How to use this calculator
- Enter deck dimensions. Length × width in feet. The calculator handles rectangular decks; for L-shaped decks, run two passes.
- Pick board width and length. 5/4 × 6 (5.5" actual) is the most common deck board. Lengths 8, 10, 12, 16 ft. Pick the longest length that minimizes seams.
- Add prices (optional). Enter price per board and per joist. Get total estimate including framing, fasteners, and concrete for posts.
- Read your bill of materials. Decking boards (with waste), floor joists, beams, support posts, concrete bags, and total estimate.
Formula
Boards = Length ÷ (Board Width / 12) × Rows + 10% waste
Worked example
A 16 × 12 ft deck with 5.5" wide × 12 ft long boards: 35 decking boards (with 10% waste), 14 joists at 16" OC, 4 posts, 8 concrete bags. At $12/board, $24/joist, $6.50/bag: about $760 in lumber + $52 in concrete = $812 in materials.
Common project sizes
Quick reference for the most common deck calculator use cases. Use these as a sanity check on your calculator inputs.
| Project | Dimensions | Result |
|---|---|---|
| 12 × 12 ft deck | 144 ft² | ~28 boards · 11 joists · 4 posts |
| 16 × 12 ft deck | 192 ft² | ~35 boards · 14 joists · 4 posts |
| 20 × 16 ft deck | 320 ft² | ~58 boards · 17 joists · 6 posts |
| 24 × 16 ft deck | 384 ft² | ~70 boards · 19 joists · 6 posts |
2026 cost reference
Typical retail price range in the United States for deck. Local pricing varies by region, supplier, and grade — confirm with two or three quotes before ordering.
Per square foot (installed)
$25 – $80
Pressure-treated pine: $25–$45/ft² installed. Cedar: $35–$55. Composite (Trex, TimberTech): $45–$80/ft². Add 30% for elevated decks, multi-level designs, or built-in seating.
How we calculate this
Assumptions baked in
Decking boards = (deck_length / board_width_in_feet) × ceil(deck_width / board_length), rounded up with 10% cut waste. Joists every 16" o.c. with 2×8 default size (10 ft span maximum at 16" o.c. per IRC Table R507.6). Beams: 2×10 doubled, every 8 ft of width. Posts: 4×4 (or 6×6 for decks over 8 ft above grade per IRC R507.4) at every 8 ft along the beam line.
Accuracy and margin of error
Board count is accurate for straight rectangular decks. Diagonal, herringbone, or picture-frame patterns add 15–25% waste — push the input by that much. Joist count ignores cantilevers (allowed up to 24" per IRC R507.6) and double joists under stair stringers — add 2 more joists for each stair landing.
Edge cases this calculator does not handle
Decks over 6 ft above grade require lateral connectors at the ledger (Simpson DTT2Z) per IRC R507.2.4 — the calculator does not count these. Decks over 30" above grade need guardrails (36" min height) — calculate post and baluster count separately. Hot tubs and outdoor kitchens are concentrated loads that often require an engineered footing/beam upgrade.
Cited sources for this page
The figures and rules above are anchored to the following normative references. We link the underlying claim to its standard — not as generic SEO trust signals, but so you can audit any number on this page against a primary source.
Residential deck design including joist span tables, post sizing, and ledger connections is documented in DCA 6 (American Wood Council) and IRC Section R507.
Source: AWC DCA 6 Prescriptive Residential Wood Deck Construction Guide
Lateral load connection between deck and house ledger is required per IRC R507.2.4 — typically via two DTT2Z hold-downs.
Source: IRC 2021 Section R507 — Decks
Tips for accurate results
- Composite decking (Trex, TimberTech) is 2.5–3.5× the price of pressure-treated pine but lasts 25–30 years vs 12–15.
- Use ground-contact rated lumber for any joist within 6 inches of soil. Standard PT only carries an above-ground rating.
- Joists 16" on center for residential. 12" OC for hot-tub or heavy load areas. Always check local code.
- Order 5–10% extra on framing and 10–15% extra on decking. Damaged ends and bad cuts add up.
- Hidden fasteners (Camo, Trex) double the labour but eliminate visible screw heads. Check whether your deck board is grooved for them.
When to consult a pro
Decks are workable DIY for ground-level builds at standard heights. Anything elevated (>30" off grade) or multi-story requires permitting and inspection in most jurisdictions, and structural calculations get nuanced. Hire a pro for: rooftop/balcony decks, pool decks (waterproofing), spans over 12 ft, and any deck that supports more than people (hot tubs, planters, outdoor kitchens).