Subfloor calculator: sheets of plywood or OSB for a floor
Enter the floor size (or several identical rooms) to count 4×8 subfloor panels, with a waste allowance and cost. Switch to meters for 2440 × 1220 mm or 2400 × 600 mm flooring boards.
Floor area: 192.0 sq ft · sheet 32.00 sq ft
You need 7 sheets of 4×8 ft (48 × 96 in) (6.00 before 5% waste)
Check: whole rows of sheets with no offcut reuse would take 6 sheets
Worked examples
| Job | Area | Panel | Waste | Sheets |
|---|---|---|---|---|
| 12 × 16 ft bedroom | 192.0 sq ft | 4×8 ft (48 × 96 in) | 5% | 7 |
| 24 × 36 ft first floor | 864.0 sq ft | 4×8 ft (48 × 96 in) | 5% | 29 |
| Three 11 × 13 ft rooms | 429.0 sq ft | 4×8 ft (48 × 96 in) | 10% | 15 |
| 4 × 5 m room, 2440 × 1220 mm sheets | 20.0 m² | 2440 × 1220 mm | 5% | 8 |
| 4 × 5 m room, 2400 × 600 mm boards | 20.0 m² | 2400 × 600 mm flooring board | 7% | 15 |
How the subfloor estimate works
A subfloor is the simplest sheet-goods job to estimate because almost every panel goes down whole. The calculator multiplies length × width × the number of rooms, adds the waste allowance and divides by the area of one panel — 32 sq ft for a 4×8, or 2.98 m² for a 2440 × 1220 mm sheet — then rounds up. A 5% allowance is enough for a plain rectangle; use 10% for L-shaped rooms, stair openings, bump-outs and floors split into many small rooms, where offcuts are too short to reach the next joist.
The result also shows a second, stricter count: whole rows of sheets laid across the floor with no offcut reused. If that number is much higher than the area count, your room size lands just past a sheet edge (a 12 ft 6 in width needs a fourth row of 4-ft sheets that is only 6 inches wide). In that case save the offcuts from the last row and use them to start the next one.
Which subfloor panel thickness?
Subfloor panels are rated by the joist spacing they can span. Read the stamp: a "Span Rated Sturd-I-Floor 24 oc" panel is made for joists up to 24 inches on center. Most new floors use 23/32-inch (sold as 3/4-inch) tongue-and-groove plywood or OSB.
| Joist spacing | Minimum span rating | Common panel |
|---|---|---|
| 16 in on center | 16 oc | 19/32 or 23/32 in T&G (18–22 mm) |
| 19.2 in on center | 20 oc | 23/32 in T&G |
| 24 in on center | 24 oc | 23/32 in minimum; 7/8 in is often recommended under nail-down hardwood and tile |
Code tables and flooring manufacturers have the final word — tile and hardwood makers often ask for a stiffer floor than the panel rating alone. In the UK and Europe, 18 mm and 22 mm P5 moisture-resistant chipboard flooring is sold in 2400 × 600 mm tongue-and-groove boards; 22 mm is the usual choice for 600 mm joist centers.
Laying out subfloor sheets
- Run the long 8-ft edge across the joists — the panel is stiffest along its length.
- Land every end joint on a joist and stagger the joints row to row, usually by 4 ft (half a sheet). Start the second row with the half sheet cut from the end of the first.
- Leave a 1/8-inch gap at panel ends and edges (most T&G panels self-space on the long edge) so the floor can swell without buckling.
- Glue to every joist with subfloor adhesive and fasten with ring-shank nails or screws, typically 6 in apart at panel ends and 12 in in the field — check the stamp and local code.
Because the starter half-sheets are reused, a staggered layout does not add waste: the area count above already covers it.
How to use the calculator
- Measure the floor. Measure length and width to the outside of the rim joists (or wall to wall for a floor over an existing structure). Treat an L-shaped floor as two rectangles.
- Pick the panel. Choose the panel size you will buy — 4×8 ft, 2440 × 1220 mm or 2400 × 600 mm boards — and a thickness rated for your joist spacing.
- Add waste. Use 5% for a plain rectangle and 10% for floors with openings or many rooms.
- Read the sheet count. Round up to whole sheets; the calculator also shows the cost when you enter a price per sheet.
Other plywood calculators
- Roof sheathing calculatorFootprint, overhang and pitch → roof deck sheets
- Wall sheathing calculatorWalls, openings and gables → sheathing panels
- Kitchen cabinetsSheets per base and wall cabinet
- BookshelvesSheets by bookcase height and width
Cutting parts rather than laying whole sheets? Area estimates undercount furniture and cabinets — use the cut list optimizer, and see plywood sizes and thicknesses for real panel dimensions.
FAQ
How many sheets of plywood do I need for a 12×12 floor?
A 12 × 12 ft floor is 144 sq ft. Divided by 32 sq ft per 4×8 sheet that is 4.5 sheets; with 5% waste it is 4.73, so buy 5 sheets.
Should subfloor sheets run parallel or perpendicular to joists?
Perpendicular. Lay the 8-ft length across the joists, with the end joints centered on joists and staggered between rows.
Is OSB or plywood better for a subfloor?
Both are code-approved when they carry the right span rating. OSB is cheaper and very uniform; plywood recovers better if it gets wet during construction and holds screws slightly better at the edges. Use tongue-and-groove edges either way.
How much waste should I allow for subfloor?
5% for a simple rectangle and 10% for floors with stairs, bump-outs or many small rooms. Whole-sheet jobs like a subfloor waste far less than furniture, where 15–25% is normal.