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Deck board calculator

Find how many deck boards to buy for a new deck surface. The calculator uses the actual board width plus the gap between boards, not the nominal size.

Updated Checked by Matholio Editorial TeamFree, no sign-up
Length (along the boards)Width

How to measure

Measure the deck along the direction the boards will run, then across it.

Deck size
Boards

A 5/4 × 6 deck board is about 5.5 in wide.

Deck boards needed

29 boards

Buy 29 boards, 16 ft long (464 linear ft)

Deck area192 sq ft
Rows of boards across26
Boards per row1
Boards with 10% waste29

Boards run along the deck length. Each row uses the actual board width plus one gap.

How many deck boards do I need?

Divide the deck width in inches by the board width plus the gap to get the rows, then multiply by the boards needed per row. A 16 × 12 ft deck with 5.5 in boards and 1/8 in gaps needs 26 rows of 16 ft boards; with 10% waste, buy 29 boards.

Worked example: a 16 × 12 ft deck

  1. Width across the boards: 12 ft × 12 = 144 in.
  2. Each row takes 5.5 in + 0.125 in = 5.625 in.
  3. Rows: 144 ÷ 5.625 = 25.6, rounded up to 26 rows.
  4. Boards per row: 16 ft deck ÷ 16 ft boards = 1.
  5. With 10% waste: 26 × 1.10 = 28.6, rounded up to 29 boards.

Deck boards needed by deck size

Using 5.5 in boards as long as the deck, with 10% waste.

Deck boards by deck size and gap
Deck Board length Wood, 1/8 in gap Composite, 1/4 in gap
12 × 12 ft 12 ft 29 29
16 × 12 ft 16 ft 29 29
20 × 12 ft 20 ft 29 29
16 × 16 ft 16 ft 39 38
20 × 16 ft 20 ft 39 38

How the calculator works

Rows = deck width (in) ÷ (board width + gap), rounded up

Boards per row = deck length ÷ board length, rounded up

Boards = rows × boards per row × (1 + waste %), rounded up

Deck boards are sold by nominal size, but the actual width is smaller: a 5/4 × 6 board is about 1 × 5.5 in, and most composite boards are also 5.5 in wide. Wood decking is usually gapped 1/8 to 1/4 in, and Trex calls for a 1/4 in gap between the sides of its composite boards.

If the deck is longer than your boards, the calculator adds butt joints in each row. Joints should land over a joist, so check that your board lengths work with the joist layout.

Frequently asked questions

What is the actual width of a deck board?

A nominal 5/4 × 6 deck board is about 1 in thick and 5.5 in wide. Most composite deck boards are also 5.5 in wide.

How far apart should deck boards be?

Wood decking is usually spaced 1/8 to 1/4 in apart. For composite decking, follow the manufacturer; Trex specifies a 1/4 in gap between board sides.

How many deck boards for a 12 × 12 deck?

With 5.5 in boards, 12 ft long and 1/8 in gaps, a 12 × 12 ft deck needs 26 rows. With 10% waste, buy 29 boards.

Does this include joists and stairs?

No. It covers the deck surface only. Framing, fascia, stair treads and railings are separate materials.

Sources

  1. Engineer Fix, What Is the Actual Width of Deck Boards?
  2. Lowe's (answer from Trex), Trex Transcend deck board spacing