Deck Material Calculator — Lumber & Hardware

What the material list covers

The list is built in the order a deck goes together. Decking is counted by rows across the deck and boards along it. Joists, ledger, and rim are the joist size at the stock length that covers the run. Beams are the ply count you chose, one run per beam. Posts are one per bay along each beam plus the post that closes it, and each post gets a footing, a base, and a cap. Hardware closes it out: hangers where the joists meet the ledger, connectors where they meet a beam, ledger fasteners, and decking screws.

Every quantity is what you would buy, not what you would use: whole boards, whole bags, and stock lengths taken from the 8, 10, 12, 14, 16, 20 ft framing list and the 8, 10, 12, 16, and 20 ft decking list. Square feet and lineal feet appear only as summary figures, never as something to carry to the yard.

Three things are deliberately outside it. This list does not size the beam: you choose the ply and the size, and the span between posts is sized by the deck beam calculator, which reads IRC 2021 Table R507.5 at the effective joist span. Stairs and railing are not in the list — the deck stair calculator and the deck railing calculator cover those. The list is quantities only — it stops at what to buy and says nothing about money.

How the decking count works

Boards run parallel to the ledger, so a row is one deck length long and rows stack across the depth. The row count is the deck depth divided by the board width plus the gap, rounded up — a 5 1/2 in board with 3/16 in gaps covers 5 11/16 in per row, so a 12 ft deep deck takes 26 rows.

Then the rows are cut from stock. A board at least a deck length long is cross-cut into whole rows: a 20 ft board makes 2 rows of a 10 ft deck. A board shorter than the deck takes more than one per row, with the joint landing on a joist — a 16 ft deck framed with 12 ft boards buys 2 boards for every row, and the leftover from each is waste rather than being spliced into the next row. Offcuts are never carried between rows, which is what keeps the count a purchase quantity.

The waste allowance multiplies what the deck needs, and the rounding up to whole boards happens once, afterwards. What the deck needs is not always a whole number: a 10 ft by 20 ft deck takes 43 rows at 2 rows a board, which is 21.5 boards, and 10% on that buys 24. Rounding to whole boards first and adding waste after would round twice and buy one more.

Download the deck board count chart (PNG) — common deck sizes against 12, 16, and 20 ft boards, on the same basis. Each size on it is written as the length along the ledger by the depth out from the house, in that order.

Where the joist and footing lines come from

This page works out neither of them. It hands your deck depth, species group and spacing to the deck joist calculator and prints the row that comes back, so the two pages can never disagree. If that calculator has no answer — the depth is past what it covers, or the cantilever you asked for is — the list says which, and leaves the joist, ledger, rim, and joist hardware blank rather than printing a number nothing stands behind.

The footing row works the same way, with one piece of geometry supplied from here: how much deck a single post carries. A beam takes half the joist span behind it plus the whole cantilever in front, and an interior post takes half a bay on each side of it; the worst post's share is what goes to the deck footing calculator, and every footing is bought alike so none is under-built. Concrete comes back as whole bags — 60 lb or 80 lb, per footing and in total.

Posts, footings, and ledger fasteners

Posts are spaced along each beam at no more than the spacing you pick, so the count is the deck length divided by that spacing, rounded up, plus one. The actual spacing is then the deck length divided by the bays, which is always at or under the maximum. The length to buy is the shortest stock post that reaches the deck height above grade: an 8 ft post for anything up to 8 ft. That is deliberately long — the post actually runs from the top of the footing to the underside of the beam, which is a beam depth, a joist depth, and a board thickness below the deck surface, so the deck height always leaves something to trim. Above 16 ft the list states no length at all and flags a special order, because no stock length would be true. A joist longer than the framing stock lengths is handled the same way: the row keeps its count and drops its length.

Ledger fasteners follow IRC 2021 Table R507.9.1.3(1), which sets the on-center spacing from the joist span. The count assumes the 1/2 in lag screw column with 1/2 in maximum sheathing — the tightest of the table's three columns, so the count is never short if you use 1/2 in through bolts instead, which the table lets you space further apart. A fastener lands at each end of the ledger plus one per space, and the table stops at a 18 ft joist span; past that the list withholds the count.

Decking screws are 2 at every place a board crosses a joist, and a spliced row crosses the joist under its joint twice — once for each board end — so every splice adds a crossing. The total is grown by the same waste allowance as the boards.