How Much Plywood for a 20x20 Floor?

How that number is worked out

These are the figures this page was built with and every line the calculator produces from them. Change anything above and all of it moves.

Floor size compared

Twenty foot wide floors either side of this one. The joist column grows faster than the sheet column, because joists are priced by the foot and sheets by the sheet.

What else matters here

A 20 by 20 floor is 400 sq ft, which is also the answer to how much plywood covers 400 square feet, because the sheet count comes off the area and nothing else. Fourteen sheets, 300 ft of joist and 10 lb of screws.

Twenty is the awkward dimension again, twice over. It divides by four, so a run of sheets on their short edge is clean, but it does not divide by eight, so a run the long way covers 16 ft and leaves 4 ft. Laid on the short edge in five rows of four feet the whole floor cuts with nothing but end trims, which is the layout worth drawing before ordering.

The waste allowance is doing real work at this size and it is worth seeing how little it takes to move the order. The area is 12.5 sheets bare. At no waste that rounds to 13, at five and at ten percent it is 14, and at fifteen percent it is 15. Two of those allowances give the same answer and the ones either side do not, which is why guessing the allowance and guessing the sheet count are not the same guess.

What this calculator assumes

• A sheet is 4 by 8, which is 32 square feet, so three and an eighth sheets cover a square.
• Openings come off in full here, not at half. A window hole in a sheathed wall is a real hole and the offcut around it is what the waste line is for.
• The nail count is the fastener schedule rather than a rule of thumb: six inches around every edge and twelve inches into the framing in the field, so a sheet at 16 inch spacing takes 64 nails and one at 24 inch spacing takes 56. Eight penny common runs about 100 nails to the pound.
• Subfloor adhesive is figured on the linear feet of joist under the sheets, at about 84 feet of quarter inch bead from a 28 ounce tube, which is the size subfloor adhesive actually comes in.
• Roof sheathing wants H clips in each bay along the unsupported joint between rafters, which is four per sheet at 24 inches on center. At 16 inches most panel thicknesses do not need them, so the list leaves them off below 19.2 inch spacing.
• A subfloor gets screwed rather than nailed on most jobs, so the list carries the same fastener count in number eight screws at about ninety five to the pound and leaves the nail line under it as the alternative. Only one of those two lines gets bought.
• Thickness does not change the count, only the price and the span rating. Check the panel span rating against your framing spacing before you buy the thin one.

Questions people ask

Other common sizes