Understanding your result
The headline is the sheets to buy, with the screws and adhesive in the line beneath it. The table gives the panel, the sheets with waste, the panel area bought, the fasteners per sheet, the screws with a spare, the boxes and the adhesive tubes.
Sheets is the floor area plus the cutting waste divided by the panel area. A 4 × 8 ft structural panel is 32 sq ft; the 1/4 in underlayment option uses 4 × 4 ft sheets, which are easier to handle in a small bathroom. Fasteners per sheet follow the schedule. For a structural panel laid with its long side across the joists, an 8 ft sheet lands on a joist line every spacing interval: the two lines at the panel ends get a screw every 6 in over the panel’s 4 ft width, and each line in between gets one every 12 in. For underlayment panels the fasteners go on a grid, every 6 in round the edges and every 8 in in the field.
The adhesive is one 28 oz tube per three sheets on 16 in joists, fewer sheets per tube at closer spacing. For the joists themselves, the floor joist calculator counts pieces; if the panel surface needs flattening before a hard floor, the self-leveling compound calculator; and under tile, the cement board calculator counts the backer that goes on top.
How we calculate this
The fastening schedule is the APA’s for single-floor and subfloor panels: 6 in on centre at supported panel edges and 12 in at intermediate supports. Underlayment schedules vary by maker; 6 in edges and an 8 in field grid is a common one. The 1.05 covers stripped and missed screws. The subfloor prep guide explains the panels, gaps and stagger, and the self-leveling guide covers the next step for a floor that is not flat.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Structural panel | 23/32 in T&G, 4 × 8 ft, Span Rating 24 oc | APA – The Engineered Wood Association, Engineered Wood Construction Guide |
| Fastening | 6 in at panel edges, 12 in at intermediate supports | APA fastening schedule for subfloor and single-floor panels |
| Underlayment fastening | 6 in edges, 8 in field grid | Typical underlayment plywood instructions |
| Joist spacing | 16 in on centre | The most common US floor framing |
| Waste | 10% | Cuts and staggered end joints in a simple room |
| Screws per box | about 450 in a 5 lb box of 2 1/2 in screws | Typical packaging |
| Adhesive | one 28 oz tube per 3 sheets on 16 in joists | Adhesive maker coverage for a 3/8 in bead |
Assumptions last reviewed October 8, 2026.
The calculator counts pieces only. It does not check that the joists and panels are stiff enough for the floor you plan; tile needs a floor that deflects no more than L/360 under load and natural stone L/720, which is a question for the span tables and the panel’s rating. It does not count blocking at unsupported panel edges, which tongue-and-groove edges usually make unnecessary, or the 1/8 in spacing gaps the APA recommends at panel ends and edges, which are not material.
Two worked examples
A 24 × 16 ft room on 16 in joists
23/32 in T&G, 10% waste, glued.
- Sheets: 384 × 1.10 ÷ 32 = 13.2, rounded up to 14 sheets
- Per sheet: 7 joist lines; 2 × 9 + 5 × 5 = 43 screws
- Screws: 14 × 43 × 1.05 = 633, two 5 lb boxes; adhesive 5 tubes
Fourteen sheets and about 630 screws. The adhesive is what makes the floor quiet: screws hold the panel down, glue stops it moving against the joist, which is where squeaks come from.
A 45 sq ft bathroom getting 1/4 in underlayment for vinyl
4 × 4 ft sheets, no adhesive.
- Sheets: 45 × 1.10 ÷ 16 = 3.1, rounded up to 4 sheets
- Per sheet: 32 edge screws + 25 field = 57
- Screws: 4 × 57 × 1.05 = 240, one box
Thin vinyl shows every nail head and every gap in the floor beneath, which is why underlayment is fastened so closely and its joints are filled and sanded before the vinyl goes down.
Where to find your inputs
Floor area. Wall to wall over the joists, including under cabinets if the subfloor runs beneath them.
Panel. The span rating and thickness stamped on the panel; buy what the joist spacing and the floor finish need.
Joist spacing. Measure centre to centre of the joists from below, or from the nail lines of the old subfloor.
Screws per box. On the box; subfloor screws are usually 2 to 2 1/2 in for 23/32 in panels.
Common mistakes
- Panels running with the joists. The long side goes across the joists, or the panel loses most of its strength.
- No stagger. End joints in neighbouring rows should be offset by at least two joists.
- Tight joints. Leave the 1/8 in gap at panel ends and edges for swelling.
- Skipping the glue. It is what keeps the floor quiet.
- Thicker panel to fix bouncy joists. Stiffness comes from the joists; add blocking or sister them.
- Underlayment nailed into the joists. It is fastened into the subfloor, with its joints offset from it.
Questions people ask
- How many sheets of plywood for a 400 square foot subfloor?
- A 4 × 8 ft sheet is 32 sq ft, so 400 sq ft is 12.5 sheets before waste and 14 with 10% for cuts and staggered joints. A 24 × 16 ft room (384 sq ft) is also 14 sheets with waste.
- How many screws per sheet of subfloor?
- On 16 inch joists, a 4 × 8 ft sheet laid across the joists crosses 7 joist lines. With screws every 6 inches on the two edge joists and every 12 inches on the five in between, that is about 43 screws a sheet. On 24 inch joists it is about 33.
- What thickness of subfloor do I need?
- 23/32 inch (sold as 3/4 inch) tongue-and-groove plywood or OSB rated 24 inches on centre is the standard for joists up to 24 inches apart. Under tile or stone on 24 inch joists, 7/8 inch or a second layer is the usual answer. The span rating stamped on the panel decides.
- Should subfloor be glued and screwed?
- Yes, glued with subfloor construction adhesive and screwed or ring-shank nailed is the common practice for a stiff, squeak-free floor. A 28 ounce tube lays a bead on the joists under about three sheets on 16 inch joists.
- Plywood or OSB for a subfloor?
- Both are accepted when they carry the same APA span rating. Plywood tolerates wetting during construction a little better and holds fasteners slightly better at the edges; OSB is usually cheaper and more uniform. Under resilient floors, an underlayment-grade plywood with a sanded face goes on top of either.
- What is underlayment plywood for?
- A thin panel, 1/4 to 1/2 inch, laid over a sound subfloor to give a smooth, clean surface for vinyl or to build up a floor to match another. It is fastened into the subfloor on a close grid, not into the joists, with its joints offset from the subfloor's.