Subfloor Calculator
Count the sheets of subfloor plywood or OSB a room needs.
What you'll need to order
- 3/4 in tongue-and-groove plywood or OSB
- subfloor adhesive
- subfloor screws
Prices vary by region and haul distance. Get quotes from at least two local suppliers.
How this calculator works
sheets = area ÷ 32 × 1.10
A 4 by 8 sheet covers 32 sq ft. Lay sheets perpendicular to the joists and stagger the end joints between rows — running all joints in one line halves the floor stiffness.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- US Dept. of Commerce Voluntary Product Standard PS 1 (Structural Plywood) 2019 — structural panel dimensions and span ratings
Cited to a specific edition on purpose — table numbers and values move between cycles. Confirm against whichever edition is enforced locally.
Worked example
With room length 20 ft, room width 16 ft, joist spacing 16 in, this calculator returns 11 .
Waste allowance
The headline figure includes a 10% allowance for waste and over-ordering. The exact requirement is shown alongside it.
Subfloor sheets for rooms of common size
Each row is a link that fills in the calculator above. Send one and the other end sees the same figures you do.
| Job | Size | Sheets needed (4x8) |
|---|---|---|
| Bedroom | 12 ft × 11 ft × 16 in | 4.54 |
| Living room | 20 ft × 16 ft × 16 in | 11.00 |
| Whole floor | 40 ft × 26 ft × 16 in | 35.75 |
| Deck-over-garage | 22 ft × 20 ft × 24 in | 15.13 |
| Small addition | 14 ft × 12 ft × 16 in | 5.78 |
Run sheets across the joists and stagger the end joints. All joints in one line halves the stiffness.
Frequently asked questions
How thick should a subfloor be?
Three-quarter inch tongue-and-groove over joists at 16 inch centres. At 24 inch spacing you need 7/8 or 1 inch, or the floor will feel springy between joists.
Plywood or OSB for a subfloor?
Both meet code and OSB costs less. Plywood dries out better after a wetting and holds fasteners slightly better at the edges, which matters if the deck will sit exposed during construction.
Should subfloor be glued as well as screwed?
Yes. Adhesive between joist and sheet is what eliminates squeaks — screws alone loosen as the lumber dries and shrinks. Use a proper subfloor adhesive, one tube per two sheets.
How far apart should subfloor screws be?
Six inches along the sheet edges and 12 inches through the field. That works out to roughly 50 screws per 4 by 8 sheet on joists at 16 inch centres.