How many deck boards you need
Board count across the width is boards = ceil(deck width in inches ÷ (board width + gap)). A 12 ft wide deck with 5.5 in boards and a 0.125 in gap needs 26 boards, each cut to the deck length. Start from the deck area given by the square footage calculator.
How to use this calculator
Enter the deck width in feet (the direction boards span across), the board width in inches and the gap. The result is the number of rows. For the railing posts, see the fence calculator. Lumber priced by volume is easier to compare in the board foot calculator.
Gaps and waste
A 1/8 in (0.125) gap lets boards drain and expand; composite often uses a wider gap. Add about 10% for cuts and defects. Setting deck footings? Size the pour with the concrete calculator.
The calculator counts rows across the width; to get lumber to buy, multiply the row count by the deck length and pick stock lengths that avoid joints mid-deck. Where joints are unavoidable, land them on a joist and stagger them so they never line up in neighbouring rows. Remember that the gap is what you have after the boards dry: pressure-treated lumber arrives wet and shrinks, so boards fitted tight will open up on their own.
Frequently asked questions
- How do I calculate deck boards?
- Divide the deck width in inches by the board width plus the gap, then round up to whole boards.
- What gap should I leave between deck boards?
- About 1/8 inch for wood to allow drainage and expansion; check the maker's spec for composite.
- How long should deck boards be?
- Each board runs the deck length, so buy lengths that match or minimize end joints.
- How much waste should I add?
- About 10% for cuts, end trimming and the occasional defective board.
- What changes if I lay the boards diagonally?
- Add about 15% more material, because every board is cut at an angle at both ends. The rows also span a longer diagonal distance, so buy longer stock than the deck width suggests.