Deck solidly.
Deck boards, joists, beams, and fasteners for any size deck. Accounts for board width, joist spacing, and the support frame.
How we calculated this
Deck boards: the calculator assumes boards run perpendicular to the joists (standard orientation). Board count is derived from deck width divided by effective board coverage (actual width plus a 1/8" gap for expansion). Total linear feet converts to 16-foot board count with 10% waste, which is standard for straight-cut installations.
Joists: spaced at the specified on-center distance, running perpendicular to the decking. Joist count is the length of the deck divided by spacing, plus one for the closing joist. All joists span the full width. Composite decking requires 12-inch or 16-inch spacing; pressure-treated 2×6 decking can span 24 inches if solid (but not preferred).
Beams are the horizontal members running under the joists, transferring loads to the posts. Most residential decks use doubled (2-ply) 2×8 or 2×10 beams. The calculator assumes 2 beams at 2 plies each, running the full length — this covers typical deck framing but is approximate. Complex deck shapes require more beams.
Posts: rough estimate of one 4×4 or 6×6 post per 80 square feet of deck area. Actual post count depends on beam span capacity, which depends on beam size and wood species. For engineered accuracy, consult the International Residential Code (IRC) 2021 prescriptive tables or have a pro design.
Fasteners: visible deck screws at 2 per board per joist (front and back of board), or hidden fasteners at 1 clip per board per joist. Hidden fasteners cost 3-5× more and add an hour per 100 sq ft of install time but deliver a cleaner finished look.
Not included: ledger board (attaches deck to house), flashing (critical for ledger waterproofing), joist hangers (one per joist end), knee braces for lateral stability, railing posts and balusters (calculate separately), and stairs. These add significantly to material cost — budget 30-40% more than the calculator's output for a complete build.
The part of your deck you never see costs more than the part you do
When my neighbor built his deck in 2023, he picked composite boards, agonized over the color for two weeks, and asked the contractor to match it with a complementary railing. The decking boards came to $3,800. Then the full invoice arrived: $11,200. The other $7,400 went to things he never thought about. Joists. Beams. Post brackets. Concrete for 8 footings. Joist hangers. A ledger board with flashing. A staircase. Two days of labor. The walking surface he spent all his time choosing was barely a third of the bill.
How we calculated these numbers▾
Framing specs follow IRC 2021 Section R507 (decks). Joist spacing and span tables from the American Wood Council (AWC) DCA6 prescriptive residential wood deck construction guide. Composite requirements from Trex (Transcend, Enhance) and TimberTech (AZEK, Pro) installation manuals. Pricing reflects 2026 installed costs from HomeGuide and Angi.
How much a deck costs by size and material
The price ranges below are wide for a reason. Deck costs depend on three things most online estimates ignore. First, height: a ground-level deck needs short posts and no stairs, while a second-story deck needs 10-foot posts and a full staircase. Second, railing: cable rail costs three times what wood balusters cost for the same linear footage. Third, geography: labor in the Northeast runs 40 to 60 percent more than in the Southeast.
DIY | Professional | |
|---|---|---|
| Decking surface (320 ft²) | $1,400–3,000 | $1,400–3,000 |
| Frame lumber (joists, beams, ledger) | $800–1,200 | Included |
| Posts + concrete footings | $300–500 | Included |
| Fasteners + joist hangers | $200–350 | Included |
| Stairs + railing | $400–900 | Included |
| Labor | $0 (3–5 weekends) | $3,000–7,000 |
| Total for 16×20 | $3,100–6,000 | $8,000–16,000 |
DIY saves 50-60% but requires framing knowledge. The ledger board connection (where the deck attaches to the house) is the most structurally critical joint and the one most DIYers get wrong.
Joist spacing: the decision you cannot change later
Joist spacing is set during framing, before a single decking board goes down. Get it wrong and the only fix is tearing up the entire surface, adding or moving joists, and re-laying everything. This is the most expensive mistake in deck building because it turns a $200 framing decision into a $2,000 rework.
Composite illustration based on typical project dimensions, regional contractor pricing, and 2026 material costs. Not a specific real project.
Fasteners: why the wrong screw ruins a $10,000 deck
Fastener callbacks account for roughly 80 percent of deck warranty claims and service calls, according to NADRA (North American Deck and Railing Association) member surveys. The problem is almost always the same: someone used regular galvanized screws on a pressure-treated deck. The copper preservative in modern PT lumber (ACQ or CA-B) corrodes standard galvanized coating within 18 to 24 months. The screw heads turn black, stain the wood, and eventually lose holding power. Stainless steel screws or screws specifically rated for ACQ-treated lumber solve this completely. They cost twice as much per box and save the entire deck.
