Deck Material Calculator

Divide your deck area by the coverage of one board course: area ÷ ((board width + gap) ÷ 12). For a 16×12 ft deck (192 sq ft) with 5.5-inch boards and a 1/4-inch gap, that is 192 ÷ 0.479 ≈ 401 linear feet of decking. Add 10% waste for straight runs (15% for diagonal) and buy the stock length with the least waste — the calculator above picks it for you automatically.

The decking formula is: linear feet = deck area in sq ft ÷ ((board width in inches + gap in inches) ÷ 12). Then convert to boards: boards = linear feet ÷ stock length, rounded up, plus waste. Framing is counted separately: joists = (deck width ÷ joist spacing) + 1, posts = (beam length ÷ post spacing) + 1 per beam line, and footings = one per post. This calculator applies all of these steps in one pass.

16 inches on center is the standard for most wood and composite decking. Use 12 inches on center for diagonal decking patterns, thinner deck boards, or heavier loads such as a hot tub. 24 inches on center is only acceptable for 2×6 decking laid flat and is not recommended for composite boards. Tighter spacing also lets joists span slightly farther.

Using prescriptive span tables (40 psf live + 10 psf dead load), a Southern pine 2×8 joist spans 11 ft 10 in at 16 inches on center, 13 ft 1 in at 12 inches on center, and 9 ft 8 in at 24 inches on center. Douglas fir-larch spans a little less (11 ft 1 in at 16 in o.c.). Always verify against your local building code — the calculator checks your span against these tables automatically.

A Southern pine 2×10 spans 14 ft 0 in at 16 inches on center (16 ft 2 in at 12 in o.c.), and a 2×12 spans 16 ft 6 in at 16 in o.c. (18 ft 0 in at 12 in o.c.), per prescriptive deck tables at 40 psf live load. If your deck projection is longer than the allowed span, add an intermediate beam line — the calculator does this automatically and warns you.

Beam size depends on the joist span it carries and the distance between posts. For example, with joists spanning 12 ft, a doubled (2-ply) Southern pine 2×12 beam can span 8 ft 7 in between posts, while a 2-ply 2×10 manages only 7 ft 4 in. Enter your post spacing in the calculator and it checks it against the prescriptive beam-span table, suggesting more posts or a bigger beam when you exceed it.

Posts per beam line = (beam length ÷ post spacing) + 1, and you need one footing per post. A 16 ft wide deck with posts every 8 ft needs 3 posts and 3 footings on its beam line. Concrete per footing = π × (diameter/2)² × depth; a 12-inch diameter footing 36 inches deep takes about 2.36 cubic feet, or 4 bags of 80 lb concrete mix.

Footings must bear on undisturbed soil below your local frost line — commonly 36 to 48 inches deep in northern US states, shallower or frost-free in the south. Enter your local frost depth in the calculator; it sizes the concrete per footing from the diameter and depth you give. When in doubt, call your local building department for the required depth.

Code requires that a 4-inch sphere cannot pass through guard openings, so with 2×2 balusters (1.5 in actual) the maximum on-center spacing is about 5.5 inches. Balusters = railing length in inches ÷ 5.5, rounded up. For 40 linear feet of railing that is 480 ÷ 5.5 ≈ 88 balusters, plus a rail post at least every 6 feet.

Plan on 2 screws at every board-to-joist intersection: screws = board courses × joists × 2. A 16×12 ft deck has 26 board courses and 13 joists, so 26 × 13 × 2 = 676 screws — about two 350-count boxes. Hidden fastener systems for composite decking use roughly one clip per intersection instead.

A filled hot tub weighs 4,000–6,000 lb — prescriptive span tables explicitly exclude such concentrated loads, so a standard deck design cannot just be assumed adequate. Turn on hot-tub mode in the calculator for footing and post upgrade guidance (18-inch footings, 6×6 posts, tighter joist spacing), but have a licensed structural engineer design the framing before you place a tub.

Leave 1/8 to 1/4 inch (3–6 mm) between boards for drainage and expansion. Use the wider 1/4-inch gap for wet pressure-treated lumber, which shrinks as it dries; kiln-dried or composite boards can use 3/16 inch. The calculator's board count adjusts automatically with the gap you enter.

Add 10% waste for straight runs with staggered butt joints — cutting and end-trimming consumes it. Bump to 15% for diagonal or herringbone patterns, which generate far more offcuts. The calculator applies your waste percentage on top of the exact coverage, then picks the stock board length (8–20 ft) with the lowest additional waste.

Yes — switch to metric and enter sizes like 140×45, 190×45 or 240×45 mm (AU/NZ) or 47×150 to 47×225 mm (UK). Span guidance maps each metric size to the nearest smaller IRC nominal size as a conservative approximation, and 300/400/450/600 mm spacings map to 12/16/24 in. table columns. Sizes below the prescriptive minimum (90×45, 47×100, 47×125 mm) get no span guidance — verify those with AS 1684, BS 5268/TRADA tables, or an engineer.

A 12×12 ft deck is 144 sq ft. With 5.5-inch boards and a 1/4-inch gap: 144 ÷ 0.479 ≈ 301 linear feet of decking, plus 10% waste ≈ 331 ft. The least-waste stock choice is 8 ft boards: 38 boards (16 ft boards tie at the same cutting waste). Enter your exact dimensions above for a full material list including joists, beams, posts and footings.

4×4 posts are fine for low decks with short post heights. Choose 6×6 posts for decks more than about 4 ft off the ground, for notched beam connections, in high-wind or seismic areas, and always under a hot tub. The calculator lets you switch post size and updates the shopping list and footing guidance accordingly.