🪵 Plywood Subfloor Calculator — Joist Spacing & Thickness

Thickness vs joist spacing: the trade-off matrix

Subfloor selection is a two-variable trade: thicker panels cost more per sheet but tolerate wider joist spacing and stiffer-floor demands. Under-spacing thickness is the expensive mistake — a bouncy floor is fixed only by tearing the finish back off. These are the APA span-rating minimums, with the trade each row makes:

* Span ratings come from APA-rated panel marks stamped on each sheet. Where the calculator’s selected thickness sits below the spacing’s minimum, the estimator flags the mismatch in the sheet-type note.

The 47.5-inch problem: why T&G sheets never cover 4 feet

A tongue-and-groove panel is 48 inches nominal, but the tongue is a machined sliver that lives inside the neighbor’s groove. Once two sheets are mated, a row of subfloor is 47.5 inches wider per sheet — not 48. Half an inch per row sounds cosmetic until it compounds: a 12 ft room wants 288 inches of covered width, and three rows of “48-inch” math promise 144 of them that do not exist. Counting nominal widths is the single most reliable way to come back from the lumber yard one sheet short.

That effective-width correction also explains the 12% waste default, which is higher than the generic 10%: seam-break waste is real waste — every row edge gets ripped once, ripped rows lose their tongue, and a ripped-offcut cannot start the next row the way a square-cut sheathing offcut can.

Direction: perpendicular is not negotiable

This is why the calculator locks the orientation: pick which walls the joists run along, and the row/column layout flips to keep every sheet’s strength axis crossing the framing. The diagram updates live so you can see the ripped edge row move before it surprises you on site.

🪵Long edge across the joists, 1/8 inch off the walls

Long edge across the joists, 1/8 inch off the wallsPhoto: Unsplash

Underlayment: the second layer decision

The underlayment switch exists because the answer changes by finish floor, and the two layers are bought differently — underlayment sheets are full 4×8 counts with no tongue loss and only a 5% handling allowance. Match your finish to the row that applies:

Wall rules: the 1/8-inch gap and the seam-break rule

Seam-break discipline is the other half: end joints stagger at least one joist bay between rows, and every end joint lands on framing — a seam hanging between joists is a future soft spot underfoot no matter how thick the panel is. The 12% waste allowance in the calculator exists precisely to make this discipline free: staggered rows consume offcuts, and the allowance covers the row-edge rips where they cannot.

Scenario cards: which job is yours?

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