PLYWOOD / OSB SHEETS
About this calculator
This plywood calculator estimates how many 4-foot-wide sheets of plywood or OSB you need for a subfloor, wall sheathing, or roof deck. Enter the length and width of the area to cover, choose the sheet length (4×8 = 32 ft², 4×9 = 36 ft², 4×10 = 40 ft²), and adjust the waste percentage. Subfloor jobs run 5–10% waste because joists land on a 16" or 24" pattern that aligns well with 4×8 sheets; complex hip roofs and walls with many openings push waste to 12–15%. The calculator rounds up to the nearest whole sheet. For T&G subfloor, allow at least one extra sheet to deal with the tongue lost on the perimeter sheet. ESTIMATE ONLY — measure the actual area and verify your fastening pattern with the engineered plans.
How to use this calculator
Enter the length and width of the area to cover in feet (subfloor, wall sheathing, or roof deck). Pick the sheet size — 4×8 (32 ft²) is the standard everyone stocks; 4×9 and 4×10 are special-order for taller walls and reduce horizontal seams. Adjust the waste factor based on your project type: 5–10% for clean rectangular subfloors that align with joist spacing; 10–12% for walls with windows and doors; 12–15% for hip roofs with lots of cuts.
The calculator returns the sheet count rounded up. For T&G subfloor, plan to buy 1 extra sheet beyond the calculation — the tongue gets ripped off the perimeter sheet against the wall, costing ~3 inches of width on those sheets.
Worked example
For a 24 × 16 ft subfloor (384 ft²) with 4×8 T&G sheets, 8% waste:
Exact need: 384 ÷ 32 = 12 sheets. With 8% waste: 12.96 → 13 sheets. Plus 1 extra for T&G perimeter loss = 14 sheets.
At ~$45 per sheet of ¾-inch T&G plywood, materials cost $630. Add subfloor adhesive (~$15 per sheet bond = $210), screws (5 lbs at ~$30), and you're at $870 for the subfloor materials of a typical 24×16 room.
For a 30 × 40 ft hip roof deck (1,200 ft²) with 4×8 sheets and 14% waste (lots of hip cuts):
Exact need: 37.5 sheets. With waste: 42.75 → 43 sheets. At ~$35 per sheet for ⅝-inch CDX roof sheathing, $1,505 in roof deck materials.
Common mistakes & waste factors
Skipping the T&G perimeter loss. T&G sheets need their tongue ripped off where they meet the wall — costs ~3 inches of width. One extra sheet covers that loss.
Confusing nominal and actual thickness. Plywood "½ inch" is actually 15/32 inch; "¾ inch" is 23/32 inch. Important when you're matching to drywall or door jamb thickness.
Buying CDX for finished applications. CDX is rough construction plywood — knots, voids, rough faces. For finished cabinets or trim, buy A-grade hardwood plywood; for paint-grade finished surfaces, BC-grade (smooth one side).
Underordering for hip roofs. Hip roofs have triangular sections that waste a lot more sheet area than gables. 12–15% waste minimum; 18% for very cut-up roofs.
Rules of thumb
Standard sheet: 4×8 = 32 ft². Less common: 4×9 = 36 ft², 4×10 = 40 ft².
Subfloor: ¾-inch T&G plywood for joists 16" o.c.; thicker (⅞ or 1 inch) for 24" o.c.
Wall sheathing: ½-inch OSB or CDX is standard for residential exterior walls.
Roof deck: ⅝-inch CDX or OSB for trusses 24" o.c. with snow load; ½-inch can work for 16" o.c. low-snow areas.
Waste: 5–10% subfloor, 10–12% walls, 12–15% hip roofs, 18%+ very cut-up roofs.
Common questions
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