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Reviewed · Prices checked October 2026 · How we calculate

Plywood Calculator: How Many Sheets of Plywood or OSB?

Choose walls, subfloor or roof, and enter the size. You get the number of 4 × 8 ft sheets to buy, the thickness usually used, and the cost at your lumberyard's price.

Inputs

What you're covering
Units
ft
ft
sq ft

Total area of doors and windows. For walls, add up the length of every wall.

Extra for waste

10% for simple walls and floors; 15% for hips, valleys, dormers or lots of cuts.

$

Panel prices swing week to week, so type your yard's price.

Shopping list

Updates as you type
27sheets

4 × 8 ft sheets for 776 sq ft of walls, 10% extra

Cost at your price—

Thickness: Wall sheathing rated 24/16: usually 7/16 in OSB or 15/32 in plywood. Your local building code and the span rating stamped on the panel decide.

Show the math
112 × 8 ft − 120 sq ft openings = 776 sq ft776 sq ft × 1.1 ÷ 32 sq ft per sheet = 26.7 → 27 sheets

The price you type is used for this estimate only; it isn't saved.

How it works

The formula

walls: area = total length × height − openings
subfloor: area = length × width
roof: area = footprint (with overhangs) × √(1 + (rise ÷ 12)²)
sheets = area × (1 + waste) ÷ 32, rounded up

Assumptions

  • Sheets are 4 × 8 ft (32 sq ft). Taller walls are often sheathed with 4 × 9 or 4 × 10 ft panels; the area still divides the same way at 36 or 40 sq ft each.
  • Waste covers cuts around openings, at roof edges, hips and valleys, and short offcuts: our rule of thumb, not a published figure.
  • Thickness follows APA (The Engineered Wood Association) span ratings: wall and roof sheathing rated 24/16, often 7/16 in OSB; Sturd-I-Floor subfloor rated 20 oc (19/32 in) for joists up to 19.2 in apart and rated 24 oc (23/32 in) for 24 in. Code and loads decide; read the stamp on the panel.
  • The roof uses the plan footprint including overhangs, times the pitch factor. Gable-end walls are part of the walls, not the roof.

Worked example

The walls of a 24 × 32 ft house: 112 ft around, 8 ft high, 120 sq ft of doors and windows

  1. 112 × 8 − 120 = 776 sq ft
  2. 776 × 1.1 ÷ 32 = 26.7 → 27 sheets

Roof sheets by house size

4 × 8 sheets for a roof deck by footprint and pitch, 10% extra
Footprint4/126/128/1212/12
24 × 32 ft28303238
28 × 40 ft41444755
30 × 50 ft55586273
40 × 60 ft8793100117

Footprint including overhangs, 10% extra.

Questions people ask

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

Multiply the house footprint, with overhangs, by the pitch factor, add 10% and divide by 32. A 28 × 40 ft footprint at 6/12 is about 1,252 sq ft of roof, so 44 sheets.

OSB or plywood?

Both are rated the same way, and code treats a 7/16 in OSB panel rated 24/16 like a 15/32 in plywood panel with the same rating. OSB is usually cheaper; plywood shrugs off getting wet during construction a little better. The span rating stamped on the panel is what matters.

What thickness for a subfloor?

Tongue-and-groove 23/32 in (sold as 3/4 in) for joists 24 in apart, or 19/32 in (5/8 in) for joists 16 or 19.2 in apart, glued and screwed. Tile floors may need more; the tile maker's deflection rules apply.

How many sheets for a 12 × 12 shed floor?

144 sq ft × 1.1 ÷ 32 = 5.0, so 5 sheets. Lay them with the long side across the joists and the end seams staggered over joists.