Deck Boards Calculator — Boards, Joists & Screws

Deck boards run across the joists, so the number you need depends on how much width each board covers once you leave a drainage gap. Add the board's face width to the gap and you have the spacing of one course; divide the deck's width by that spacing and round up, and you have the board count. A 10 ft wide deck with 5.5 in boards and a 1/4 in gap fits about 21 courses, each one running the full length of the deck.

Joists carry those boards and are spaced a set distance apart on centre — commonly 16 in, sometimes 12 in for composite decking. Divide the deck's length by the spacing and add one for the joist at the far end. Fasteners follow from there: the standard pattern is two screws where each board crosses each joist, so the screw count is the board count times the joist count times two. Those three numbers — boards, joists, screws — are what you actually carry to the till.

The tool counts full-length boards, which is the simplest way to think about a small deck, and also reports the total linear feet of decking so you can buy in whatever stock lengths your yard carries. It sizes and counts material; it does not check whether your joists or beams can carry the load. Confirm member sizes and spans against your local code or a span table.