Deck Material Calculator

Start with the decking: divide the deck width in inches by 5.625 (a 5.5-inch board plus a 1/8-inch gap), round up for rows, then multiply by the number of board-lengths in the deck length and add 10% waste. Joists are floor(length in inches ÷ 16) + 1 on 16-inch centers. Beams are doubled 2×10s — one line up to 12 feet wide, two beyond. Posts go about every 8 feet of perimeter, with 1.5 bags of concrete each. The calculator above runs all of this instantly and adds an approximate screw count, giving you a single shopping list.

A 12×12 deck needs 22 board rows (144 inches ÷ 5.625, rounded up) × 1 board per 12-foot row = 22 boards, plus 10% waste = 25 boards of 12-foot decking. If you use 16-foot boards instead, you still need 22 rows but only one board per row — still 22 boards plus waste (25), with fewer butt joints but a higher per-board price. Either way, buy from the same lumber lot so the color and grain match, and cull any badly warped boards at the yard before they reach your truck.

Most residential decks use 2×8 joists on 16-inch centers for spans up to about 11–12 feet, stepping up to 2×10 for longer spans — but the only correct answer comes from a span table (the American Wood Council's DCA 6 is the standard reference) and your local code. Composite decking usually requires 12-inch centers regardless of span. When in doubt, upsize the joist rather than pushing a span limit: the cost difference is small and a bouncy deck is forever.

Figure 1.5 bags of 80-lb concrete mix per 6×6 post as a planning number — that fills a typical 12-inch-diameter hole to northern-Illinois frost depth with the post in it. Gate posts, beam posts carrying heavy loads, and oversized holes take 2 or more bags. In Waukegan and Lake County, holes must reach 36–42 inches deep to beat frost heave, which is deeper (and thirstier for concrete) than the same deck would need in a warm climate. Buy a couple of extra bags; returning unopened bags beats a stalled Saturday.

Sixteen-foot boards mean fewer butt joints — on a 16-foot-long deck, each row is a single board with no joint at all, which looks cleaner and is slightly stronger. The trade-offs: 16-footers cost more per board, are harder to transport (measure your truck bed and the yard's delivery options), and are more prone to warping over their length, so cull carefully. Twelve-foot boards are easier to handle and cheaper per piece but put a butt joint in every long row. For decks 12 feet or shorter in the long direction, 12-foot boards are the obvious pick.

As a rule of thumb, two screws per board-to-joist intersection: multiply board rows × joists × 2. A 16×12 deck (26 rows × 13 joists) needs roughly 676 screws — about two 5-lb boxes. That assumes face-screwing; hidden clip systems for composite and hardwood decking use roughly one clip per joist per board instead, plus a starter row of face screws. Buy exterior-rated screws (coated or stainless) — drywall screws will rust, bleed, and eventually snap in an outdoor deck.

The 10% waste allowance covers the boards lost to end cuts, notches around posts, ripped edge pieces, and the occasional warped or split board you cull from the pile. It is the right number for simple rectangular decks with 12- or 16-foot boards. Bump it to 15% for decks with stairs, angled corners, or picture-frame borders, which generate far more offcuts — and for diagonal decking patterns, which waste the most of all. Running short mid-project is worse than having leftovers: dye lots vary, and a second-batch board can be a visibly different shade.

Yes — 2×6 decking is thicker, stiffer, and more forgiving of slightly wide joist spacing, which is why it is common on docks, boardwalks, and high-traffic commercial decks. The trade-offs are cost (more lumber per square foot), weight (heavier to handle), and a chunkier look that some homeowners find less refined than 5/4×6 decking. Standard 5/4×6 boards are the residential norm and perfectly adequate on 16-inch centers. Whichever you choose, keep the same 1/8-inch gap for drainage and expansion.