Quick lookup
Deck materials for common sizes (5/4×6 boards, 16 in OC joists)
Assumes pressure-treated 2×8 joists at 16 in on center, 6×6 posts at 8 ft spacing, 50 lb concrete bag per post footing. Cost is PT lumber only — composite is 2-3x more.
| Deck size | Sq ft | Boards (16 ft) | Joists | Posts | PT cost |
|---|---|---|---|---|---|
| 8 × 10 ft | 80 | 18 | 8 | 4 | $725 |
| 10 × 12 ft | 120 | 26 | 10 | 6 | $1,090 |
| 12 × 12 ft | 144 | 26 | 10 | 6 | $1,310 |
| 12 × 16 ft (standard) | 192 | 26 | 13 | 6 | $1,745 |
| 14 × 20 ft | 280 | 32 | 16 | 9 | $2,545 |
| 16 × 16 ft | 256 | 32 | 13 | 9 | $2,325 |
| 16 × 20 ft (large) | 320 | 32 | 16 | 9 | $2,910 |
| 20 × 24 ft (entertaining) | 480 | 39 | 19 | 12 | $4,365 |
Add 5-10% extra boards for cuts and a budget for joist hangers ($1 each, one per joist end), ledger flashing, hidden fasteners or 2.5 in deck screws, plus stair stringers if needed.
Quick answer
How much material do I need for a deck?
Deck materials break down into 5 buckets: decking (the boards you walk on), joists (the framing under), posts (vertical supports), concrete (for footings), and railing. Our calculator builds the complete list with realistic spacing (16 inch joist OC, 1/8 inch board gap, 8 ft post spacing) and cost estimates.
The math
The formulas explained
Deck boards
Number of boards across the width: ⌈(width × 12) / (boardWidth + gap)⌉. For a 12 ft wide deck with standard 5.5 inch boards and 1/8 inch gaps, you need ⌈144 / 5.625⌉ = 26 boards. Each board equals the length of the deck.
Joists
Spaced 16 inches on center: joists = ⌈(width × 12) / 16⌉ + 1. For a 12 ft wide deck: ⌈144/16⌉ + 1 = 10 joists, each the length of the deck.
Posts
Spaced max 8 ft apart along beam lines: postsPerBeam = ⌈length / 8⌉ + 1. Attached decks have one beam line; freestanding need two.
Concrete footings
12 inch diameter × 3 ft deep footings take 4 bags of 60 lb concrete each.
Choose your boards
Decking material comparison
| Material | Cost + lifespan | |
|---|---|---|
| Pressure-treated pine | $2–3/board ft | 10–15 years. Needs staining every 2-3 years. |
| Cedar | $4–6/board ft | 15–25 years. Naturally rot resistant. Premium look. |
| Composite (Trex, TimberTech) | $7–10/board ft | 25–50 years. No staining. Splinter-free. |
| PVC decking | $10–15/board ft | 30–50 years. Most stain resistant. |
| Tropical hardwood (Ipe) | $10–18/board ft | 40+ years. Heaviest, hardest to work with. |
| Aluminum | $15–20/board ft | 50+ years. Cool to touch even in sun. |
Use cases
Common deck sizes
Small entry deck (8' × 8')
16 deck boards, 7 joists, 6 posts. About $1,200-2,000 in materials.
Standard dining deck (12' × 12')
26 deck boards, 10 joists, 8 posts. About $2,500-4,000.
Large entertaining deck (20' × 16')
35 deck boards, 13 joists, 12 posts. About $5,000-8,000.
Wraparound porch (40' × 8')
18 deck boards × 40 ft each, 7 joists × 40 ft. $4,000-7,000.
Pool deck (20' × 30')
65 deck boards, 23 joists, 22 posts. $8,000-13,000.
Rooftop deck (15' × 20', composite)
Add 30-50% to material cost; specialized framing required.
From a deck builder
Pro tips
- Pull permits. Deck codes are strict (ledger attachment, footing depth, railing height). DIY without permits creates issues at sale time.
- Use joist hangers — never just toenail joists to the rim. Hangers are code in most jurisdictions.
- Place ledger board flashing correctly: under siding, above the ledger. Water in the wall kills more decks than rot in the boards.
- Use ground-contact rated pressure treated for any wood within 6 inches of soil. Regular PT will rot.
- Spacing 1/8 inch between boards drains better than tight installation. Composite uses tighter spacing per manufacturer.
- Hidden fasteners (like Camo or Trex Hideaway clips) cost more but eliminate visible screw heads.
- Stairs eat material — add 15-20% to estimates if your deck has stairs.
Footings are the foundation
Related calculators
Decks need concrete for footings, paint or stain for finish, and usually a fence or roof structure nearby.
Last reviewed: · Methodology based on US building code standards, contractor pricing surveys, and manufacturer specifications.