Deck Boards Calculator
Calculate how many deck boards you need to cover a deck. Works for any board width and gap size.
How to use this tool
- Enter the deck length and width in feet.
- Enter the actual face width of your decking boards in inches.
- Set the gap size (typically 1/4 in) and a 10% waste factor.
- The calculator returns total area and number of boards to buy.
Find out exactly how many deck boards to buy before building your deck.
Formula
Deck area = length (ft) × width (ft)
Board spacing (ft) = (board face width (in) + gap (in)) ÷ 12
Rows = ⌈ deck width (ft) ÷ board spacing (ft) ⌉
Boards needed = ⌈ rows × (1 + waste% ÷ 100) ⌉
How it works
This calculator finds how many full-length deck boards are required to cover a rectangular deck. It converts board width and gap to feet, then divides the deck width by that combined spacing to find the number of board rows needed. A waste factor is applied on top to cover end cuts and defects, and the result is rounded up to a whole board count. The calculation assumes boards run the full length of the deck parallel to one another; diagonal or herringbone patterns would increase waste beyond the entered percentage.
Worked example
Worked example
- Deck area = 16 ft × 12 ft = 192 ft².
- Board spacing = (5.5 in + 0.25 in) ÷ 12 = 5.75 ÷ 12 ≈ 0.4792 ft.
- Rows = ⌈12 ft ÷ 0.4792⌉ = ⌈25.04⌉ = 26 rows.
- Apply 10% waste: 26 × 1.10 = 28.6 boards.
- Round up: ⌈28.6⌉ = 29 boards.
You need 29 deck boards for a 16 ft × 12 ft deck using 5.5 in boards with a 0.25 in gap and 10% waste.
Key terms
- Face width
- The visible width of a deck board after it is installed; a nominal 6-inch board typically has an actual face width of 5.5 inches.
- Gap (board spacing)
- The deliberate space left between adjacent deck boards to allow for drainage and wood expansion; 1/4 inch is standard for pressure-treated lumber.
- Waste allowance
- Extra boards ordered to cover end cuts, defects, and miscuts; 10% is standard for straight-lay decks, 15–20% for diagonal patterns.
- Board row
- A single deck board running the full length of the deck from one end to the other; the total number of rows determines the board count.
Frequently asked questions
- How many deck boards do I need?
- Divide the deck width by (board width + gap) to get the number of rows, then multiply by the deck length to see total board linear footage. Add 10% for waste.
- What is the standard gap between deck boards?
- A 1/4-inch gap between boards is standard for composite and pressure-treated lumber. This allows drainage and accounts for wood expansion.