A 12 x 12 room needs about 5 sheets of 4 x 8 plywood for a subfloor, allowing 10 percent for cuts. Each 4 x 8 sheet is 32 square feet, so the count is floor area times 1.1, divided by 32, rounded up. Here are the counts for other rooms and the fasteners and glue that go with them.
Sheets by room size (4 x 8, 10% waste)
| Floor size | Sheets | Screws (35/sheet) | Adhesive tubes |
|---|---|---|---|
| 12 x 12 ft | 5 | 175 | 2 |
| 16 x 12 ft | 7 | 245 | 3 |
| 24 x 16 ft (addition) | 14 | 490 | 5 |
| 24 x 24 ft (garage loft) | 20 | 700 | 7 |
The plywood subfloor calculator runs any floor size and returns the sheet count, subfloor screws, and adhesive tubes in one pass.
What thickness for a subfloor?
Thickness follows the joist spacing:
- Joists 16 inches on center: 23/32 inch (nominal 3/4 inch) tongue-and-groove is the standard. 5/8 inch is the bare minimum and will feel bouncier.
- Joists 19.2 or 24 inches on center: step up to 7/8 inch or use a rated Sturd-I-Floor panel marked for the wider span.
Buy tongue-and-groove panels. The interlocking edge supports the seam between joists, which is exactly where a square-edge subfloor would flex and squeak.
Glue and screw it, do not just nail it
The quiet-floor recipe is construction adhesive plus screws, not nails alone:
- Adhesive: run a bead of subfloor construction adhesive along every joist before setting each panel, about one 28 oz tube per three sheets. The glue is what stops seasonal squeaks.
- Screws: 6 inches apart along the panel edges and 12 inches through the field, roughly 35 per sheet. Use coarse-thread subfloor screws, not drywall screws, which snap under floor load.
- The 1/8 inch gap: leave it at panel ends and tongue-and-groove seams so the wood can expand without buckling. The T&G joint already builds in the side gap for you.
Subfloor vs underlayment
These are two different layers people mix up. The subfloor is the structural 3/4 inch panel screwed to the joists. Underlayment is a thinner smooth panel (or a foam or cement board) added on top only when the finish floor needs a flawless surface, like sheet vinyl or some tile. Nail-down hardwood and carpet go straight over a sound subfloor with no underlayment at all.
Once the subfloor is down, the carpet calculator and tile calculator size the finish floor, and the wall framing calculator handles any walls going up on top of it.