Planning a deck but unsure how many boards to buy? Deck materials are expensive, so accurate calculations save hundreds of dollars. Let's break down exactly what you need.
Quick Answer: Use a Deck Calculator
Our Deck Calculator calculates all materials instantly:
- Deck boards (accounting for waste)
- Joists and beams
- Posts and footings
- Fasteners (screws/nails)
Average deck (12×16 ft): 17.7-19.5 m², ~30 deck boards (16 ft long, 5.5" wide)
Step-by-Step: Calculate Deck Materials
Step 1: Measure Your Deck
Deck dimensions needed:
- Length (ft)
- Width (ft)
- Deck height above ground
- Joist spacing (typically 16" on center)
Example: 3.7 m × 4.9 m deck = 17.8 m²
Step 2: Calculate Deck Boards
Standard deck board sizes:
- Width: 140 mm (actual, marketed as 150 mm)
- Length: 2.4 m, 3 m, 3.7 m, 4.9 m, 6.1 m
- Thickness: 25 mm (actual)
Formula:
Deck area Ă· Board coverage + 15% waste
Example (3.7Ă—4.9 m deck with 4.9 m boards):
- Board coverage: 4.9 m × 0.14 m = 0.69 m² per board
- Boards needed: 17.8 Ă· 0.69 = 25.8 boards
- Add 15% waste: 26.2 Ă— 1.15 = 30 boards
Step 3: Calculate Joists
Joist spacing options:
- 12" on center: Maximum strength, more expensive
- 16" on center: Standard for most decks
- 24" on center: Minimum spacing, composite only
Formula for 16" spacing:
(Deck width in mm) Ă· 400 + 1
Example (3.7 m wide deck): (3,700 mm) Ă· 400 mm + 1 = 10 joists (4.9 m long)
Step 4: Calculate Posts and Footings
Post spacing: Maximum 2.4 m apart
Posts needed:
(Deck length Ă· 8) + 1 per beam line
Example (12Ă—16 deck):
- 2 beam lines (front and back)
- 4.9 m Ă· 2.4 m = 2 + 1 = 3 posts per beam
- Total: 6 posts (6Ă—6, 2.4 m tall)
Step 5: Calculate Fasteners
Deck screws:
- Hidden fasteners: 20 per board
- Face screws: 32 per board (2 screws per joist)
Example (30 boards): 30 boards Ă— 32 screws = 960 screws (~0.9 kg)
Deck Material Costs
Pressure-treated lumber (most affordable):
- Deck boards: $2-4 per linear foot
- Joists (2Ă—8): $8-12 each (4.9 m)
- Posts (6Ă—6): $25-40 each (2.4 m)
- 12Ă—16 deck total: $1,200-2,000
Cedar (mid-range):
- Deck boards: $4-7 per linear foot
- Joists: Same as PT
- 12Ă—16 deck total: $2,500-4,000
Composite (premium):
- Deck boards: $6-12 per linear foot
- Aluminum framing available
- 12Ă—16 deck total: $4,000-8,000
Pro Tips for Deck Building
1. Buy 15% extra decking
Accounts for cuts, defects, and mistakes. Every deck has waste.
2. Mix board lengths
Use different lengths to minimize waste. Stagger seams for strength and appearance.
3. Check board quality
Inspect every board before installing. Return warped, split, or damaged boards immediately.
4. Consider orientation
Run boards perpendicular to house for proper drainage and strength.
5. Use proper spacing
Leave 1/8" gap between boards for drainage and wood expansion. Use spacer nails.
6. Choose the right fasteners
Hidden fasteners: Cleaner look, more expensive
Face screws: Faster install, less expensive, visible
Common Deck Calculation Mistakes
Mistake #1: Forgetting waste factor
Always add 10-15% for cuts and defects.
Mistake #2: Wrong joist spacing
16" on center is standard. Verify your deck board requirements.
Mistake #3: Not accounting for stairs
Stairs use additional boards and stringers. Calculate separately.
Mistake #4: Undersizing posts
Taller decks need larger posts. Follow local building codes.
Deck Size Material Estimates
3×3.7 m deck (11.1 m²):
- Deck boards: 18-22 (4.9 m)
- Joists: 8 (2Ă—8Ă—3.7 m)
- Posts: 4-6
- Estimated cost: $800-1,500 (PT)
3.7×4.9 m deck (17.8 m²):
- Deck boards: 28-32 (4.9 m)
- Joists: 10 (2Ă—8Ă—4.9 m)
- Posts: 6-8
- Estimated cost: $1,200-2,000 (PT)
4.9×6.1 m deck (29.7 m²):
- Deck boards: 48-55 (6.1 m)
- Joists: 13 (2Ă—10Ă—6.1 m)
- Posts: 10-12
- Estimated cost: $2,500-4,000 (PT)
Frequently Asked Questions
How many deck boards do I need for 18.6 m²?
Approximately 28-32 boards (4.9 m long, 140 mm wide) including 15% waste.
What is the best spacing for deck joists?
400 mm on center is standard for wood decking. Composite decking may allow 600 mm spacing—check manufacturer specs.
How do I calculate deck board spacing?
Use 1/8" spacing between boards. Install spacer nail temporarily while fastening adjacent boards.
Can I use 2Ă—6 boards for decking?
Yes, but 5/4Ă—6 boards (actual 1" thick) are standard and more economical. 2Ă—6 is overkill unless spanning long distances.
How far apart should deck posts be?
Maximum 2.4 m apart. Closer spacing (1.8 m) provides better stability for taller decks.
How much overhang should deck boards have?
2.5-5 cm past the outside joist for professional appearance and water runoff.
Calculate Your Deck Materials Now
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- Exact deck boards needed
- Joist and beam quantities
- Post and footing requirements
- Fastener estimates
- Material cost estimates
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Build your dream deck with confidence—start with the right materials count.
Updated: February 2026
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