How Many Deck Boards Do I Need? Board Width, Gap, Waste and Fasteners

Divide the deck width by the board width plus the gap and round up for rows. A 12 ft × 12 ft deck with 5.5 in boards and a 3/16 in gap has 26 rows, or 29 boards of 12 ft with 10% waste.

Multiply the rows by the length the boards run, add about 10% for waste and divide by your board length. For the 12 ft × 12 ft deck with 5/4×6 boards (5.5 in actual) that is 26 rows and 312 linear feet. A 16 ft × 20 ft deck with the boards running along the 20 ft side needs 34 rows and 38 boards of 20 ft. Linear feet are length only, as how to calculate linear feet explains, and if a supplier prices lumber by the board foot, see how to calculate board feet or the board foot calculator.

How do you calculate the number of deck boards?

rows            = ceil( deck width across the boards (in) ÷ (board width + gap) )
total length    = rows × length the boards run (ft)
boards to buy   = ceil( total length × (1 + waste) ÷ board length (ft) )

1. Decide which way the boards run. The dimension along the boards is the run. The dimension across them is the width the rows must fill, converted to inches.
2. Use the actual board width, not the nominal one. Each row takes one board plus one gap.
3. Round the rows up. A partial row still needs a board.
4. Multiply the rows by the run to get the total length of board.
5. Add waste. Ten percent is the usual allowance for cuts, trimmed ends and defects, and it is a convention rather than a standard. Use about 15% for diagonal layouts. Multiply by 1.10 or 1.15, do not add the percentage to the count.
6. Divide by the board length you will buy (8, 10, 12, 16 or 20 ft) and round up.

For the area side of the job, how to calculate square feet covers decks that are not simple rectangles. This guide counts boards for a straight layout. It does not design a deck.

Board width and gap

A deck board sold as 5/4×6 or 2×6 measures 5.5 in wide. The 5/4 board is 1 in thick and the 2×6 is 1-1/2 in thick. Using 6 in leaves you short. Many composite profiles are also 5.5 in wide, but check the product label.

The gap leaves room for drainage and for the boards to move. Dry wood is often laid with about 1/8 in, composite with about 1/4 in, and pressure-treated boards installed wet can go closer because they shrink as they dry. These are conventions. Composite makers publish their own gapping guides, and Trex says to follow them for both width-to-width and end-to-end gaps. Type in the number your maker gives. This guide uses 3/16 in, between the two common values, so that you can see how the count changes.

With no gap at all, the same 144 in would need 144 ÷ 5.5 = 26.18 rows, so a gap matters. On a 16 ft width, going from 1/4 in to 1/8 in adds a whole row.

Worked example: a 12 × 12 ft deck

Boards run along one 12 ft side, 5.5 in wide with a 3/16 in gap, in 12 ft lengths, with 10% waste.

• Width across the boards: 12 × 12 = 144 in.
• Pitch: 5.5 + 0.1875 = 5.6875 in.
• Rows: 144 ÷ 5.6875 = 25.32, so 26 rows.
• Total length: 26 × 12 = 312 ft.
• With 10% waste: 312 × 1.10 = 343.2 ft.
• Boards of 12 ft: 343.2 ÷ 12 = 28.6, so 29 boards.

Boards of 16 ft would need 343.2 ÷ 16 = 21.45, so 22, if you cut them and use the offcuts. Boards of 8 ft would need 43.

Worked example: a 16 × 20 ft deck

The same board and gap, 10% waste, with the boards running along the 20 ft side.

• Width across the boards: 16 × 12 = 192 in.
• Rows: 192 ÷ 5.6875 = 33.76, so 34 rows.
• Total length: 34 × 20 = 680 ft.
• With 10% waste: 680 × 1.10 = 748 ft.
• Boards of 20 ft: 748 ÷ 20 = 37.4, so 38 boards. In 16 ft lengths, with the 20 ft runs spliced over a joist and the offcuts reused, it is 748 ÷ 16 = 46.75, so 47.

Turn the boards the other way, along the 16 ft side, and the rows fill 240 in: 240 ÷ 5.6875 = 42.20, so 43 rows. Total length is 43 × 16 = 688 ft, or 756.8 ft with waste. That is 8 ft more board before waste than the first layout, so the choice should rest on the lengths you can buy and where the joints fall, not on the board count. A diagonal layout needs about 15%: 680 × 1.15 = 782 ft, or 40 boards of 20 ft.

Quick table by deck size

These counts use 5.5 in boards, a 3/16 in gap and 10% waste. The first dimension is the width across the boards and the second is the run. The board columns divide the total length by the stock length, so they assume offcuts are reused and joints land over joists.

What happens to the last row?

The rows rarely fill the width exactly. On the 12 ft deck, 25 full rows use 25 × 5.6875 = 142.19 in of the 144 in, which leaves 1.81 in. The 26th board would be ripped to a strip under 2 in wide. Plan for it before you start:

• Rip the first and last boards evenly to about 3.7 in each (24 full boards, 25 gaps of 3/16 in and two strips: 24 × 5.5 + 25 × 0.1875 = 136.69 in, leaving 7.31 in to split).
• Widen the gap slightly, within the maker’s limit. Exactly 25 boards across 144 in would need 24 gaps of 6.5 ÷ 24 = 0.27 in.
• Adjust the deck width a little if it has not been built yet.

With the first option you still use 26 boards, so the count does not change. Only a wider gap or a narrower deck would save a row.

Joists and fasteners

Every board is fastened where it crosses a joist. Count the crossings: rows × joists. A joist layout count is floor(run in inches ÷ spacing) + 1, with the joists spaced along the run and running across the boards. At 16 in on center, the 12 ft run has floor(144 ÷ 16) + 1 = 10 joists, so the 12 × 12 ft deck has 26 × 10 = 260 crossings. The 16 × 20 ft deck has floor(240 ÷ 16) + 1 = 16 joists and 34 × 16 = 544 crossings.

Face-screwed boards take two screws per crossing and hidden-fastener clips take one, which is a common convention. That gives 520 screws or 260 clips for the 12 × 12 ft deck, and 1,088 screws or 544 clips for the 16 × 20 ft deck. Add about 10% for drops and bent screws, and follow the board maker’s fastener guide, because the type and spacing depend on the board.

Joist spacing of 12, 16 or 24 in on center depends on the board. Trex says 24 in applies only to 2×6 Trex boards in residential or commercial applications at 100 psf or less. Check your board maker’s guide for the spacing it allows. The counts here are only the number of members at a spacing you choose.

Assumptions and limits

• Straight layout with all the boards running one way. Diagonal, herringbone and picture-frame layouts need more, about 15% for diagonal. Stairs, fascia, railings and trim are not counted.
• The total length is divided by the stock length, which assumes you can use offcuts and that joints land over joists. A deck run that is not a multiple of the board length leaves waste that the 10% may not cover.
• Layout, not design. Deck framing, footings, ledger attachment and railings are structural and regulated by code (IRC R507). Joist size, span, beam and footing sizes are not covered here. Get a permit, follow your local code, and consult your building department or an engineer.

Common questions