How the calculator works
Choose what you are covering, enter its size, then pick the sheet size.
- Floor, subfloor or ceiling: length × width.
- Walls: the total length of the walls end to end × the wall height, less the area of doors and windows if you will not sheathe over them.
- Gable roof: length × width of the footprint, overhangs included, × the slope factor √(1 + (pitch ÷ 12)²), which turns the level footprint into the sloped area of both sides.
- Sheets: area ÷ sheet area, plus waste (10% by default), rounded up.
- Full-sheet layout: for floors, the calculator also counts whole sheets laid in a grid with no offcuts reused, the most a simple rectangular floor should take.
Plywood sheet chart
4 × 8 ft sheets (32 square feet each) for common floor sizes.
| Floor | Exact sheets | With 10% waste | Full-sheet grid |
|---|---|---|---|
| 8 × 10 ft (80 sq ft) | 2.5 | 3 | 3 |
| 10 × 12 ft (120 sq ft) | 3.75 | 5 | 6 |
| 12 × 12 ft (144 sq ft) | 4.5 | 5 | 6 |
| 12 × 16 ft (192 sq ft) | 6 | 7 | 6 |
| 16 × 20 ft (320 sq ft) | 10 | 11 | 10 |
| 20 × 24 ft (480 sq ft) | 15 | 17 | 15 |
| 24 × 24 ft (576 sq ft) | 18 | 20 | 18 |
When the room is a multiple of the sheet, the grid count is exact and the waste allowance covers damaged sheets and mistakes. When it is not, offcuts from one row can often start the next, so the real count lands between the two.
Nominal and actual plywood thickness
| Sold as | Actual thickness (typical) | Common uses |
|---|---|---|
| 1/4 in | 7/32 in (about 6 mm) | Drawer bottoms, cabinet backs, underlayment |
| 3/8 in | 11/32 in | Wall sheathing, curved work |
| 1/2 in | 15/32 in | Wall and roof sheathing |
| 5/8 in | 19/32 in | Roof decking, subfloors at 16 in on center |
| 3/4 in | 23/32 in (18 mm in hardwood plywood) | Subfloors, cabinets, shelves, furniture |
Example: sheathing a 10 × 12 ft shed
The shed has 8-foot walls with a 3 × 6 ft 8 in door and two 2 × 3 ft windows. The 5 in 12 gable roof has a 1-foot overhang all around, so its footprint is 14 × 12 ft. The floor is 10 × 12 ft. All sheets are 4 × 8, with 10% waste.
- Walls: 2 × (10 + 12) = 44 feet of wall × 8 feet = 352 square feet. Less the door (20 sq ft) and windows (12 sq ft): 320 square feet, so 10 sheets exactly and 11 with waste.
- Roof: 14 × 12 = 168 square feet × 1.083 = 182 square feet, so 5.7 sheets and 7 with waste.
- Floor: 10 × 12 = 120 square feet, 3.75 sheets and 5 with waste. Laid as whole sheets in a grid it would take 6, but cutting a 2-foot strip off one row covers the gap.
- Order: 11 + 7 + 5 = 23 sheets. Sheathing straight over the door and windows and cutting them out later would take 13 sheets for the walls instead of 11, but the cutouts can be used for blocking.
Tips
- Run sheets across the framing. Lay the long side of each sheet across the joists or rafters, and stagger the end joints by at least one framing bay.
- Leave expansion gaps. Space panels about 1/8 inch at edges and ends so they can swell without buckling, unless the panel maker says otherwise.
- Mind the stamp. The grade stamp gives the span rating, exposure rating and which side faces up.
- Glue and screw subfloors. Construction adhesive on every joist and ring-shank nails or screws prevent squeaks.
- Buy for the cut list, not just the area. For cabinets and furniture, sketch the parts on a 4 × 8 grid; grain direction and part sizes often add more waste than the area suggests.
Frequently asked questions
How many sheets of plywood do I need?
Divide the area in square feet by the sheet's area, 32 square feet for a 4 × 8 sheet, then add about 10% for waste and round up. A 12 × 16 ft floor is 192 square feet, which is exactly 6 sheets; with 10% extra, buy 7.
How many sheets of plywood do I need for a roof?
Multiply the roof's footprint, overhangs included, by the slope factor for its pitch, then divide by 32. A 14 × 12 ft shed roof at 5 in 12 is 168 × 1.083 = 182 square feet, about 5.7 sheets, so 7 with 10% extra. Steeper roofs have more area for the same footprint.
How many sheets of plywood do I need for walls?
Multiply the total length of the walls by their height, subtract doors and windows if you will not sheathe over them, and divide by 32 for 4 × 8 sheets. Walls taller than 8 feet often use 4 × 9 or 4 × 10 sheets, which cover the plate and rim in one piece.
What is the actual thickness of 3/4 plywood?
Usually 23/32 inch. Plywood is sold by nominal size and is thinner after sanding, so 1/2-inch sheathing is about 15/32 inch, 5/8 is 19/32 inch, and 3/8 is 11/32 inch. Hardwood and Baltic birch plywood is often sold in millimeters, such as 18 mm for 3/4 inch.
What thickness of plywood should I use for a subfloor?
For joists 16 inches on center, 19/32 or 5/8-inch tongue-and-groove panels rated for flooring are common, and 23/32 or 3/4-inch for 24-inch spacing or tile. Check the span rating stamped on the panel, which gives the largest joist spacing it is made for, and follow the tile or flooring maker's requirements.
Should I use plywood or OSB?
Both are used for subfloors, wall sheathing and roof decking, and code treats panels with the same span rating alike. OSB is usually cheaper and comes in more sizes. Plywood holds up better if it gets wet and dries out again, and many builders prefer it for subfloors under tile.
Last updated October 21, 2026.