Deck Board Calculator

Deck boards are counted by rows, not by area, because a board only ever covers a fixed strip of width. What matters is how many strips fit across the deck and how long each strip has to be.

The width of a strip is the board plus one gap, and the gap matters more than people expect. A 5.5 inch board with a quarter-inch gap covers 5.75 inches, so a 12 foot wide deck needs 26 rows rather than the 25 you would get from board width alone.

Everything after that is multiplication. Rows times deck length gives linear footage, add waste, divide by the length of board you can buy. Fasteners come from area, because screw count depends on how many joists the boards cross rather than on how the boards are cut.

1. Step 1 How many rows rows = ceil(deck_width_inches ÷ (board_width + gap))
2. Step 2 Linear footage linear_ft = rows × deck_length
3. Step 3 Add cutting waste linear_ft × (1 + waste%) 10% for a rectangular deck, 15% or more for diagonal or picture-frame layouts.
4. Step 4 Convert to boards boards = ceil(linear_ft_with_waste ÷ board_length)
5. Step 5 Fasteners and framing screws = deck_area ÷ 100 × screws_per_100_sqft joists = ceil(deck_length_inches ÷ spacing) + 1

Example: a 16 × 12 ft deck with 5.5 in boards and 1/4 in gaps needs 26 rows. That is 416 linear feet, or 458 with 10% waste — 29 boards at 16 ft. Add about 672 screws and 13 joists at 16 in spacing.

• Boards are assumed to run along the deck length in a straight, parallel layout. Diagonal decking wastes considerably more — raise the waste figure to 15–20% — and picture-frame borders need extra long boards for the perimeter.
• Board widths are actual, not nominal. A 5/4 × 6 deck board and a 2 × 6 are both really 5.5 inches wide.
• The 1/4 inch default gap suits kiln-dried and composite boards. Wet-treated lumber shrinks as it dries and should be laid at about 1/8 inch, otherwise the finished gaps end up too wide.
• Screw estimates assume two fasteners per board per joist at 16 inch spacing. Hidden fastener systems and clip systems use different quantities entirely — check the manufacturer's coverage figure.
• The joist count is for the deck footprint only. It does not size the joists, beams, posts or footings, all of which depend on span, load and local code and need a proper span table or an engineer.
• No allowance is made for stairs, railings, fascia or skirting. Those are separate material lists.
• Butt joints are assumed acceptable where the deck is longer than the boards available. Staggering them across rows and landing every joint on a joist is a structural requirement, not a preference.