8") is the minimum with edge clips and 24/16 (7/16") the minimum without. For floors, 24/16 or 32/16 subfloor suits joists 16" apart, 40/20 joists 20" apart and 48/24 joists 24" apart.

Single-layer floors (Sturd-I-Floor)

APA Rated Sturd-I-Floor is one panel that serves as both subfloor and underlayment. Its stamp gives the joist spacing directly; the same IRC table sets the thickness:

Leave a 1/8" gap at panel ends and edges, as panel makers recommend.

Walls — IRC Table R602.3(3)

• 24/0, 3/8" minimum — studs up to 16" apart; 6d common nails 6" apart at edges, 12" in the field. Wall-16 and Wall-24 rated panels are allowed instead of 24/0.
• 24/16, 7/16" minimum — studs 16" or 24" apart; 8d common nails 6" apart at edges, 12" in the field.

The table also limits each panel by design wind speed and exposure category, so look up the row for your area — this is a summary, not a substitute for the table or a structural engineer.

Sources

• IRC Table R503.2.1.1(1), roof and subfloor sheathing: UpCodes — floor sheathing, UpCodes — floor and roof sheathing
• IRC Table R602.3(3), wall sheathing: UpCodes — IRC 2024 chapter 6, wall construction
• APA Engineered Wood Construction Guide (E30): floors, walls
• APA — trademarks and grade stamp anatomy
• Boise Cascade — plywood Sturd-I-Floor (panel spacing)

Related tools

Count wall framing with the stud calculator, get the roof slope from the roof pitch calculator, and size a deck with the deck calculator.