Free Deck Board Spacing Calculator — Even Gaps & Layout (2026)

How to Use This Calculator

1. 1Enter the deck width in feet (the dimension across which boards will be laid).
2. 2Enter the board width in inches (5.5 inches is standard for 5/4x6 decking).
3. 3Enter your desired gap size in inches (1/4 inch is typical for dry wood).
4. 4Click Calculate to get even board count and actual uniform gap.

About This Material

Proper deck board spacing is critical for drainage, wood movement, and deck longevity. Gaps between boards allow water to drain through, air to circulate underneath, and the boards to expand and contract with temperature and humidity changes. For pressure-treated wood installed dry, use 3/16 to 1/4-inch gaps. If the wood is freshly treated and still wet (common right off the delivery truck), install boards tightly with no gap -- the wood will shrink as it dries, creating natural gaps of approximately 1/8 to 3/16 inch. Cedar and redwood should have 1/8 to 3/16-inch gaps. Composite decking (Trex, TimberTech, Azek) expands and contracts with temperature. Most manufacturers specify 1/8 to 3/16-inch gaps between board ends and 1/16 to 1/8-inch side gaps. Always follow the specific manufacturer's spacing chart, which varies by installation temperature. The standard deck board is 5/4x6 (actual 1 inch x 5.5 inches) in either pressure-treated wood ($1.50-3.00 per linear foot) or composite ($3.00-8.00 per linear foot). For a 12-foot deck width with 5.5-inch boards and 1/4-inch gaps, you need approximately 25 boards. This calculator distributes boards evenly so every gap is identical -- preventing the common problem of the last board having a different gap than the rest, which looks unprofessional.

Installation Tips

• •Use a consistent spacer (16d nail for composites, carpenter's pencil for wider wood gaps) between every board.
• •Install the first board perfectly parallel to the house -- every subsequent board follows this reference line.
• •For wet pressure-treated lumber, butt boards tightly together -- they will shrink to create natural gaps.
• •For composites, follow the manufacturer's spacing chart based on installation temperature -- gaps change with season.

Common Mistakes to Avoid

• ⚠Spacing wet pressure-treated boards with gaps -- when the wood dries, gaps become too large and catch shoe heels.
• ⚠Not accounting for composite thermal expansion -- decks installed tight in winter may buckle in summer heat.
• ⚠Inconsistent gaps -- uneven spacing is highly visible and looks unprofessional. Always use a spacer.
• ⚠Gaps too wide (over 3/8 inch) -- can catch heels, allow debris accumulation, and let small objects fall through.

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