How the plywood calculator works
Sheathing is counted in whole panels, so the job is to divide the surface by the area of one sheet and round up after adding a cutting allowance. The calculator multiplies length by width, removes any large opening you enter, converts everything to the same unit and divides by the panel you selected.
Sheets = (length × width − openings) × (1 + waste %) ÷ sheet area, rounded upA North American 4 × 8 ft panel covers exactly 32 ft² (2.97 m²). European mills cut 2440 × 1220 mm (2.98 m²), which is the metric twin of 4 × 8, and 2500 × 1250 mm (3.125 m²), common for OSB and structural boards in Germany, Switzerland and France. Taller 4 × 9 and 4 × 10 ft panels let you cover a 9 or 10 ft wall in one piece without horizontal joints.
Worked example
A new subfloor for a 5 m × 4 m room covers 20 m². In 2440 × 1220 mm sheets that is 20 ÷ 2.977 = 6.72 sheets. Adding 10% for trimming at the walls and around joists gives 7.39, so buy 8. In North America, a 20 × 16 ft floor is 320 ft², exactly 10 sheets of 4 × 8; with the same 10% allowance, order 11. The spare sheet is not wasted money: staggering the end joints means the last sheet in every other row is cut, and offcuts shorter than two joist bays are rarely usable on a floor.
Typical thickness by use
| Use | Metric (typical) | US (typical) | Approx. weight per sheet |
|---|---|---|---|
| Subfloor, joists at 400 mm / 16 in | 18 mm tongue-and-groove | 19/32–23/32 in T&G | 18 mm OSB about 33 kg; 23/32 in OSB about 70–75 lb |
| Subfloor, joists at 600 mm / 24 in | 22 mm tongue-and-groove | 23/32–7/8 in T&G | 22 mm OSB about 40 kg |
| Roof sheathing, rafters at 600 mm / 24 in | 15–18 mm | 7/16 in OSB with edge clips, or 1/2 in plywood | 7/16 in OSB about 45 lb |
| Wall sheathing, studs at 400 mm / 16 in | 9–12 mm | 7/16 in OSB or 15/32 in plywood | 12 mm softwood plywood about 18 kg |
North American panels carry a span rating stamp such as 32/16 (32 in roof span, 16 in floor span) or "24 oc" on floor panels. Treat the table as the common starting point only: the grade, span and fastening required for your roof, floor or shear wall are set by the local building code, and structural or high-snow-load jobs should be checked by a building inspector or engineer.
Laying tips and common mistakes
- Run the long side across the supports. Plywood and OSB are much stiffer along the strength axis, marked on the panel, so the 8 ft or 2440 mm edge should cross the joists or rafters.
- Leave expansion gaps. Panel makers call for about 3 mm (1/8 in) between sheets; butting them tight causes buckled floors and ridged roofs after the first wet week.
- Stagger the joints. Start every second row with a half sheet so end joints do not line up on one joist.
- Measure the roof on the slope. A roof at 6/12 pitch is about 12% larger than its footprint; multiply the plan area by 1.118, or use the rafter length as the width.
- Glue and screw floors. A bead of construction adhesive on each joist, with fasteners about 150 mm (6 in) apart on edges and 300 mm (12 in) in the field, prevents squeaks later.
A reproducible calculation with complete inputs
For a reproducible plywood & osb sheet calculator example, use the metric input system and enter Surface length: 5 m; Surface width or wall height: 4 m; Openings to leave out: 0 m²; Sheet size: 2440 × 1220 mm; Waste allowance: 10 %. These settings describe one specific scenario, not a new default for every job. The result follows the method and allowances already explained above. Keep the entire set of values when comparing it with a hand calculation; omitting a selected material or an allowance can change the result even when the dimensions match. The following rows show the unrounded calculation values alongside the precision used by the interface.
| Result | Calculation value | Displayed value | Unit |
|---|---|---|---|
| Sheets to buy | 8 | 8 | sheets |
| Area to cover | 20 | 20.0 | m² |
| Sheets before waste | 6.7186240258 | 6.72 | sheets |
| Area of one sheet | 2.9768 | 2.98 | m² |
Frequently asked questions
How many sheets of plywood do I need for a 12x12 room?
A 12 × 12 ft floor is 144 ft², which is 4.5 sheets of 4 × 8. With a 10% allowance you need 4.95, so buy 5, or 6 if the room is out of square.
How many square feet does a sheet of plywood cover?
A standard 4 × 8 ft sheet covers 32 ft². A 4 × 9 covers 36 ft² and a 4 × 10 covers 40 ft².
What is the metric size of a sheet of plywood?
The common European sheet is 2440 × 1220 mm (2.98 m²), the metric equivalent of 4 × 8 ft. OSB is also sold at 2500 × 1250 mm (3.125 m²) and in narrower tongue-and-groove floor boards.
Is OSB or plywood better for subfloor?
Both are accepted for subfloors when rated for the joist spacing. OSB is cheaper and very uniform; plywood recovers better if it gets wet during construction and holds screws near the edges slightly better.
How much waste should I add for plywood?
Add about 5% for straight walls, 10% for floors and simple gable roofs, and 15% for hip roofs, dormers or rooms with many angles.