Calculate exactly how many deck boards you need before buying — over-buying wastes money, under-buying means a second trip to the lumber yard mid-project.
Step 1: Calculate Deck Area
Deck Area (sq ft) = Length (ft) × Width (ft)
For L-shaped or irregular decks, break into rectangles and sum the areas.
Example: 16 ft × 20 ft rectangular deck = 320 sq ft
Step 2: Choose Your Decking Board Size
Common decking board nominal sizes and their actual dimensions:
| Nominal Size | Actual Width | Actual Thickness |
|---|---|---|
| 2×4 | 3.5 inches | 1.5 inches |
| 2×6 | 5.5 inches | 1.5 inches |
| 5/4×6 | 5.5 inches | 1 inch |
| 2×8 | 7.25 inches | 1.5 inches |
Composite decking boards vary by brand — measure actual width from the manufacturer spec sheet.
Step 3: Calculate Boards Needed (Without Gaps)
Boards Needed = Deck Length (in) ÷ Board Width (in)
Or if decking runs perpendicular to joists:
Boards Needed = Deck Width (in) ÷ Board Width (in)
Step 4: Account for Board Spacing (Gaps)
Most decks require a ⅛ inch to ¼ inch gap between boards for drainage and expansion:
Effective Board Width = Actual Board Width + Gap
Boards Needed = Deck Run (inches) ÷ Effective Board Width
Example: 16 ft (192 inch) run, using 5/4×6 boards (5.5" wide) with ¼" gap:
- Effective width = 5.5 + 0.25 = 5.75 inches
- Boards = 192 ÷ 5.75 = 33.4 → round up to 34 boards
Step 5: Calculate Total Linear Feet
Total Linear Feet = Boards Needed × Board Length
Boards come in standard lengths: 8, 10, 12, 14, 16, and 20 ft.
Choose lengths that minimise waste. For a 20 ft deck, use 20 ft boards (no cuts). For a 16 ft deck, use 16 ft boards.
Example: 34 boards × 20 ft = 680 linear feet
Step 6: Add Waste Factor
Always add 10–15% for waste from cuts, defects, and layout:
Total with Waste = Linear Feet × 1.10 to 1.15
For decks with diagonal layouts, increase waste factor to 15–20% (diagonal cuts waste more material at the ends).
Example: 680 × 1.10 = 748 linear feet (or 748 ÷ 20 = 38 boards of 20 ft)
Joist Spacing and Framing Materials
Standard joist spacing for 5/4 composite/wood decking:
- 16 inches on centre: Most common for 5/4 decking
- 12 inches on centre: Required for some composite brands and diagonal installations
- 24 inches on centre: Only suitable for 2×6 boards (check manufacturer spec)
Number of joists:
Joists = (Deck Length ÷ Spacing) + 1
Example: 20 ft deck, 16" spacing:
- Joists = (240 ÷ 16) + 1 = 15 + 1 = 16 joists
Fasteners Estimate
Screws: 2–3 screws per board per joist crossing
Screws = Boards × Joists × Screws per Crossing
Example: 34 boards × 16 joists × 2 screws = 1,088 screws
Hidden fasteners (for composite decking): approximately 1 fastener per board width per joist — similar count.
Full Material List for 16×20 ft Deck (5/4×6 decking, 16" joist spacing)
| Material | Quantity |
|---|---|
| 5/4×6 decking boards (20 ft) | 38 |
| 2×8×12 joists | 18 |
| 2×8×20 rim joists | 2 |
| 2×8×16 rim joists | 2 |
| Post anchors or footings | 6–8 |
| 4×4×10 posts | 6–8 |
| 3" exterior screws (1 lb = ~100) | 12 lbs |
| Joist hanger hardware | 32 |
| Beam (if required) | Varies |
Cost Estimate
Typical material costs per square foot of deck (2025):
- Pressure-treated lumber: $15–$25/sq ft of deck area
- Composite decking: $30–$50/sq ft
- Hardwood (ipe, cedar): $25–$40/sq ft
Labour (professional install): $15–$35/sq ft additional.
Use our lumber calculator to get an exact count of all lumber pieces for your specific deck dimensions and joist layout.