Decking Calculator — Boards, Joists & Cost
Estimate deck boards, joists, screws, and total cost for any deck. Supports 2×6 wood and composite in standard 8/10/12/16/20 ft (or metric 2.4/3/3.6/4.9 m) lengths, with 12″ or 16″ (300/400 mm) joist spacing.
Decking Calculator
Deck Board Sizes and Materials
| Material | Nominal × Actual | Common Lengths | Cost (per lf) |
|---|---|---|---|
| Pressure-treated pine | 2×6 (1.5″ × 5.5″ / 38×140 mm) | 8, 10, 12, 16, 20 ft (2.4–6 m) | $1.50–$3.00 ($4.90–$9.85/m) |
| Cedar / redwood | 2×6 (1.5″ × 5.5″ / 38×140 mm) | 8, 10, 12, 16 ft (2.4–4.9 m) | $3.00–$5.00 ($9.85–$16.40/m) |
| Composite (Trex/TimberTech) | 1″ × 5.5″ or 7.25″ (25×140/184 mm) | 12, 16, 20 ft (3.7–6.1 m) | $3.50–$8.00 ($11.50–$26.25/m) |
| Ipe / tropical hardwood | 1″ × 5.5″ (25×140 mm) | 6–20 ft (1.8–6.1 m) | $6.00–$12.00 ($19.70–$39.40/m) |
| PVC (Azek/Wolf) | 1″ × 5.5″ (25×140 mm) | 12, 16, 20 ft (3.7–6.1 m) | $5.00–$10.00 ($16.40–$32.80/m) |
Composite resists rot and insects but needs closer joist spacing (12″ / 300 mm) for diagonal layouts. Ipe is the most durable hardwood but requires pre-drilling and oiling annually.
Joist Spacing Guide
| Decking Type | Straight (perpendicular) | Diagonal (45°) |
|---|---|---|
| Pressure-treated 2×6 | 16″ (400 mm) OC | 12″ (300 mm) OC |
| Cedar/redwood 2×6 | 16″ (400 mm) OC | 12″ (300 mm) OC |
| Composite (5/4 thick) | 16″ (400 mm) OC | 12″ (300 mm) OC |
| Composite (1″ thin) | 12″ (300 mm) OC | 8″ (200 mm) OC |
| Ipe (1″) | 16″ (400 mm) OC | 12″ (300 mm) OC |
Joist size by span: 2×6 up to 8 ft (2.4 m), 2×8 up to 10 ft (3 m), 2×10 up to 14 ft (4.3 m), 2×12 up to 18 ft (5.5 m). Use the IRC 2021 R507 deck table or local code for specific species and live load.
How to Calculate Decking Materials
- Measure deck area: length × width in sq ft (or m²).
- Pick board direction: typically the long dimension — this determines board length needed.
- Board coverage: board width (in feet) × board length = sq ft per board. For 5.5″ × 12 ft = 5.5÷12 × 12 = 5.5 sq ft.
- Boards needed: ceil(area ÷ coverage × (1 + waste)). Add 10% standard, 15% diagonal/picture-frame.
- Joists: ceil(deck length ÷ joist OC) + 1. For 16 ft at 16″ OC: ceil(16 ÷ 1.33) + 1 = 13 joists.
- Screws: 2 per joist crossing. boards × ceil(boardLength ÷ joistOC) × 2.
Imperial example: 16×12 ft deck, 5.5″ × 12 ft boards, 16″ OC, 10% waste → 192 sq ft ÷ 5.5 = 35 boards × 1.10 = 39 boards. Joists: 13. Screws: 39 × 18 = 702.
Metric example: 4.9×3.7 m deck, 140 mm × 4.9 m boards, 400 mm OC, 10% waste → 18 m² ÷ 0.69 m² = 27 boards × 1.10 = 30 boards. Joists: 13. Screws: 30 × 26 = 780.
Decking Cost Comparison
| Material | Installed (per sq ft) | Installed (per m²) | Lifespan |
|---|---|---|---|
| Pressure-treated pine | $15–$25 | $160–$270 | 15–25 years |
| Cedar / redwood | $25–$35 | $270–$375 | 20–30 years |
| Composite (Trex Enhance) | $25–$35 | $270–$375 | 25–30 years (warranty) |
| Composite (Trex Transcend / TimberTech AZEK) | $35–$45 | $375–$485 | 50 years (warranty) |
| Ipe / tropical hardwood | $35–$60 | $375–$645 | 40–75 years |
| PVC (Azek) | $30–$50 | $320–$540 | 50 years (warranty) |
Installed cost includes boards, joists/framing, footings, hardware, and labor. DIY material-only cost is roughly 1/3 to 1/2 of installed. Add ~$500–$1,500 for a permit and engineered drawings on decks over 30″ (76 cm) above grade.
Frequently Asked Questions
How many deck boards for a 16×12 deck?
For a 16×12 ft (4.9×3.7 m) deck using 5.5″-wide boards (2×6 actual) running the 12-ft direction: about 35 boards at 12 ft, plus 10% waste = 39 boards. If running boards the 16-ft direction, about 26 boards at 16 ft.
How far apart should deck joists be?
16 in (400 mm) on center is standard for wood decking. Use 12 in (300 mm) OC for composite decking, diagonal patterns, or thinner boards. Always check the decking manufacturer’s specification — composites can require closer spacing for warranty coverage.
How much does a deck cost to build?
Pressure-treated wood: $15–$25/sq ft installed ($160–$270/m²). Composite: $25–$45/sq ft ($270–$485/m²). Cedar/redwood: $25–$35/sq ft ($270–$375/m²). A 16×12 ft (4.9×3.7 m) deck (192 sq ft / 17.8 m²) typically runs $2,900–$8,600 depending on material.
What size boards for decking?
2×6 (actual 1.5″×5.5″ / 38×140 mm) is the most common for wood decks. Composite boards typically come in 5.5″ (140 mm) or 7.25″ (184 mm) widths. For joists, use 2×8 for spans up to 10 ft (3 m), 2×10 for up to 14 ft (4.3 m).
Related Calculators
- Board Foot Calculator — pricing rough lumber for the deck frame
- Stair Calculator — building deck stairs
- Concrete Calculator — need footings for the deck?
- Fence Calculator — adding a railing or fence?
- Paver Calculator — ground-level patio alternative?
- Area Unit Converter — convert between square feet, square meters, acres, hectares
- All Construction Calculators
Board counts assume a single direction with no diagonal cuts. Add 5% extra for picture-frame borders. Joist counts assume a single beam — multi-beam decks need additional posts and footings. Always check local codes (US: IRC R507; UK: BS 5268) for joist spans, ledger attachment, and footing depth. Spacing between deck boards: 1/8″ to 1/4″ (3–6 mm) for drainage and expansion.
