How the deck board spacing calculator works
Enter deck width, board width, and a desired gap. The calculator estimates how many whole boards fit while distributing the remaining width across the gaps.
Formula
Boards = floor((deck width + desired gap) ÷ (board width + desired gap))
Actual gap = (deck width − boards × board width) ÷ (boards − 1)
Example
For a 12 ft width with 5.5 inch boards and a 1/8 inch desired gap, the calculator estimates the board count and resulting average gap.
Assumptions and material notes
This is a planning calculation for evenly distributed boards. Follow the decking manufacturer's recommended spacing and account for board movement, moisture, drainage, and installation conditions.
Frequently asked questions
Why can the actual gap differ from my desired gap?
The deck width may not fit an exact number of boards at the desired gap, so the remaining width is distributed among the gaps.
Does this include perimeter framing?
No. It only estimates field deck boards.
Should I use the manufacturer gap?
Yes.
Can I use decimals?
Yes.
What if the calculated board count is too low?
Check the result against the actual deck edges and framing; perimeter boards and layout changes can require additional material.
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Disclaimer: Estimates are for planning purposes only. Actual material requirements may vary based on site conditions, material specifications, installation methods, waste, and local requirements. Always verify quantities with your supplier or qualified professional before purchasing materials.