Deck Calculator
Board count, joist count, and fastener count for any residential deck. Four materials (pressure-treated, cedar, composite, PVC), three patterns with waste filled in, three joist sizes. Imperial and metric units.
Results
Estimates only. Board count assumes 5.5″ actual width with 1/8″ gap. Fastener counts use industry rules of thumb (350 deck screws per 100 sq ft). Always verify joist span and fastener spec against the manufacturer's data and your local code.
How to use this calculator
- 01Choose Imperial or Metric so deck dimensions, board spacing, and results use the units you are working from.
- 02Enter the deck length and width. Length is the direction the deck boards run; width is the direction joists span.
- 03Pick the board size and board pattern. The pattern fills in a starting waste percentage that you can adjust for cuts and layout.
- 04Set joist spacing, joist size, and fastener type so board count, joist count, and fastener count match the framing plan.
- 05Add an optional price per board only when you want a rough material cost, then use Deck Boards as the main ordering quantity.
Use this page for deck boards, joists, and fasteners; size the concrete under the posts separately with the Deck Footing Calculator.
Deck size reference
Board, joist, and linear-foot counts for common deck sizes, computed at 5/4×6 boards in a perpendicular pattern with 16 inch (40 cm) joist spacing and no waste applied. Your actual order should add the calculator’s waste percentage on top.
| Deck size | Boards (5/4×6) | Joists (16″ OC) | Linear ft |
|---|---|---|---|
| 8 ft × 10 ft | 18 | 9 | 180 |
| 12 ft × 16 ft | 26 | 13 | 416 |
| 16 ft × 20 ft | 35 | 16 | 700 |
| 20 ft × 24 ft | 43 | 19 | 1032 |
Frequently asked questions
How to calculate deck boards?
Board count = ceil(deck width × 12 ÷ effective board width), where effective width = actual board width + 1/8 inch (3 mm) 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 (3.66 m) wide deck: 144 ÷ 5.625 = 25.6 → 26 boards. Multiply by 1 + waste% for cut overhead. Perpendicular pattern 5%, diagonal 15%, picture frame 10%. Each board runs the deck length.
How to calculate square footage of a deck?
Multiply length by width: square footage = deck length × deck width. For a 12 ft × 16 ft (3.66 m × 4.88 m) deck: 12 × 16 = 192 sq ft (17.8 m²). For L-shaped or multi-level decks, split into rectangles, calculate each section, and sum. Include stairs (treads × tread depth × stair width) if you're staining or finishing them. The calculator above handles the area math.
What's the difference between 5/4×6 and 2×6 deck boards?
5/4×6 ("five-quarter") is the standard deck board: nominal 1.25 × 6 inches, actual 1 × 5.5 inches (25 × 140 mm). Lighter, cheaper, 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 spans up to 24 inches (60 cm) between joists, costs more per board, and feels more solid underfoot. Same actual width means the same board count for any deck size.
Pressure-treated vs cedar vs composite vs PVC?
Pressure-treated: cheapest ($), 15-20 year lifespan, high maintenance (annual stain), prone to splintering and warping. Cedar: mid-range ($$), 20-25 year lifespan, naturally rot-resistant, mild maintenance, looks high-end. Composite (Trex, TimberTech): expensive ($$$), 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 board count. The same area takes the same number of boards.
What's the right joist spacing for deck boards?
5/4 deck boards: 16 inch (40 cm) on-center max, per IRC residential code. 2× decking: up to 24 inches (60 cm) OC. For a more solid feel and less 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, so check the product data sheet. Their numbers can override IRC.
How many deck screws do I need?
Industry rule: 350 deck screws per 100 sq ft (~38 per 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 take longer to install but produce a cleaner finish.
How much does it cost to build a deck?
Pressure-treated DIY for a 12 × 16 ft (3.66 m × 4.88 m, 192 sq ft / 17.8 m²) 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 in materials. Pro labor adds $2,500-5,000. Composite scales 2-3× higher. Site complexity (slope, demolition, permits) can double the cost. The Deck Footing Calculator and Deck Stain Calculator handle related costs.
Do I need a permit to build a deck?
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 (9.3-18.6 m²). Permits typically require a site plan, foundation plan with footing size and depth, structural drawing (joist size and span, beam sizing, post layout), and a guardrail spec for elevated decks. Inspections happen at footing pour, framing complete, and final. Skipping the permit creates resale issues (you must disclose) and may force expensive retrofitting.
Diagonal vs perpendicular vs picture-frame board patterns?
Perpendicular: boards run across the deck perpendicular to the joists. The standard pattern, lowest waste (5%), simplest cuts. Diagonal: boards run at 45° to the joists. Adds visual interest, takes 15% waste because of the angled end-cuts, and needs joist spacing tightened by 1 step (16″ / 40 cm becomes 12″ / 30 cm). Picture frame: perimeter boards form a border around an interior field. Higher-end look, 10% waste, more cuts but no joist-span penalty. The calculator fills in the waste percentage based on your pattern.
Can I use this calculator with metric measurements?
Yes. Pick Metric in the unit selector at the top and inputs switch to meters. Common conversions: 12 × 16 ft deck = 3.66 m × 4.88 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 sticks across pages and tabs via localStorage. Joist spacing input shows both imperial and metric labels for clarity.