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.
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.
Last reviewed: 2026-09-27 · How we calculate