Deck boards, joists, and footings from dimensions
Enter your deck's length and width, joist spacing, and decking board width. The calculator estimates the number of decking boards, the number of joists (including a rim joist at each end), concrete footings based on beam spans, and a lumber cost estimate. Board width should include the fastener gap — a standard 5.5" composite or 5.5" wood board spans 5.5" installed with gaps.
A 16 ft × 12 ft deck with 5.5" boards running the 16 ft direction: board rows = 12 ÷ (5.5/12) = 26.2 → 27 rows. Using 16 ft boards, that's 27 boards. Joists at 16" OC spanning the 12 ft width: ceil(16 ÷ 1.333) + 1 = 13 joists × 12 ft each. With beams at mid-span and ledger, figure 4–6 footings depending on beam config. At $8/board and $14/joist: decking ≈ $216, joists ≈ $182.
What joist spacing should I use? 16" OC is standard for wood decking. Use 12" OC for composite (check manufacturer specs — many require it perpendicular to house) and for diagonal layouts; 24" OC only for 2×6 wood boards.
How far can a 2x8 joist span? Roughly 12–14 ft at 16" OC for Southern pine; check the IRC span tables or your local code for your species and grade.
How many footings do I need? Typically one every 4–8 ft along each beam line, sized by soil bearing capacity. Most ground-level decks need 4–8; elevated multi-beam decks need more.
Should decking run parallel or perpendicular to the house? Boards normally run perpendicular to joists and parallel to the house for a conventional look — the calculator assumes boards run parallel to the deck length.