Rows are a fence-post problem, not a division
The count of boards across a deck is not the width divided by the board width. Each row occupies its face width plus one gap, except the last row, which has no gap after it. So the arithmetic is the covered width plus one gap, divided by the board width plus the gap, rounded up. On a 12 foot span with 5-1/2 inch boards and a 3/16 gap that is 144.1875 divided by 5.6875, which is 25.35, rounded to 26 rows.
Round the other way and you come up short: twenty-five rows lay up 142 inches and leave a two inch strip of joist showing. Twenty-six rows lay up 147-11/16 inches, which is 3-11/16 more than the span. That surplus is where deck builders earn their money. You rip the last board down, or shave a sixteenth off every gap, or let the final board run proud of the rim, which is often what you wanted anyway. Any of the three is fine. What is not fine is discovering the discrepancy when you are one board from the house.
Gap, and why pressure treated is different
Wet pressure-treated lumber comes off the truck full of water. Laid tight, it shrinks as it dries and opens a gap of roughly 1/8 to 1/4 inch on its own. Laid with a gap, it shrinks to a gap wide enough to drop a phone through. Kiln-dried-after-treatment stock, cedar and composite do not do this, and they get an installed gap.
| Material | Install gap | What happens after |
|---|---|---|
| Wet pressure treated | Tight to 1/16" | Shrinks open to 1/8-1/4" |
| KDAT, cedar, redwood | 1/8" | Small seasonal movement |
| Composite / PVC | Per the maker, often 1/8-3/16" side, 1/8-1/4" end | Expands and contracts with temperature, not moisture |
Composite end gaps matter more than side gaps because the movement is along the length. A 16 foot composite board installed at 40 degrees and baking at 140 will grow enough to buckle if the ends are tight. The maker's table is not a suggestion, and it is the reason this calculator asks for a gap rather than assuming one.
Fasteners, and why the number is a range
Two screws per board per joist crossing is the conventional face-screw count, and it is what this calculator uses. Butt joints get four, two into each board end, into a doubled or sistered joist. Hidden clips are one per crossing for most systems, but the systems genuinely differ: some use a clip at every joist, some add a starter clip per row and a fascia screw pattern, and grooved-edge boards from different makers are not interchangeable. Buy by the coverage figure printed on the bucket rather than by a screw count, then check it against the crossing count here.
Screws also fail differently from what people expect. The usual problem is not the count, it is length and material — stainless or coated structural screws for treated lumber, long enough to get 1-1/4 inch into the joist, and predrilled at board ends so nothing splits. A perfectly counted box of the wrong screw is still the wrong screw.
What the joist number covers
The joist count here is the number of joists in the field, spaced along the direction the boards run: the span divided by the spacing, plus one. It excludes the ledger, the rim joists, the beams, any blocking or bridging, the doubled joists under butt joints, and the extra framing a stair opening or a picture-frame border needs. On a real deck the framing takeoff runs noticeably above this figure — the point of the number is to tell you whether your intended spacing matches the board you have chosen, not to buy your frame.
Span is the part that has to come from a table. Joist size, species, grade and spacing together decide how far a joist can reach, and decking span ratings decide how far apart the joists can sit. A composite rated for 16 inch perpendicular installation laid diagonally needs 12. Get those two constraints from the published tables before the frame goes up, because nothing about the board count will warn you.
Questions people ask
How many deck boards for a 12x16 deck?
With 5-1/2 inch boards running the 16 foot direction and a 3/16 inch gap, you need 26 rows across the 12 foot width, so 26 boards of 16 feet, or 416 lineal feet. Change the gap to 1/4 inch and it is still 26 rows, because the rounding absorbs it. Run the boards the other way and it becomes 34 rows of 12 feet, or 408 lineal feet — near enough the same material, but 34 boards to carry instead of 26, and a very different look. Board direction is mostly an aesthetic and framing decision, not a material saving.
Should the boards run the long way or the short way?
Whichever direction avoids butt joints is usually the better build, because every butt joint needs a doubled joist under it and is a place water sits. If your deck is longer than the longest board you can buy, you will have joints regardless, in which case stagger them deliberately across the surface rather than letting them fall in a line. The other consideration is the joists, which must run perpendicular to the boards. If the deck is attached to the house, joists normally run away from the ledger, which fixes the board direction to parallel with the house wall.
Does this account for a picture frame border or stairs?
No. A picture-frame border changes the field dimensions, needs blocking around the whole perimeter to land the mitred border boards, and adds its own linear footage. Stairs are a separate calculation entirely — treads, risers, stringers and a landing. Take the field of the deck with this calculator, then add the border and stair material as separate items. If you are picture-framing, subtract roughly two board widths plus two gaps from each dimension before entering it here, and add the perimeter length back as border stock.
How much extra should I order?
The offcut figure here already tells you what the stock length is costing you, and that is the dominant waste on most decks. Beyond that, add for defects and for the cuts you will regret: five percent on straight rectangular decks in decent stock, ten if the deck has angles, a border, or if you are working in pressure-treated where a certain fraction of every bundle is too crooked to use. Composite has almost no defect waste but no forgiveness either, since a miscut board cannot be planed straight.