Deck Calculator

Board count = ceil(deck width × 12 ÷ effective board width), where effective width = actual board width + 1/8 inch gap. Both 5/4×6 and 2×6 have a 5.5 inch (14 cm) actual width, so effective = 5.625 inches (14.3 cm). For a 12 ft (3.6 m) wide deck: 144 ÷ 5.625 = 25.6 → 26 boards needed. Multiply by 1 + waste% to add the cut overhead — perpendicular pattern 5%, diagonal 15%, picture frame 10%. Each board runs the deck length.

Multiply length by width: square footage = deck length × deck width. For a 12 ft × 16 ft (3.6 m × 4.9 m) deck: 12 × 16 = 192 sq ft (17.8 m²). For L-shaped or multi-level decks, divide into rectangles, calculate each section, and sum. Always include stairs (treads × tread depth × stair width) if you're staining or finishing them. Our calculator handles the area math automatically.

5/4×6 ("five-quarter") is the standard deck board: nominal 1.25 × 6 inches, actual 1 × 5.5 inches (25 × 140 mm). It's lighter, cheaper, and easier to work with — but spans a max 16 inches (40 cm) between joists. 2×6 is heavier framing lumber: actual 1.5 × 5.5 inches (38 × 140 mm). It can span up to 24 inches (60 cm) between joists, costs more per board, and feels more solid underfoot. Same actual width means same board count for any deck size.

Pressure-treated: cheapest ($), 15-20 year lifespan, high maintenance (annual stain), prone to splintering and warping. Cedar: mid-range ($$), 20-25 year lifespan, naturally rot-resistant, mild maintenance, premium look. Composite (Trex, TimberTech): premium ($$$), 25-30 year lifespan, low maintenance, plastic feel. PVC (Azek, AeraTech): top tier ($$$$), 30+ year lifespan, very low maintenance, most realistic wood-look surfaces. Material choice doesn't affect the calculator's board count — the same area takes the same number of boards.

5/4 deck boards: 16 inch (40 cm) on-center max — code-driven for IRC residential. 2× decking: up to 24 inches (60 cm) OC. For premium feel and reduced bounce, tighten to 12 inches (30 cm). Diagonal patterns require 12 inch OC for 5/4 boards because the unsupported diagonal span is longer than the joist spacing. Composite and PVC manufacturers often specify their own max spans — always check the product data sheet, which can override IRC.

Industry rule: 350 deck screws per 100 sq ft (9.3 m²) at 2 screws per joist crossing. For a 12 × 16 ft (192 sq ft / 17.8 m²) deck: 192 ÷ 100 × 350 = 672 screws, or about 9 lb (4 kg). Buy a 5 lb (2.3 kg) box plus a 1 lb (0.5 kg) box for an average deck. Hidden fasteners use roughly one fastener per board-joist crossing — for the same deck: 26 boards × 13 joists = 338 fasteners. Hidden fasteners are slower to install but produce a cleaner finish.

Pressure-treated DIY for a 12 × 16 ft (192 sq ft) deck: lumber $800-1,200 (boards $25 × 26 = $650, joists $20 × 13 = $260, ledger and beam $200, posts $100), fasteners $80, footings $400, hardware $150 — total $1,500-2,000 materials. Pro labor adds $2,500-5,000. Composite scales 2-3× higher. Site complexity (slope, demolition, permits) can double the cost. Use our Deck Footing Calculator and Deck Stain Calculator for related costs.

Almost always yes. Most US jurisdictions require a permit for any deck above 30 inches (76 cm) above grade, attached to a house, or larger than 100-200 sq ft. Permits typically require: site plan, foundation plan with footing size and depth, structural drawing (joist size and span, beam sizing, post layout), guardrail spec for elevated decks. Inspections happen at footing pour, framing complete, and final. Skipping the permit creates resale issues (must disclose) and may force expensive retrofitting.

Perpendicular: boards run across the deck perpendicular to the joists — the standard, lowest waste (5%), simplest cuts. Diagonal: boards run at 45° to the joists — adds visual interest, requires 15% more waste because of angled end-cuts, and needs joist spacing tightened by 1 step (16″ becomes 12″). Picture frame: perimeter boards form a border around an interior field — premium look, 10% waste, more cuts but no joist-span penalty. Our calculator pre-fills the right waste % based on your pattern choice.

Yes — pick Metric in the unit selector at the top and inputs switch to meters. Common conversions: 12 × 16 ft deck = 3.6 × 4.9 m; 16 inch joist spacing = 40 cm OC; 24 inch = 60 cm. Board widths are universally 14 cm (5.5 in) actual. Total area outputs in m². Your unit choice persists across pages and tabs via localStorage. Inputs like joist spacing show both Imperial and metric labels for clarity.