How to count plywood or OSB sheets
Start by measuring the length and width of the surface you're covering — a floor, a wall, or a roof plane — and multiply them to get the total square footage. If the area isn't a simple rectangle, break it into rectangular sections, measure each one separately, and add the totals together.
Next, add a waste allowance. Full sheets rarely fit an area perfectly: edges need trimming, cuts around openings leave offcuts that are too small to reuse, and the occasional piece gets miscut. A 10% allowance covers most jobs, though a simple, obstruction-free rectangle can get away with 5%, and a complex layout with lots of angles or cutouts is safer at 15%.
Finally, divide the area (with waste added) by the coverage of one sheet — 32 sq ft for a standard 4×8 sheet — and round up, since sheets are sold whole. Worked example: a 20 ft × 10 ft floor is 200 sq ft. With a 10% waste allowance that becomes 220 sq ft, and 220 ÷ 32 = 6.875, which rounds up to 7 sheets.
Plywood vs OSB, and common thicknesses
Plywood is made of thin wood veneers glued together in alternating grain directions, which gives it strength in both directions and makes it more resistant to swelling if it gets wet. OSB (oriented strand board) is made of compressed wood strands bonded with resin — it's typically less expensive and comes in the same standard sheet sizes, but it can swell more at cut edges if it's exposed to moisture for long periods. Both are structurally rated for sheathing and subfloor use, and building codes generally treat them as interchangeable when installed to the same specification.
Thickness depends on where the sheet is going and how much load or span it needs to handle — these are general starting points, not a substitute for your local code or an engineer's spec:
| Use | Plywood | OSB |
|---|---|---|
| Roof sheathing | 1/2" – 5/8" | 7/16" – 5/8" |
| Wall sheathing | 3/8" – 1/2" | 7/16" – 1/2" |
| Subfloor (16" joist spacing) | 5/8" – 3/4" | 23/32" – 3/4" |
| Underlayment | 1/4" – 1/2" | Not typically used |
Reducing waste with a smart cut layout
How you lay out the sheets matters as much as how many you buy. Running full sheets across the longest, most obstruction-free stretch first uses up whole sheets efficiently and leaves the irregular offcuts for shorter, trickier sections where a partial piece fits anyway.
For structural sheathing and subfloors, stagger the seams between rows — line up joints in a brick-like pattern rather than stacking them directly on top of each other from one row to the next. This spreads the joints across the framing instead of concentrating them along a single line, which is both a common code requirement and good practice for a stronger, less bouncy floor or roof.
Keep any offcuts larger than about a square foot — they're often exactly the size needed for a closet floor, a small patch, or the last piece around a chimney chase, and using them means one less sheet cut into for a small job.