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Calculate deck materials: boards, joists, posts, beams, screws and cost from size, board dimensions and spacing.
Calculate deck materials: boards, joists, posts, beams, screws and cost from size, board dimensions and spacing.
Enter values above and click Calculate — results will appear here with the formula explained.
Deck boards run along length; number across width is width divided by board width plus gap (1/8 inch), times rows along length. Joists are placed every 16 inches (12 for diagonal, composite). Waste 10% for cuts. Screws are 2 per board per joist plus 10% extra. For metric, convert: 1 ft = 0.3048 m, board width in cm.
Nominal versus actual lumber dimensions trip up most first-time estimators: a nominal '6-inch' deck board is actually 5.5 inches wide, and nominal 2×8 joists are 1.5×7.25 inches. Always enter actual dimensions — a half-inch error across 40 boards compounds into whole missing boards. Add the 1/8-inch drainage gap between boards to the effective width when counting across.
Joist spacing follows the decking material and pattern. Straight-laid wood decking on 16-inch centers is the standard; composite boards and diagonal patterns (45°) typically require 12-inch centers because both flex more and span shorter under angled loads. Cantilever limits matter too: joists generally shouldn't overhang beams by more than about one-quarter of the joist span — check local span tables rather than guessing.
Posts, beams and footings carry everything the calculator's board count implies. Posts every 6–8 feet on doubled beams is the common residential pattern, with concrete footings extending below the local frost line (12 inches in warm climates, 36–48+ inches where ground freezes). Undersized footings cause the seasonal heaving that opens gaps and loosens railings — the most expensive 'savings' in deck building.
Fasteners deserve their own budget line because they are numerous and corrosion-sensitive. Two screws per board per joist crossing is the standard (hidden clip systems use their own counts), stainless or coated for pressure-treated lumber to resist chemical corrosion, and always 10% extra for drops, splits and misfires. A 300-sq-ft deck easily needs 1,000+ fasteners — buying in bulk boxes beats small packs by wide margins.
Cost control comes from design choices made before buying: 16-foot boards on a 16-foot deck eliminate butt joints and waste; picture-frame borders consume extra material but hide end grain beautifully; stairs cost roughly 3–4 boards per step plus stringers. Price the full list here, then add 10–15% contingency for delivery damage, miscuts and the inevitable extra hardware-store run.
Calculate deck materials: boards, joists, posts, beams, screws and cost from size, board dimensions and spacing. Formula: Deck area = length×width. Example: 16×20 ft deck (320 sq ft), 5.5-inch boards, 16 ft long, 16-inch joists, 10% waste: about 44 boards, 13 joists, 1,430 screws, cost $1,540 at $35/board.
Deck boards run along length; number across width is width divided by board width plus gap (1/8 inch), times rows along length. Joists are placed every 16 inches (12 for diagonal, composite). Waste 10% for cuts. Screws are 2 per board per joist plus 10% extra. For metric, convert: 1 ft = 0.3048 m, board width in cm.
Nominal versus actual lumber dimensions trip up most first-time estimators: a nominal '6-inch' deck board is actually 5.5 inches wide, and nominal 2×8 joists are 1.5×7.25 inches. Always enter actual dimensions — a half-inch error across 40 boards compounds into whole missing boards. Add the 1/8-inch drainage gap between boards to the effective width when counting across.
Joist spacing follows the decking material and pattern. Straight-laid wood decking on 16-inch centers is the standard; composite boards and diagonal patterns (45°) typically require 12-inch centers because both flex more and span shorter under angled loads. Cantilever limits matter too: joists generally shouldn't overhang beams by more than about one-quarter of the joist span — check local span tables rather than guessing.
Posts, beams and footings carry everything the calculator's board count implies. Posts every 6–8 feet on doubled beams is the common residential pattern, with concrete footings extending below the local frost line (12 inches in warm climates, 36–48+ inches where ground freezes). Undersized footings cause the seasonal heaving that opens gaps and loosens railings — the most expensive 'savings' in deck building.
Fasteners deserve their own budget line because they are numerous and corrosion-sensitive. Two screws per board per joist crossing is the standard (hidden clip systems use their own counts), stainless or coated for pressure-treated lumber to resist chemical corrosion, and always 10% extra for drops, splits and misfires. A 300-sq-ft deck easily needs 1,000+ fasteners — buying in bulk boxes beats small packs by wide margins.
Cost control comes from design choices made before buying: 16-foot boards on a 16-foot deck eliminate butt joints and waste; picture-frame borders consume extra material but hide end grain beautifully; stairs cost roughly 3–4 boards per step plus stringers. Price the full list here, then add 10–15% contingency for delivery damage, miscuts and the inevitable extra hardware-store run.
16×20 ft deck (320 sq ft), 5.5-inch boards, 16 ft long, 16-inch joists, 10% waste: about 44 boards, 13 joists, 1,430 screws, cost $1,540 at $35/board. A composite variant of the same deck at 12-inch spacing needs roughly 30% more joists and pricier boards — often $3,000+ in materials — trading upfront cost for decades of low maintenance.
Formulas are standard public references (see our methodology). External standards are cited in the text where they apply.
Last reviewed: September 2026 · Report an error