How many sheets of plywood?
Divide the area by the sheet area and add waste: a 4 × 8 sheet covers 32 sq ft, a 4 × 9 36 and a 4 × 10 40. The calculator handles the three common jobs:
| Job | Area | Sheets |
|---|---|---|
| Subfloor, 24 × 32 ft | 768 sq ft | 24 |
| Wall sheathing, 140 ft × 8 ft, 150 sq ft of openings | 970 sq ft | 30.3 |
| Roof, 42 × 30 ft footprint at 6/12 | 1,409 sq ft | 44 |
Sheathing calculator for walls
For wall sheathing, enter the total wall length and the height from sill to top plate, and the window and door area to deduct. Many framers sheathe over the openings and cut them out afterwards, which turns the openings into waste; use 0 for openings if you work that way. Studs must be laid out so sheet edges land on them: 16 or 24 in centres line up with 4 ft sheets[2]. The stud calculator gives the layout marks.
Roof sheathing
Roof sheathing covers the sloped area, which is the footprint (including overhangs) times the rafter factor for the pitch. Panels run with the long edge across the rafters, and end joints are staggered. Rafter spacing comes from the rafter span tables[3]; the rafter calculator gives the rafter length and the roof calculator the roofing squares.
Subfloor calculator
For a subfloor, enter the floor's length and width. Subfloor panels run across the joists with staggered end joints that land on joists. Joist size and spacing come from the floor span tables[1]; check them with the joist span calculator. Add 5–10% waste for cuts at walls, stairs and openings.
Subfloor thickness and sheathing span ratings
Subfloor thickness follows the joist spacing: at 16 in centres the minimum is a 19/32 or 5/8 in combination subfloor-underlayment panel (16 oc) or 7/16 in rated sheathing (24/16); at 24 in it is 23/32 or 3/4 in (24 oc), under IRC Table R503.2.1.1(1) in the 2021 and 2024 codes[1]. The span rating stamped on the panel is what counts. On rated sheathing the left number is the longest roof span and the right number the longest floor span; a single number, as on Sturd-I-Floor panels, is the floor span.
| Span rating | Minimum thickness | Floor span | Roof span, edge support / none |
|---|---|---|---|
| 24/16 | 7/16 in | 16 in | 24 / 24 in |
| 32/16 | 15/32 or 1/2 in | 16 in | 32 / 28 in |
| 16 oc (subfloor-underlayment) | 19/32 or 5/8 in | 16 in | 24 / 24 in |
| 20 oc | 19/32 or 5/8 in | 20 in | 32 / 32 in |
| 24 oc | 23/32 or 3/4 in | 24 in | 48 / 36 in |
- Joists at 19.2 in need a panel rated for at least a 20 in floor span, such as 20 oc.
- Roof sheathing at 24 in needs at least a 24/16 panel (7/16 in). Edge support means blocking, panel clips or tongue-and-groove edges.
- Rafter and joist spacing come from the rafter span chart and the joist span tables, so pick the spacing first and the panel second.
Frequently asked questions
How many sheets of plywood do I need?
Area ÷ 32 for 4 × 8 sheets, plus about 10% waste. 1,000 sq ft takes 31.25 sheets, so 35 with waste.
How many sheets of plywood for a roof?
Footprint (with overhangs) × rafter factor ÷ 32. At 6/12 the factor is 1.118.
How many square feet is a sheet of plywood?
A 4 × 8 sheet is 32 sq ft; 4 × 9 is 36 and 4 × 10 is 40.
Is an OSB calculator different from a plywood calculator?
No. OSB and plywood both come in 4 × 8 ft panels, so the sheet count is the same.
What thickness subfloor do I need?
For joists at 16 in: a 19/32 or 5/8 in combination subfloor (16 oc) or 7/16 in 24/16 rated sheathing. For joists at 24 in: 23/32 or 3/4 in (24 oc). The span rating on the panel must be at least the joist spacing.
How many sheets for a roof sheathing calculator?
Roof area (footprint × the pitch factor) ÷ 32 for 4 × 8 sheets, plus about 10%. A 1,260 sq ft footprint at 6/12 is 1,409 sq ft: 44 sheets, 49 with 10% waste.