Deck Board Calculator
Boards, joists, posts & hardware
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Formula & explanation
Deck area is length × width. Linear feet of decking comes from dividing the width by the board width (in feet) to get the number of boards, times the deck length — plus waste. Joists run across the deck at the chosen spacing (16\" OC is standard). Posts support the beam, typically every 6 feet.
Example calculation
12 × 16 ft deck, 5.5\" boards, 16\" joist spacing, 15% waste:
Area = 192 sq ft × 1.15 = 221 sq ft. Boards = (12÷0.458)×16×1.15 ≈ 484 linear ft. Joists = ceil(16÷1.33)+1 = 13.
Buying & project-planning tips
- Use galvanized or stainless deck screws — regular steel will streak and corrode in pressure-treated lumber.
- Space deck boards 1/8–1/4" for drainage — gap wider for wet lumber, narrower for kiln-dried.
- Attached decks are structural — get a permit and follow ledger flashing requirements to prevent rot.
- Joist span tables limit how far joists can run between beams — size beams and posts to your local code.
- Composite decking may require 12" OC joist spacing — check the manufacturer's spec.
Frequently asked questions
How far apart should deck joists be?+
16 inches on-center is standard for 2×6/2×8 joists. Use 12" OC for composite decking or heavy loads, 24" only for 2×6 with lighter residential loads per span tables.
Do I need a permit for a deck?+
Attached decks and any deck over 30 inches above grade almost always require a permit. Freestanding low decks may not — check your local building department.
Composite or wood decking?+
Composite costs more upfront but needs no staining and lasts longer. Wood (pressure-treated, cedar) is cheaper but requires regular maintenance. Composite may need closer joist spacing.
How do I space deck boards?+
1/8–1/4 inch gap for drainage. Wet (green) lumber needs a wider gap because it shrinks as it dries; kiln-dried lumber needs less.