Hidden clips | Face screws | Nails | |
|---|---|---|---|
| Look | Clean, no visible fasteners | Screw heads visible | Nail heads visible |
| Cost | $0.75–1.50/ft² | $0.25–0.50/ft² | $0.10–0.20/ft² |
| Board removal | Individual boards removable | Individual removal possible | Boards split during removal |
| Best for | Composite, premium cedar | PT, standard cedar | Framing only, never for decking |
Hidden clips add $200-500 to a typical deck but are standard for composite and allow individual board replacement if one gets damaged.
Permits and inspections
Permits run $100 to $500 depending on your municipality. The inspection checks four things that fail catastrophically when done wrong: the ledger board connection to the house (the leading cause of deck collapses), footing depth below frost line, joist hanger installation, and railing post connections. An unpermitted deck creates insurance gaps and complicates home sales. The inspector is not the enemy. The inspector is the person who confirms your deck will not collapse with 20 people on it during a July cookout.
Footings: what holds the whole thing up
Every deck post sits on a concrete footing that extends below the frost line. That means 36 to 48 inches deep in northern climates, 12 to 18 inches in the South. Each footing is a hole about 12 inches in diameter filled with concrete, roughly 0.6 cubic feet per footing or 2 bags of 80-lb mix.
A typical 16 × 20 deck needs 6 to 9 footings depending on beam spacing and load. At 2 bags per footing, that is 12 to 18 bags of concrete, roughly $90 to $135 in materials. The labor is the expensive part, not the concrete itself. Use the concrete calculator for exact footing volumes and the stair calculator for code-compliant stairs from deck to ground.
One thing contractors sometimes skip on lower decks: they set posts directly on poured concrete pads instead of using post base brackets. This puts the wood in direct contact with a surface that wicks moisture. Within 5 to 8 years the post base rots from the inside out. A $4 galvanized post base bracket on every footing keeps wood above the concrete and adds decades to post life. If your contractor's quote does not include post brackets, ask why.
Sources
- IRC 2021 — Deck Design Provisions — Code-compliant joist and beam spans
- Trex — Composite Decking Installation Guide — Board spacing and fastener recommendations
Frequently asked
How many deck boards do I need for a 12×16 deck?
For a 12×16 ft deck (192 sq ft) using 5.5"-wide boards (2×6 nominal), you need about 28 boards at 16 feet long, or 42 boards at 12 feet long. Composite boards (5.25" actual) need slightly more. Use the calculator above for exact numbers based on your board width.
What joist spacing should I use?
Standard pressure-treated 2×6 decking: 16" on center for straight boards, 12" for diagonal layouts. All composite decking: 16" maximum (some require 12"). Wider spacing causes bounce, long-term sag, and voids the composite warranty. Always check the composite manufacturer's spec — they vary.
Do I need hidden fasteners?
Not required, but preferred for composite decking for a cleaner look (no screw heads visible). For pressure-treated lumber, standard deck screws are fine and significantly cheaper. Hidden fasteners cost $0.15-0.30 per clip vs $0.03 per screw. Labor is similar with modern systems.
How far can my beams span?
Depends on beam size and wood species. Typical residential: doubled 2×8 spans 8 feet, doubled 2×10 spans 10 feet, doubled 2×12 spans 12 feet. Longer spans require LVL or triple plies. The calculator assumes typical spacing but doesn't verify span — always cross-check with IRC tables.
What's the difference between 16' and 12' board lengths?
16-foot boards produce fewer end-to-end seams on long decks, which looks cleaner and is slightly stronger. 12-foot boards are easier to handle solo and cheaper per board. For a 16-foot-long deck, 16-foot boards have no butt joints at all — just one continuous board per row.
Do I need ledger flashing?
Critical and not optional for decks attached to the house. A proper ledger installation includes: house wrap cut and folded behind the ledger, metal Z-flashing over the top edge, rubber flashing at both ends. Poor flashing causes hidden water damage to house walls that can be $10,000+ to repair.
Can I calculate without the frame?
The calculator's boards-only count (ignoring joists and beams) is accurate for replacing existing decking where the frame is already in place. For new decks, you need the full materials list. For decks being resurfaced over old framing, inspect the joists and beams for rot — replace anything soft.
How many posts do I need?
Rough rule: one post per 80-100 sq ft of deck for basic spacing. Actual post count depends on beam capacity. Most decks have 4-6 posts on the outside perimeter (beam support) plus a ledger attachment to the house on the fourth side. Freestanding decks need posts on all four sides.
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