Count the rows, then the joists
A takeoff for decking is two counts, not one: the boards you walk on, and the joists that hold them up. The boards go by rows. Figure how wide one board plus its gap really is, divide the deck's width by that to get the number of rows, then multiply by the deck's length to get the linear feet of decking. Convert to physical boards by dividing by the length of board you buy, and add for waste. Joists are separate: they run under the boards, spaced 16 inches on center for a straight layout, and the count is the deck length divided by that spacing, plus one for the far end.
The single most common mistake is estimating from square footage and forgetting the gap. Boards have to breathe — a gap sheds water and gives the board room to move — so a nominal "6-inch" board is really 5½ inches wide, and with a 1/8-inch gap it eats 5.625 inches of your deck per row. Skip that and you'll come up a few rows short on anything but a tiny deck. The second mistake is buying a board length that fights your deck: if your run is 17 feet and you buy 16-foot boards, every row needs a butt joint and a short filler, and every one of those joints has to land on a joist. Buy the longest board that spans your deck in one piece and the waste and the labor both drop.
Gap and brand change the divisor
Board width and gap are the levers, and they differ by product — which is why this tool keeps a preset per material. Standard 5/4 and 2× lumber and Trex all run 5½ inches wide; the gap is where they split, since Trex's guidance sets a 1/4-inch gap while dry lumber is usually laid at 1/8 inch. A wider gap means fewer rows, so it genuinely moves the board count. Going diagonal changes two things at once: the joists tighten from 16 to 12 inches on center, adding framing, and the angled end cuts push waste from 10% toward 15%. Board length changes the count through butt joints, and picture-frame borders or a breaker board down the middle add boards this simple rectangle model doesn't — add those separately. The Trex 1/4-inch vs 3/16-inch conflict, and why this tool uses the live page with worked math, is walked through in How Many Deck Boards Do You Need.
One thing this calculator deliberately doesn't do: size your joists or beams. How far a joist can span before it needs support is a structural question driven by lumber size, species, and load — that's the job of the IRC span tables or an engineer, not a board count. This tool tells you how many joists at a given spacing; it does not tell you whether a 2×8 is big enough to carry them. Board-width, gap, and joist-spacing presets come from each manufacturer’s published install guidance, cited below — not affiliated with Trex or TimberTech.