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Blueprint Calc

Deck Calculator

Plan deck boards, joists, fasteners, and a rough material cost for pressure-treated, cedar, or composite decking. Gives you a shopping list, not a guess.

How much deck material do I need?

Measurements in feet. Defaults to a 16x12 pressure-treated deck.

You need
  • Deck boards
    30 boards
    16 ft each, 192.00 sq ft deck area + 10% waste
  • Joists (2x8)
    13 joists, ~13 × 12 ft boards
    16" OC, 12.00 ft span
  • Deck screws
    2 × 5 lb boxes
    ~780 screws total
  • Joist hangers
    13 hangers
    one per joist end at the ledger/beam
  • Estimated material cost
    $695
    boards + joists + fasteners + hangers. Excludes posts, concrete, railing, stairs, labor.

This covers the deck surface only. You still need posts, footings (concrete), a ledger board, flashing, stairs, and railing — plus permits in most jurisdictions.

What you'll need to buy

Prices approximate · Links go to retailer search results
Estimated total: ~$695 (materials, before tax)

Product links go to Home Depot search results — pick the best match for your project. Prices shown are rough averages.

How we calculated it

This calculator covers the deck surface — boards, joists, fasteners, and hangers. It does not size posts, beams, footings, railing, or stairs, which depend on deck height, local code, and soil conditions.

  1. Deck area = length × width (sq ft)
  2. Board coverage = (5.5" board width − 1/8" gap) × board length / 144 = sq ft per board
  3. Board count = deck area ÷ board coverage × 1.10 waste, rounded up
  4. Joist count = long side ÷ joist spacing (16" or 12") + 1
  5. Fasteners: 2 screws per joist-board crossing, or 1 hidden clip per crossing for composite
  6. Material cost ≈ boards × price + joists × price + fasteners + hangers

Critical caveats: most jurisdictions require a permit for decks over 30" high or attached to the house. Structural framing (beams, posts, footings, ledger attachment) is not a DIY shortcut area — get a permit, follow the IRC, and have it inspected. Cost estimates are materials only, ballpark, regional pricing varies ±30%.

Joist span by size and spacing (PT Southern yellow pine, wet-service, IRC R507.6)

Maximum span before you need a beam mid-span. Species and grade affect these numbers — always check the official span table for your lumber.

Joist size 12" OC 16" OC 24" OC
2 × 6 9'11" 9'0" 7'10"
2 × 8 13'1" 11'10" 10'3"
2 × 10 16'2" 14'8" 12'10"
2 × 12 18'0" 17'5" 15'3"

Composite decking — regardless of joist size — typically uses straight-lay at 16 OC, diagonal at 12 OC per Trex/TimberTech spec.

Deck board count by deck size (5.5 in board, 16 ft length, straight lay)

Includes 10% waste. Larger decks scale linearly; go diagonal and add ~15% for cuts.

Deck size Area Board count (16 ft) Board count (12 ft)
8 × 10 ft 80 sq ft 13 17
10 × 12 ft 120 sq ft 19 25
12 × 16 ft 192 sq ft 30 40
14 × 20 ft 280 sq ft 44 58
16 × 24 ft 384 sq ft 60 79
20 × 30 ft 600 sq ft 93 124

Deck board effective width = 5.5 in minus 1/8 in gap = 5.375 in. A 16 ft board covers ~7.17 sq ft.

Frequently asked questions

How many deck boards do I need for a 12x16 deck?
A 12x16 deck is 192 sq ft. With 5.5" wide boards (5.375" after a 1/8" gap) and 16 ft lengths, you need about 26 boards to cover it, plus 10% waste — so 29 boards total. Different board widths or lengths change the number.
How far apart should deck joists be?
16" on-center (OC) is standard for pressure-treated, cedar, and for most composite boards laid straight (parallel to the joists). Trex and TimberTech spec 12" OC only for diagonal-lay patterns, some heavier product lines, or hot climates where thermal movement is higher. Always confirm against the specific board manufacturer's install guide before framing.
What size joists do I need?
For exterior decks (IRC R507.6, wet-service loading), pressure-treated Southern yellow pine 2x8 joists at 16" OC span about 11'10". 2x10 spans ~14'. 2x12 spans ~16'6". Note these are deck-specific — interior floor span tables (R502.3.1) are longer because loading and exposure are different. Longer spans need a beam mid-way. Always check the exact table for your lumber species and grade.
Pressure-treated, cedar, or composite — how do I decide?
Pressure-treated is cheapest ($10-15/board) but needs annual staining. Cedar is mid-price ($25-35/board), naturally rot-resistant, ages to silver gray. Composite is expensive ($50-80/board) but zero maintenance for 25+ years. Total-cost-of-ownership, composite often wins over 20 years.
How much does it cost to build a deck?
Ballpark materials: $15-30/sq ft for pressure-treated, $25-45/sq ft for cedar, $35-60/sq ft for composite. Add $20-40/sq ft for labor if hiring a pro. A 12x16 composite deck runs $8,000-$20,000 depending on region, railing style, and stairs.
Do I need a permit to build a deck?
Almost always yes for decks over 30" off the ground or attached to the house. Most jurisdictions also require footing inspections. Call your local building department before buying materials — fines and tear-down orders are expensive.
How deep should deck footings be?
Below the frost line — usually 36"-48" in cold climates, 12"-18" in warm ones. Check your local code. Typical minimum is a 10-12" diameter concrete pier at frost depth, often in a Sonotube form.
Should deck boards be screwed or nailed?
Screws. Coated deck screws (stainless for cedar, exterior-rated for pressure-treated) hold better, resist pop-out over time, and come out cleanly for replacement. Nails back out within a few seasons from freeze-thaw cycles.
What is hidden fastening on a deck?
A clip system that fastens deck boards from the side instead of the top, leaving a surface with no visible screw heads. Required for most composite boards under warranty, optional for wood. Costs more (about $1-2 per sq ft extra) but looks cleaner.
How long does a deck last?
Pressure-treated: 15-25 years with annual maintenance. Cedar: 20-30 years. Composite: 25-30+ years (manufacturer warranty typical). The framing (joists, beams, posts) lasts as long as the fasteners and flashing hold up — keep water away from connections.

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